Control of light speed From slow light to superluminal light
快慢有度高中作文素材
快慢有度高中作文素材Life is all about finding a balance between speed and slowness. The fast pace of modern society often pushes us to move quickly, to achieve more in less time. However, we also need to take momentsto slow down, to enjoy the present moment and appreciate the little things in life.生活总是要在快慢之间找到平衡。
现代社会的快节奏往往推动我们快速行动，在更短的时间内取得更多成就。
然而，我们也需要放慢脚步，享受当下的时刻，欣赏生活中的小事物。
The fast pace of life can sometimes be overwhelming, leading to stress, anxiety, and burnout. It is important to recognize when we need to slow down, to take a break and recharge our batteries. Slowing down can help us feel more grounded, more present, and more connected to ourselves and those around us.生活的快节奏有时会让人感到不堪重负，导致压力、焦虑和倦怠。
重要的是要意识到何时需要放慢脚步，休息一下，恢复精力。
放慢步伐可以帮助我们感到更加踏实，更加在当下，与自己和周围的人更加连接。
On the other hand, there are times when speed is necessary and beneficial. In a world where information travels at the speed of light, being able to keep up with the pace of change is crucial. Speed can be a catalyst for progress, innovation, and growth. It can push us out of our comfort zones and challenge us to reach new heights.另一方面，有时速度是必要和有益的。
Glider Flying Handbook说明书
Glider Flying Handbook2013U.S. Department of TransportationFEDERAL AVIATION ADMINISTRATIONFlight Standards Servicei iPrefaceThe Glider Flying Handbook is designed as a technical manual for applicants who are preparing for glider category rating and for currently certificated glider pilots who wish to improve their knowledge. Certificated flight instructors will find this handbook a valuable training aid, since detailed coverage of aeronautical decision-making, components and systems, aerodynamics, flight instruments, performance limitations, ground operations, flight maneuvers, traffic patterns, emergencies, soaring weather, soaring techniques, and cross-country flight is included. Topics such as radio navigation and communication, use of flight information publications, and regulations are available in other Federal Aviation Administration (FAA) publications.The discussion and explanations reflect the most commonly used practices and principles. Occasionally, the word “must” or similar language is used where the desired action is deemed critical. The use of such language is not intended to add to, interpret, or relieve a duty imposed by Title 14 of the Code of Federal Regulations (14 CFR). Persons working towards a glider rating are advised to review the references from the applicable practical test standards (FAA-G-8082-4, Sport Pilot and Flight Instructor with a Sport Pilot Rating Knowledge Test Guide, FAA-G-8082-5, Commercial Pilot Knowledge Test Guide, and FAA-G-8082-17, Recreational Pilot and Private Pilot Knowledge Test Guide). Resources for study include FAA-H-8083-25, Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-2, Risk Management Handbook, and Advisory Circular (AC) 00-6, Aviation Weather For Pilots and Flight Operations Personnel, AC 00-45, Aviation Weather Services, as these documents contain basic material not duplicated herein. All beginning applicants should refer to FAA-H-8083-25, Pilot’s Handbook of Aeronautical Knowledge, for study and basic library reference.It is essential for persons using this handbook to become familiar with and apply the pertinent parts of 14 CFR and the Aeronautical Information Manual (AIM). The AIM is available online at . The current Flight Standards Service airman training and testing material and learning statements for all airman certificates and ratings can be obtained from .This handbook supersedes FAA-H-8083-13, Glider Flying Handbook, dated 2003. Always select the latest edition of any publication and check the website for errata pages and listing of changes to FAA educational publications developed by the FAA’s Airman Testing Standards Branch, AFS-630.This handbook is available for download, in PDF format, from .This handbook is published by the United States Department of Transportation, Federal Aviation Administration, Airman Testing Standards Branch, AFS-630, P.O. Box 25082, Oklahoma City, OK 73125.Comments regarding this publication should be sent, in email form, to the following address:********************************************John M. AllenDirector, Flight Standards Serviceiiii vAcknowledgmentsThe Glider Flying Handbook was produced by the Federal Aviation Administration (FAA) with the assistance of Safety Research Corporation of America (SRCA). The FAA wishes to acknowledge the following contributors: Sue Telford of Telford Fishing & Hunting Services for images used in Chapter 1JerryZieba () for images used in Chapter 2Tim Mara () for images used in Chapters 2 and 12Uli Kremer of Alexander Schleicher GmbH & Co for images used in Chapter 2Richard Lancaster () for images and content used in Chapter 3Dave Nadler of Nadler & Associates for images used in Chapter 6Dave McConeghey for images used in Chapter 6John Brandon (www.raa.asn.au) for images and content used in Chapter 7Patrick Panzera () for images used in Chapter 8Jeff Haby (www.theweatherprediction) for images used in Chapter 8National Soaring Museum () for content used in Chapter 9Bill Elliot () for images used in Chapter 12.Tiffany Fidler for images used in Chapter 12.Additional appreciation is extended to the Soaring Society of America, Inc. (), the Soaring Safety Foundation, and Mr. Brad Temeyer and Mr. Bill Martin from the National Oceanic and Atmospheric Administration (NOAA) for their technical support and input.vv iPreface (iii)Acknowledgments (v)Table of Contents (vii)Chapter 1Gliders and Sailplanes ........................................1-1 Introduction....................................................................1-1 Gliders—The Early Years ..............................................1-2 Glider or Sailplane? .......................................................1-3 Glider Pilot Schools ......................................................1-4 14 CFR Part 141 Pilot Schools ...................................1-5 14 CFR Part 61 Instruction ........................................1-5 Glider Certificate Eligibility Requirements ...................1-5 Common Glider Concepts ..............................................1-6 Terminology...............................................................1-6 Converting Metric Distance to Feet ...........................1-6 Chapter 2Components and Systems .................................2-1 Introduction....................................................................2-1 Glider Design .................................................................2-2 The Fuselage ..................................................................2-4 Wings and Components .............................................2-4 Lift/Drag Devices ...........................................................2-5 Empennage .....................................................................2-6 Towhook Devices .......................................................2-7 Powerplant .....................................................................2-7 Self-Launching Gliders .............................................2-7 Sustainer Engines .......................................................2-8 Landing Gear .................................................................2-8 Wheel Brakes .............................................................2-8 Chapter 3Aerodynamics of Flight .......................................3-1 Introduction....................................................................3-1 Forces of Flight..............................................................3-2 Newton’s Third Law of Motion .................................3-2 Lift ..............................................................................3-2The Effects of Drag on a Glider .....................................3-3 Parasite Drag ..............................................................3-3 Form Drag ...............................................................3-3 Skin Friction Drag ..................................................3-3 Interference Drag ....................................................3-5 Total Drag...................................................................3-6 Wing Planform ...........................................................3-6 Elliptical Wing ........................................................3-6 Rectangular Wing ...................................................3-7 Tapered Wing .........................................................3-7 Swept-Forward Wing ..............................................3-7 Washout ..................................................................3-7 Glide Ratio .................................................................3-8 Aspect Ratio ............................................................3-9 Weight ........................................................................3-9 Thrust .........................................................................3-9 Three Axes of Rotation ..................................................3-9 Stability ........................................................................3-10 Flutter .......................................................................3-11 Lateral Stability ........................................................3-12 Turning Flight ..............................................................3-13 Load Factors .................................................................3-13 Radius of Turn ..........................................................3-14 Turn Coordination ....................................................3-15 Slips ..........................................................................3-15 Forward Slip .........................................................3-16 Sideslip .................................................................3-17 Spins .........................................................................3-17 Ground Effect ...............................................................3-19 Chapter 4Flight Instruments ...............................................4-1 Introduction....................................................................4-1 Pitot-Static Instruments ..................................................4-2 Impact and Static Pressure Lines................................4-2 Airspeed Indicator ......................................................4-2 The Effects of Altitude on the AirspeedIndicator..................................................................4-3 Types of Airspeed ...................................................4-3Table of ContentsviiAirspeed Indicator Markings ......................................4-5 Other Airspeed Limitations ........................................4-6 Altimeter .....................................................................4-6 Principles of Operation ...........................................4-6 Effect of Nonstandard Pressure andTemperature............................................................4-7 Setting the Altimeter (Kollsman Window) .............4-9 Types of Altitude ......................................................4-10 Variometer................................................................4-11 Total Energy System .............................................4-14 Netto .....................................................................4-14 Electronic Flight Computers ....................................4-15 Magnetic Compass .......................................................4-16 Yaw String ................................................................4-16 Inclinometer..............................................................4-16 Gyroscopic Instruments ...............................................4-17 G-Meter ........................................................................4-17 FLARM Collision Avoidance System .........................4-18 Chapter 5Glider Performance .............................................5-1 Introduction....................................................................5-1 Factors Affecting Performance ......................................5-2 High and Low Density Altitude Conditions ...........5-2 Atmospheric Pressure .............................................5-2 Altitude ...................................................................5-3 Temperature............................................................5-3 Wind ...........................................................................5-3 Weight ........................................................................5-5 Rate of Climb .................................................................5-7 Flight Manuals and Placards ..........................................5-8 Placards ......................................................................5-8 Performance Information ...........................................5-8 Glider Polars ...............................................................5-8 Weight and Balance Information .............................5-10 Limitations ...............................................................5-10 Weight and Balance .....................................................5-12 Center of Gravity ......................................................5-12 Problems Associated With CG Forward ofForward Limit .......................................................5-12 Problems Associated With CG Aft of Aft Limit ..5-13 Sample Weight and Balance Problems ....................5-13 Ballast ..........................................................................5-14 Chapter 6Preflight and Ground Operations .......................6-1 Introduction....................................................................6-1 Assembly and Storage Techniques ................................6-2 Trailering....................................................................6-3 Tiedown and Securing ................................................6-4Water Ballast ..............................................................6-4 Ground Handling........................................................6-4 Launch Equipment Inspection ....................................6-5 Glider Preflight Inspection .........................................6-6 Prelaunch Checklist ....................................................6-7 Glider Care .....................................................................6-7 Preventive Maintenance .............................................6-8 Chapter 7Launch and Recovery Procedures and Flight Maneuvers ............................................................7-1 Introduction....................................................................7-1 Aerotow Takeoff Procedures .........................................7-2 Signals ........................................................................7-2 Prelaunch Signals ....................................................7-2 Inflight Signals ........................................................7-3 Takeoff Procedures and Techniques ..........................7-3 Normal Assisted Takeoff............................................7-4 Unassisted Takeoff.....................................................7-5 Crosswind Takeoff .....................................................7-5 Assisted ...................................................................7-5 Unassisted...............................................................7-6 Aerotow Climb-Out ....................................................7-6 Aerotow Release.........................................................7-8 Slack Line ...................................................................7-9 Boxing the Wake ......................................................7-10 Ground Launch Takeoff Procedures ............................7-11 CG Hooks .................................................................7-11 Signals ......................................................................7-11 Prelaunch Signals (Winch/Automobile) ...............7-11 Inflight Signals ......................................................7-12 Tow Speeds ..............................................................7-12 Automobile Launch ..................................................7-14 Crosswind Takeoff and Climb .................................7-14 Normal Into-the-Wind Launch .................................7-15 Climb-Out and Release Procedures ..........................7-16 Self-Launch Takeoff Procedures ..............................7-17 Preparation and Engine Start ....................................7-17 Taxiing .....................................................................7-18 Pretakeoff Check ......................................................7-18 Normal Takeoff ........................................................7-19 Crosswind Takeoff ...................................................7-19 Climb-Out and Shutdown Procedures ......................7-19 Landing .....................................................................7-21 Gliderport/Airport Traffic Patterns and Operations .....7-22 Normal Approach and Landing ................................7-22 Crosswind Landing ..................................................7-25 Slips ..........................................................................7-25 Downwind Landing ..................................................7-27 After Landing and Securing .....................................7-27viiiPerformance Maneuvers ..............................................7-27 Straight Glides ..........................................................7-27 Turns.........................................................................7-28 Roll-In ...................................................................7-29 Roll-Out ................................................................7-30 Steep Turns ...........................................................7-31 Maneuvering at Minimum Controllable Airspeed ...7-31 Stall Recognition and Recovery ...............................7-32 Secondary Stalls ....................................................7-34 Accelerated Stalls .................................................7-34 Crossed-Control Stalls ..........................................7-35 Operating Airspeeds .....................................................7-36 Minimum Sink Airspeed ..........................................7-36 Best Glide Airspeed..................................................7-37 Speed to Fly ..............................................................7-37 Chapter 8Abnormal and Emergency Procedures .............8-1 Introduction....................................................................8-1 Porpoising ......................................................................8-2 Pilot-Induced Oscillations (PIOs) ..............................8-2 PIOs During Launch ...................................................8-2 Factors Influencing PIOs ........................................8-2 Improper Elevator Trim Setting ..............................8-3 Improper Wing Flaps Setting ..................................8-3 Pilot-Induced Roll Oscillations During Launch .........8-3 Pilot-Induced Yaw Oscillations During Launch ........8-4 Gust-Induced Oscillations ..............................................8-5 Vertical Gusts During High-Speed Cruise .................8-5 Pilot-Induced Pitch Oscillations During Landing ......8-6 Glider-Induced Oscillations ...........................................8-6 Pitch Influence of the Glider Towhook Position ........8-6 Self-Launching Glider Oscillations During Powered Flight ...........................................................8-7 Nosewheel Glider Oscillations During Launchesand Landings ..............................................................8-7 Tailwheel/Tailskid Equipped Glider Oscillations During Launches and Landings ..................................8-8 Aerotow Abnormal and Emergency Procedures ............8-8 Abnormal Procedures .................................................8-8 Towing Failures........................................................8-10 Tow Failure With Runway To Land and Stop ......8-11 Tow Failure Without Runway To Land BelowReturning Altitude ................................................8-11 Tow Failure Above Return to Runway Altitude ...8-11 Tow Failure Above 800' AGL ..............................8-12 Tow Failure Above Traffic Pattern Altitude .........8-13 Slack Line .................................................................8-13 Ground Launch Abnormal and Emergency Procedures ....................................................................8-14 Abnormal Procedures ...............................................8-14 Emergency Procedures .............................................8-14 Self-Launch Takeoff Emergency Procedures ..............8-15 Emergency Procedures .............................................8-15 Spiral Dives ..................................................................8-15 Spins .............................................................................8-15 Entry Phase ...............................................................8-17 Incipient Phase .........................................................8-17 Developed Phase ......................................................8-17 Recovery Phase ........................................................8-17 Off-Field Landing Procedures .....................................8-18 Afterlanding Off Field .............................................8-20 Off-Field Landing Without Injury ........................8-20 Off-Field Landing With Injury .............................8-20 System and Equipment Malfunctions ..........................8-20 Flight Instrument Malfunctions ................................8-20 Airspeed Indicator Malfunctions ..........................8-21 Altimeter Malfunctions .........................................8-21 Variometer Malfunctions ......................................8-21 Compass Malfunctions .........................................8-21 Glider Canopy Malfunctions ....................................8-21 Broken Glider Canopy ..........................................8-22 Frosted Glider Canopy ..........................................8-22 Water Ballast Malfunctions ......................................8-22 Retractable Landing Gear Malfunctions ..................8-22 Primary Flight Control Systems ...............................8-22 Elevator Malfunctions ..........................................8-22 Aileron Malfunctions ............................................8-23 Rudder Malfunctions ............................................8-24 Secondary Flight Controls Systems .........................8-24 Elevator Trim Malfunctions .................................8-24 Spoiler/Dive Brake Malfunctions .........................8-24 Miscellaneous Flight System Malfunctions .................8-25 Towhook Malfunctions ............................................8-25 Oxygen System Malfunctions ..................................8-25 Drogue Chute Malfunctions .....................................8-25 Self-Launching Gliders ................................................8-26 Self-Launching/Sustainer Glider Engine Failure During Takeoff or Climb ..........................................8-26 Inability to Restart a Self-Launching/SustainerGlider Engine While Airborne .................................8-27 Self-Launching Glider Propeller Malfunctions ........8-27 Self-Launching Glider Electrical System Malfunctions .............................................................8-27 In-flight Fire .............................................................8-28 Emergency Equipment and Survival Gear ...................8-28 Survival Gear Checklists ..........................................8-28 Food and Water ........................................................8-28ixClothing ....................................................................8-28 Communication ........................................................8-29 Navigation Equipment ..............................................8-29 Medical Equipment ..................................................8-29 Stowage ....................................................................8-30 Parachute ..................................................................8-30 Oxygen System Malfunctions ..................................8-30 Accident Prevention .....................................................8-30 Chapter 9Soaring Weather ..................................................9-1 Introduction....................................................................9-1 The Atmosphere .............................................................9-2 Composition ...............................................................9-2 Properties ....................................................................9-2 Temperature............................................................9-2 Density ....................................................................9-2 Pressure ...................................................................9-2 Standard Atmosphere .................................................9-3 Layers of the Atmosphere ..........................................9-4 Scale of Weather Events ................................................9-4 Thermal Soaring Weather ..............................................9-6 Thermal Shape and Structure .....................................9-6 Atmospheric Stability .................................................9-7 Air Masses Conducive to Thermal Soaring ...................9-9 Cloud Streets ..............................................................9-9 Thermal Waves...........................................................9-9 Thunderstorms..........................................................9-10 Lifted Index ..........................................................9-12 K-Index .................................................................9-12 Weather for Slope Soaring .......................................9-14 Mechanism for Wave Formation ..............................9-16 Lift Due to Convergence ..........................................9-19 Obtaining Weather Information ...................................9-21 Preflight Weather Briefing........................................9-21 Weather-ReIated Information ..................................9-21 Interpreting Weather Charts, Reports, andForecasts ......................................................................9-23 Graphic Weather Charts ...........................................9-23 Winds and Temperatures Aloft Forecast ..............9-23 Composite Moisture Stability Chart .....................9-24 Chapter 10Soaring Techniques ..........................................10-1 Introduction..................................................................10-1 Thermal Soaring ...........................................................10-2 Locating Thermals ....................................................10-2 Cumulus Clouds ...................................................10-2 Other Indicators of Thermals ................................10-3 Wind .....................................................................10-4 The Big Picture .....................................................10-5Entering a Thermal ..............................................10-5 Inside a Thermal.......................................................10-6 Bank Angle ...........................................................10-6 Speed .....................................................................10-6 Centering ...............................................................10-7 Collision Avoidance ................................................10-9 Exiting a Thermal .....................................................10-9 Atypical Thermals ..................................................10-10 Ridge/Slope Soaring ..................................................10-10 Traps ......................................................................10-10 Procedures for Safe Flying .....................................10-12 Bowls and Spurs .....................................................10-13 Slope Lift ................................................................10-13 Obstructions ...........................................................10-14 Tips and Techniques ...............................................10-15 Wave Soaring .............................................................10-16 Preflight Preparation ...............................................10-17 Getting Into the Wave ............................................10-18 Flying in the Wave .................................................10-20 Soaring Convergence Zones ...................................10-23 Combined Sources of Updrafts ..............................10-24 Chapter 11Cross-Country Soaring .....................................11-1 Introduction..................................................................11-1 Flight Preparation and Planning ...................................11-2 Personal and Special Equipment ..................................11-3 Navigation ....................................................................11-5 Using the Plotter .......................................................11-5 A Sample Cross-Country Flight ...............................11-5 Navigation Using GPS .............................................11-8 Cross-Country Techniques ...........................................11-9 Soaring Faster and Farther .........................................11-11 Height Bands ..........................................................11-11 Tips and Techniques ...............................................11-12 Special Situations .......................................................11-14 Course Deviations ..................................................11-14 Lost Procedures ......................................................11-14 Cross-Country Flight in a Self-Launching Glider .....11-15 High-Performance Glider Operations and Considerations ............................................................11-16 Glider Complexity ..................................................11-16 Water Ballast ..........................................................11-17 Cross-Country Flight Using Other Lift Sources ........11-17 Chapter 12Towing ................................................................12-1 Introduction..................................................................12-1 Equipment Inspections and Operational Checks .........12-2 Tow Hook ................................................................12-2 Schweizer Tow Hook ...........................................12-2x。
Blue Skies-歌词
Blue Skies歌手：LenkaWe've been waiting我们一直在等待Anticipating change coming our way 改变的到来You're my baby你始终是我的宝贝Through the bad nights无论糟糕的夜晚to the brighter days还是美好的白天I know that something good我知道is waiting just around the corners 转角处就会有希望There's a new day dawning新的一天即将开始There's a new life for us新生活将向我们展开Got to keep on holding on只要再继续坚持一下for just a little longer, I know我知道That it's gonna be blue skies那就会是For you and I你我的蓝天We'll step out of the shadows我们即将走出阴影and walk into the light走进光亮Yeah, it's gonna be blue skies是的，那就会是For you and I你我的蓝天But my heart beats slow但我的心跳经过as the storm carries on up high暴风雨的洗礼变慢了And the clouds roll by乌云总会散去I can feel it我感觉到It's coming fast and fierce and wild 它很快就会到来迅猛而狂野I can see it我就知道Everytime I look into those eyes每一次当我看到那些眼神的时候Tell me is it really gonna storm again请告诉我，暴风雨真的又要来了吗Will the sky turn dark, will the rain begin? 天空又会变得灰暗，然后下起雨来I wanna be with you wherever无论电闪雷鸣lightering strikes, cause I know我都会和你在一起因为我知道That it's gonna be blue skies那就会是For you and I你我的蓝天We'll step out of the shadows我们即将走出阴影and walk into the light走进光亮Yeah, it's gonna be blue skies是的，那就会是For you and I你我的蓝天But my heart beats slow但我的心跳经过as the storm carries on up high暴风雨的洗礼变慢了And the clouds roll by乌云总会散去And the clouds roll by希望乌云快一点散去I can't stand rain for one more day我无法再忍受这雨了I know we can make it go away我坚信我们可以驱散它Can you feel it?你能感觉到Like a fever, burning till it breaks它像发烧一样吗？燃烧直至破碎Blue skies你我For you and I的蓝天We'll step out of the shadows我们即将走出阴影and walk into the light走进光亮Yeah, it's gonna be blue skies是的，那就会是For you and I你我的蓝天But my heart beats slow但我的心跳经过as the storm carries on up high(And the clouds roll by) 暴风雨的洗礼变慢了(乌云总会散去)Blue skies, Blue skies (And the clouds roll by)你我的蓝天乌云总会散去Blue skies, Blue skies (And the clouds roll by)你我的蓝天乌云总会散去Blue skies, Blue skies (And the clouds roll by)你我的蓝天乌云总会散去。
光速不变原理的英文
光速不变原理的英文The Principle of the Constancy of the Speed of Light.The principle of the constancy of the speed of light, also known as the special theory of relativity, is a fundamental concept in physics that revolutionized our understanding of space and time. This theory, proposed by Albert Einstein in 1905, states that the speed of light ina vacuum is constant and independent of the motion of the observer or the source of light.Before delving into the intricacies of this principle,it's crucial to understand what light is and how it behaves. Light is a form of electromagnetic radiation that travels through space as waves. These waves oscillateperpendicularly to their direction of propagation, andtheir speed is determined by the properties of the medium through which they travel. In a vacuum, light travels at a constant speed, denoted by the symbol 'c', approximately equal to 299,792,458 meters per second.The significance of the principle of the constancy of the speed of light lies in its implications for physics and cosmology. According to this principle, the speed of lightis the same for all observers, regardless of their relative motion. This means that if two observers are movingrelative to each other, they will measure the speed oflight to be the same, even though their measurements of distance and time will differ.This principle challenges the classical concepts of absolute space and time, introducing the idea of relativity. In the classical view, space and time were considered absolute and unchanging, with all observers agreeing on the measurements of distance and time. However, Einstein's theory suggests that space and time are relative and can be affected by the motion of observers.One of the consequences of the principle of the constancy of the speed of light is time dilation. This phenomenon occurs when time appears to slow down for an observer moving relative to another observer. For example,if an astronaut travels in a spacecraft at a high speed, the time they experience will be slower than the time experienced by someone on Earth. This is because the astronaut's frame of reference is moving relative to the Earth, and the principle of the constancy of the speed of light dictates that the speed of light remains constant regardless of the observer's motion.Another consequence is length contraction. This refers to the phenomenon where an object moving relative to an observer appears to be shorter than it actually is. This is because the moving object's length in the direction of motion is reduced due to the relative motion between the object and the observer.The principle of the constancy of the speed of light has far-reaching implications in physics and cosmology. It underpins many theories and experiments, including the famous Michelson-Morley experiment, which aimed to detect the existence of an ether, a hypothetical medium through which light travels. The negative results of this experiment led to the development of special relativity andthe abandonment of the ether theory.The principle of the constancy of the speed of light also forms the foundation of Einstein's general theory of relativity, which extends the ideas of special relativity to include gravity. General relativity suggests thatgravity is a manifestation of the curvature of spacetime caused by the presence of matter and energy.In conclusion, the principle of the constancy of the speed of light is a fundamental concept in physics that has revolutionized our understanding of space and time. It challenges the classical view of absolute space and time, introducing the idea of relativity and revolutionizing our understanding of the universe. The implications of this principle are vast and far-reaching, touching upon areas such as time dilation, length contraction, and the curvature of spacetime.。
闪光灯使用PPD
As photographers we're always looking for perfect light.And yet, the quality of available light isn't always ideal.It is rarely perfect. But I feel that in using flash wisely, I'm able to enhance or over-ride the available light.作为摄影者总是追求完美的光线，但是现场可用光的质量却并非总是尽如人意。
可以说光线完美的情况是少之又少。
但是合理的利用闪光灯你就能够加强甚至覆盖现场的可用光。
With careful use of flash, I am more in control of light, and hence the way my photos will look - than if I had just accepted the existing ambient light.合理的使用闪光灯你就会对光线的控制更加自如，使你的照片比不用闪光灯时更加耐看。
Instead of waiting for perfect light, I use what I have ...and add flash to make the best of the situation.因此，与其等待好光线出现不如利用你现有的东西---用闪光灯创造出最后的拍摄条件。
A few things we need to get out of the way first :事先要交待几点：- These pages were originally written to help other photographers who struggle with on-camera flash. But it was also written as a reaction against the snobbery of the purists who insist on using available light only - even when it looks terrible.这篇文章是写给那些使用外闪的人看的，当然也是写给即使现场光很差也不肯用闪灯的顽固派看的。
拖拉机常用词翻译
拖拉机常用词翻译1，2，3 speed PTO 1，2，3速动力输出2WD two wheel drive 两轮驱动3-point hitch 三点悬挂4 ultra-slow speed 4个超低速档4WD four wheel drive 四轮驱动540E （540 ECO）经济模式的540转动力输出Accelerator control lever 手油门手柄accelerator pedal 油门踏板accessory 附件adaptor 适配器adhere 粘附adhesive 粘合剂adjustable 可调的agribusiness 农业综合企业ail filter 空滤器air cooling 分冷的air inlet [intake] manifold 进气歧管air manifold 空气歧管align 使成一线alternator 发电机ambient temperature 温度ammeter电流表anticlockwise 逆时针anti-freeze 防冻液armature 电枢armrest 扶手assembly 装配Auxiliary control valve lever 多路控制手柄auxiliary socket 辅助套筒Axle casing桥外壳back hoe 后挖掘机backlash 间隙backrest 靠背backup 候补的Bale 打包baler 打包机，压捆机band 带子，组合barometer，air gauge气压表basic gearbox 基本（型）变速箱battery terminal电瓶极柱battery 电瓶battery 电瓶，蓄电池bearing 轴承bevel gear pair 锥齿轮副bleed 排空气blinker lamp 信号灯block-type synchromesh mechanism 滑块式同步器blown fuse 保险丝断bolt flange 有凸缘的螺栓bolts and nuts 螺钉和螺母bond 粘结(剂)bonnet 机罩boost charge 快充bore 缸径bore 孔,腔, 内腔bottom plow 铧式犁box blade 箱式平地机bracket 支架brake pedal 制动踏板broad caster撒种机，播种机built-in .内置的, 固定的, 嵌入的bulb 灯泡bush insulation 绝缘衬套bushing 衬套butt 粗大一端cable 电缆calcium chloride solution 氯化钙溶液cam 凸轮camshaft 凸轮轴capacity 容量，能力carbon 碳caution 警告Cement floor 水泥地面centrifugal pump 离心泵cetane 十六烷chassis 底盘chattering （机器）咯咯作响chisel plow 錾式犁circuit 回路circumference 周围clamp 夹具clamp 夹子clayey 粘土的clevis pin 叉杆销clip 夹子clockwise 顺时针clutch cover 离合器盖clutch disc boss 离合器摩擦片花键毂clutch disc 摩擦片clutch liner 离合器片clutch pedal 离合器踏板clutch release bearing 离合器分离轴承clutch 离合器cold starting device 冷启动装置collar 轴环combine instrument 组合仪表combine 组合combustion 燃烧compact tractor 紧凑拖拉机Compatibility 兼容性component 部件compress 压缩compression ratio 压缩比condenser 冷凝器cone-shape 锥形conical 锥形的connecting rod 连杆constant mesh transmission 啮合套换挡变速箱，常啮合式变速箱contact 联系coolant temperature guage 冷却液温度表coolant 冷却液coupling 连接盘，联合器crack 裂缝crank 有毛病的, 不正常的;曲柄crankshaft case 曲轴箱crankshaft 曲轴Creeper lever 爬行档手柄creeper 爬行档crimp 卷曲cultivate 培养，耕作cultivator 中耕机cushion 软垫cylinder block 机体cylinder head cove 缸头罩盖cylinder head 气缸盖damping 阻尼Dashboard 仪表板data 数据dealer 经销商dealership 商品经销特许权; 商品特许经销商Decal 贴签defective 故障的deflection 偏斜, 偏转, 偏差deflector 反射器[板], 反射镜Delivery 传输deluxe seat 豪华座椅demo 演示demonstrate 示范, 证明, 论证depress 压下detach 分开, 分离detergent 清洁剂diaphragm spring 膜片弹簧Diesel Engine 柴油机Differential lock switch 差速锁开关differential lock 差速锁diffirential差速器dimension 尺寸dipped headlight，low beam近光灯dipstick 量油计Direction indicator light 转向灯disc brake盘式制动disc-harrow 圆盘耙displacement 排量distilled water 蒸馏水distributor 批发商distributor分配器；分配装置Double action balanced steering cylinder 双作用平衡式转向油缸double thermostat 双节温器double-disk clutch with mechanical PTO 带机械式动力输出的双片离合器down silencer 下行排气管drag link 转向纵拉杆drag 牵引; 拖沓; 拖延; 迟缓的行动drain plug 放油塞draught angle牵引角drawbar 拖车，牵引钩dry disc brake 干式盘式制动dual clutch 双作用离合器dual strobe light 双过滤灯dumping angle 倾翻角durability 耐久dynamic 动力的ECU Environmental Control Unit 环境控制装置electro-hydraulic control 电液操纵electrolyte 电解液eliminate 排除engine block 发动机体Engine coolant temperature gauge 发动机水温表engine model 发动机型号Engine RPM and PTO RPM indicator 发动机和动力输出转速表engine type 发动机型式engine 发动机equivalent 相等的escutcheon 铭牌Essential 本质，实质establish 建立evacuator 抽空装置evaporate 蒸发exaggeration 夸大exclusively 专有地execute 执行，实施exhaust manifold 排气支管exhaust tappet gap. cold engine排气门间隙（冷态）exhaust valve 排气门explosive 易爆炸的exposure 暴露fan with 6 blades. Diameter 340 六片叶风扇。
初二物理光的折射知识点总结
初二物理光的折射知识点总结英文回答：Refraction is a phenomenon that occurs when light passes from one medium to another, causing it to change direction. It is a fundamental concept in physics and has various practical applications in our daily lives. In this summary, I will discuss the key points about the refraction of light.Firstly, when light travels from one medium to another, such as from air to water or from air to glass, it changes its speed. This change in speed causes the light to bend or deviate from its original path. This bending of light is known as refraction.The amount of bending that occurs during refraction depends on the properties of the two media involved. One important property is the refractive index, which is a measure of how much a medium can slow down light. Therefractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in that medium. For example, the refractive index of water is approximately 1.33, which means that light travels about 1.33 times slower in water than in vacuum.Snell's Law is a mathematical equation that describes the relationship between the angles of incidence and refraction. It states that the ratio of the sine of the angle of incidence to the sine of the angle of refractionis equal to the ratio of the speeds of light in the two media. Mathematically, it can be written as n1 sinθ1 = n2 sinθ2, where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively.One interesting phenomenon related to refraction is the total internal reflection. This occurs when light travels from a medium with a higher refractive index to a medium with a lower refractive index and the angle of incidence is greater than a certain critical angle. In this case, instead of refracting, the light is completely reflectedback into the original medium. Total internal reflection is the principle behind the functioning of optical fibers, which are used for transmitting information through light signals.Refraction also explains why objects appear to be displaced when viewed through a transparent medium. This displacement, known as apparent depth, is caused by the bending of light as it travels from the object to our eyes. The apparent depth is different from the actual depth of the object and can be calculated using the formula apparent depth = real depth / refractive index.In conclusion, the refraction of light is a fascinating phenomenon that occurs when light passes from one medium to another. It involves the bending of light and is governed by the refractive indices and angles of incidence and refraction. Understanding the principles of refraction is crucial in various fields, such as optics, telecommunications, and even everyday activities like wearing glasses or swimming underwater.中文回答：折射是光线从一个介质传播到另一个介质时发生的现象，导致光线改变方向。
如何拍出好看的照片英语作文
如何拍出好看的照片英语作文英文回答：How to Take Great Photographs.Composition.Rule of Thirds: Divide the frame into thirds horizontally and vertically, and place the main subject at one of the intersections.Leading Lines: Use lines in the scene to draw the viewer's eye towards the subject.Negative Space: Use empty space around the subject to create a sense of balance and depth.Background: Pay attention to the background to ensure it doesn't distract from the subject.Lighting.Natural Light: Natural light is often ideal for photography, as it provides a soft and even illumination.Golden Hour: The hour after sunrise or before sunset creates warm, golden lighting.Indirect Light: Diffuse light from a cloudy day or through a window can eliminate harsh shadows.Artificial Light: Use flash or studio lighting to control the lighting conditions when necessary.Aperture and Depth of Field.Aperture: The opening in the lens that controls the amount of light entering.Depth of Field: The range of distances that appear sharp in the image.Wide Aperture (low f-number): Creates a shallow depthof field, blurring the background and isolating the subject.Narrow Aperture (high f-number): Creates a large depth of field, keeping most of the image sharp.Shutter Speed.Shutter Speed: The amount of time the camera's shutter remains open.Fast Shutter Speed: Freezes motion and prevents blurriness.Slow Shutter Speed: Creates motion blur and can beused for creative effects.Tripod: Necessary for slow shutter speeds to minimize camera shake.ISO.ISO: The camera's sensitivity to light.Lower ISO: Less sensitive to light, produces images with less noise.Higher ISO: More sensitive to light, useful in low-light situations but can introduce noise.Framing and Perspective.Different Perspectives: Experiment with different vantage points, such as high, low, and side angles.Cropping: Use cropping to eliminate unwanted elements and focus on the main subject.Framing: Use natural elements, such as trees or buildings, to frame the subject.Additional Tips.Practice: The more you practice, the better you willbecome at photography.Study Other Photographs: Analyze great photographs to learn from the techniques and composition.Don't Be Afraid to Experiment: Try different settings and angles to find what works best.Edit Your Photos: Use photo editing software to enhance the quality and impact of your images.中文回答：如何拍出好看的照片。
四年级英语交通信号单选题50题
四年级英语交通信号单选题50题1. When we cross the road, we should stop when we see the ____.A. blue lightB. red lightC. green lightD. white light答案：B。
解析：在交通信号灯中，红灯亮起时我们应该停止过马路，这是基本的交通规则。
选项A蓝色灯光不是交通信号灯的颜色。
选项C绿灯亮起时是可以通行的信号。
选项D白色灯光也不是交通信号灯的颜色。
2. The light that tells us we can go is the ____.A. yellow lightB. red lightC. green lightD. black light答案：C。
解析：绿灯是表示我们可以通行的信号。
选项A黄灯通常是警示信号，意味着红灯即将亮起。
选项B红灯是停止的信号。
选项D黑色不是交通信号灯的颜色。
3. Look! The ____ is flashing. We should wait.A. green lightB. yellow lightC. red lightD. purple light答案：B。
解析：黄灯闪烁时，我们应该等待。
选项A绿灯是通行信号。
选项C红灯是停止信号，但这里说的是闪烁的灯，红灯一般是常亮。
选项D紫色不是交通信号灯的颜色。
4. If you see a ____ while driving, you can keep moving.A. red lightB. yellow lightC. green lightD. orange light答案：C。
解析：当驾驶时看到绿灯就可以继续行驶。
选项A红灯时要停车。
选项B黄灯是警示要准备停车。
选项D橙色不是交通信号灯的颜色。
5. At the crossroads, when the ____ turns on, all cars must stop.A. green lightB. red lightC. yellow lightD. grey light答案：B。
六年级下册英语作文以前和现在的变化30字
六年级下册英语作文以前和现在的变化30字全文共6篇示例，供读者参考篇1The Past and Present: A World of TransformationLife has changed so much since I was a little kid. Back then, everything seemed simpler, yet also more mysterious. The world was full of unknowns, waiting to be explored and understood. Now, as a 6th grader, I've gained a deeper comprehension of how things work, but I also realize how much more there is to learn.One of the most significant changes I've witnessed is in technology. When I was younger, our household had a desktop computer that took forever to boot up, and the internet was excruciatingly slow. Nowadays, we have sleek laptops and smartphones that can access information at lightning speed. The way we communicate has also evolved drastically. I remember when my parents had to use landlines to make calls, and texting was a novel concept. Now, we have video calls and instant messaging apps that keep us connected 24/7.The way we learn and access information has transformed too. In my earlier school years, we relied heavily on textbooks and encyclopedias. If we needed to research a topic, we had to visit the library and sift through mountains of books. Now, with the internet at our fingertips, we can find information on virtually any subject with just a few keystrokes. Online educational resources, like interactive lessons and virtual field trips, have also made learning more engaging and immersive.Entertainment has undergone a significant metamorphosis as well. When I was little, we had cable TV with a limited number of channels. Now, we have streaming services that offer an endless array of movies, shows, and documentaries on demand. Video games have also become more sophisticated, with realistic graphics and intricate storylines that transport us to fantastical worlds.Even the way we socialize and make friends has changed. In the past, we relied on face-to-face interactions and playdates to build relationships. Now, social media platforms have become a central part of our lives, allowing us to connect with people from all around the world. However, this has also brought new challenges, such as cyberbullying and the pressure to present a perfect online persona.Despite all these changes, some things have remained constant. Family and friends are still the pillars of my life, providing love, support, and cherished memories. The joy of playing outside, reading a good book, or creating art hasn't diminished, even in our digital age.As I look back on my childhood and compare it to the present, I'm filled with a sense of awe and gratitude. The world has evolved in ways I could never have imagined, opening up new possibilities and opportunities. At the same time, I recognize the importance of preserving the timeless values and traditions that have shaped who I am.Looking ahead, I can only imagine the incredible transformations that await us in the future. Perhaps we'll have flying cars, holographic classrooms, or even colonies on other planets. Or maybe the changes will be more subtle, but no less profound.One thing is certain: the world will continue to change, and it's up to us to adapt and grow alongside it. We must embrace innovation while staying grounded in our core values and principles. We must be willing to learn and unlearn, to question and challenge the status quo.As a 6th grader, I'm at a pivotal point in my life, poised between the innocence of childhood and the complexities of adulthood. The past has shaped me, but the present and future hold limitless possibilities. With an open mind, a curious spirit, and a willingness to embrace change, I'm ready to embark on this incredible journey of growth and discovery.篇2The Changing TimesIt's crazy how much things have changed since I was a little kid! I can hardly believe how different life is now compared to back then. From the way we entertain ourselves to how we communicate and even what we eat - everything feels so futuristic and high-tech nowadays. Let me take you on a trip down memory lane to the "old days" and examine just how radically my daily experiences have evolved.When I was really little, like 4 or 5 years old, the hottest toy around was these little plastic figurine collections. I remember I was obsessed with Bakugan and had an enormous case full of the little transforming balls and character figures. The biggest thrill was popping open a new one to build up my collection. Nowadays, physical toys like that have taken a total backseat tovideo games, apps, and virtual reality experiences. My biggest entertainment fix comes from games on my tablet or streaming shows. I have a VR headset that literally transports me into other worlds and dimensions - how crazy is that?! The graphics and immersive gameplay is lightyears beyond those simple plastic figures.Speaking of technology, the way kids communicate and stay connected has made a 180 degree turn. I feel like a fossil reminiscing about the old days of passing handwritten notes and playing outside to see your friends. These days, we don't even have to leave the house to hang out and chat. Everything happens online through messaging apps, social media, group video calls, you name it. I'm always just a couple taps away from my whole friend group. On one hand it's pretty awesome to have that level of constant connection, but I'll admit there are times I miss the simplicity of disconnecting and being in the moment.The changes aren't just limited to toys and tech though - even basic stuff like what we eat is totally different. I remember coming home from school completely ravenous and my Mom would just hand me an entire loaf of cheap white bread slathered in butter or peanut butter. That simple carb-bomb was kid heaven back in the day. Nowadays, my parents are so muchmore health-conscious about what they feed me. We eat way more fruits, veggies, whole grains, and lean proteins. And forget peanut butter - my Mom is always pushing these crazy "alternative nut butters" made from almonds, cashews, or sunflower seeds. Don't get me wrong, the health food actually tastes great, but part of me misses cramming my face with simple white bread.One thing that certainly hasn't changed though is how much I looked forward to weekends and holidays for a break from school. Those beautiful buffer days without homework, tests, or having to be anywhere specific were true bliss. The freedom to binge movies, play outside all day, or just veg out. That feeling of escapism and relaxation was unmatched. These days, school breaks mean a lot more activities, scheduled commitments, and general chaos. Between travel plans, extra-curricular activities, social obligations, and my parent's eagerness to keep me productively occupied, there's hardly any downtime anymore. I love being busy and trying new experiences, but yeesh, I could use a little more of that distraction-free alone time to recharge.Now that I'm getting older, my relationship with my parents has shifted big time too. When I was a cute, chubby-cheeked kindergartner, my parents could hardly be separated from me.We were joined at the hip, with my Mom and Dad supervising my every movement and keeping me extremely sheltered. It's not that they weren't loving and warm, but they were very overprotective and nit-picky about everything I did. Normal kid behaviors like tumbling around, making messes, or wandering too far were often met with serious scolding. These days, my parents have chilllllled out so much and give me way more independence. I take care of myself after school some days, keep my own schedule, and have a lot more freedom in general. Of course, I still have rules, a curfew, and get periodic checkings, but the leash is significantly longer than it used to be. With their loosening grip has come more expected responsibility and maturity too...it's a total give and take.I can't forget how my interests and hobbies have changed over time too. As a kid, I was obsessed with anything involving vehicles - toy cars, trucks, planes, you name it. Every birthday篇3The Changing World Around MeWow, has the world ever changed a lot since I was a little kid! It's crazy to think about all the differences between how things used to be and how they are now. From the way we learn andplay to the gadgets and gizmos we use every day, so much has transformed right before my eyes over the past several years. Let me tell you all about the biggest changes I've witnessed!Back when I was in kindergarten and 1st grade, schools were pretty old-fashioned. We had bulky, heavy textbooks for every subject instead of the slim tablet computers we use nowadays. Reading those gigantic tomes gave me a sore back every day from lugging them around in my backpack! The classroom whiteboards and projectors were kind of low-tech too compared to the massive interactive displays we have now. Teachers had to use dry erase markers instead of the cool digital styluses that let you write, draw, and control the entire screen. We also didn't have any of those online learning programs and educational apps that make studying so much more fun and engaging these days.Recess and P.E. class looked a lot different back then as well. The playgrounds had pretty basic equipment - just some swings, a couple of slides, and those old-school merry-go-rounds that seemed to spin at the speed of light! Nowadays the playgrounds are like miniature theme parks with climbing walls, zip lines, and expansive play structures shaped like pirate ships or castles. Sports equipment has evolved too - we used to play withbeat-up old balls and cones, but now we've got all sorts of high-tech gear like electronic scorekeepers and special balls that track every move.My, how technology has transformed since I was a tiny kid! I can barely believe we used to have those chunky old desktop computers that took forever to start up. Half the time they'd freeze up or crash too. Tablets and smartphones were basically unheard of in those days - we had to use paper maps to find our way around instead of GPS! Cameras were bulky separate gadgets rather than being built right into your phone. Video games were pretty pixelated and looked almost 2D compared to the lifelike graphics we've got now. We didn't even have Alexa, Siri, or any other AI assistants to answer our questions!Entertainment, fashion, and pop culture have changed drastically too over the past several years. Back then, the most popular toys were things like stuffed animals and action figures. Sure, we had video games, but the characters and settings were nowhere near as imaginative as the wild stuff kids are into nowadays. It's like every other week there's some fresh new fad - new characters, stories, fashion crazes, you name it. I remember when all us kids were obsessed with those colorful animal characters a few years back. Now that's totally fallen out of styleand everybody's into those weird little collectible doodads. It's impossible to keep up!Going to the movies was simpler in the old days too. Everything was only available to watch in 2D unless you wanted to pay extra for 3D. None of the ultra-immersive experiences they've got now - 360 surrounding screens, motion seats,wind/fog/scent effects. Even at home, all we had was basic cable TV packages or DVDs instead of the bajillion streaming services and channels out there today. Keeping track of what's popular on TikTok, YouTube, SuperStreamFlix and all those other platforms is a full-time job now!I don't know about you, but I think all these changes have been awesome! The technology is just way cooler, learning and playing is way more fun, and pop culture has gotten super creative. I feel a little bad for all the kids who grew up way back when and missed out on all the new advancements. Kids today are so lucky to just take all this wild modern stuff for granted.At the same time, a part of me sort of misses how simple things used to be, you know? It was kind of nice not being bombarded by endless new fads and trends 24/7. Having chunkier old-school gadgets made it easier to step away from the digital world for a while too. And the classic toys and gamesfelt more timeless rather than getting replaced by something new every few months.Still, overall I'm psyched about everything that's transformed in the past several years! Who knows what incredible new advancements we'll see by the time we're all in high school or college? Maybe we'll finally have those flying cars and hoverboards straight out of science fiction. Or we could get wildly advanced virtual reality that lets us step into篇4The Changing TimesWow, so much has changed in the last few years! When I think back to how things were when I was a little kid, it's like a whole different world. From the way we play and learn to the cool new technology we have access to, the present is vastly different from the past in so many ways. Let me tell you all about the biggest changes I've noticed.First up, let's talk about school and learning. Back in the day when I was in 1st or 2nd grade, we still used good old pencils, paper, textbooks, and notebooks for all our work. While those things are still around, we now have a lot more digital tools to help us learn. Every student at my school has an iPad or laptopthat we use for doing research, taking notes, making presentations, and even taking some tests. Our classrooms have huge interactive whiteboards instead of old chalkboards or dry erase boards. We can watch educational videos, explore virtual reality environments, and so much more! Learning has become a much more interactive and engaging experience compared to just reading from textbooks like in the old days.Technology has also totally transformed how we play and have fun after school. I remember when I was really little, my friends and I would just go outside and use our imaginations to play make-believe games or sports with basic equipment like balls, jump ropes, and chalk for drawings. While kids still do those kinds of things sometimes, we also have amazing new options for entertainment. Video games have gotten so incredibly realistic with mind-blowing graphics and immersive experiences in virtual worlds. I'm obsessed with my Xbox and Playstation - the games are like interactive movies! We can also watch incredible shows and movies on streaming services without even needing to wait for things to come on regular TV. And don't even get me started on how much fun social media can be for keeping up with friends and trends. Our parents definitely didn't have access to any of that growing up.Speaking of parents, their lives are incredibly different now too compared to when they were kids. My dad always tells me about how they only had basic cellphones for calling people when he was young. Can you imagine? Now our smartphones are like handheld computers that can do practically anything! We can get directions, take photos and videos, watch stuff, video chat with loved ones, play games, listen to music, read books...the list goes on and on. My mom reminisces about having to walk or take the bus everywhere since families usually only had one car to share. Nowadays it's super common for kids my age to get rides whenever we need them, whether it's from parents' car services or ride sharing apps. My parents also talk about how the internet and social media completely changed how people socialize, work, get information, and do just about everything. The older generations had to rely on things like newspapers, libraries, landline phones, and snail mail to communicate and get information. How archaic!Another big shift I've noticed is with food and how we eat. In the past, most families would cook and eat meals at home every night. There were way fewer options for ordering food or dining out. Now thanks to delivery apps and all the amazing restaurants around, my family rarely cooks at home anymore. Sometimes I'll even get food delivered right to my school for lunch! Mygrandparents talk about how they only had access to basic fruits and veggies when they were kids since exotic imported foods weren't really a thing yet. These days, we can get delicious fruits and cuisine from all around the world no matter where we live. People also used to eat a lot more meat and unhealthy stuff before obesity, heart disease, and issues like that became major concerns. Now, veganism and healthy eating is a huge trend that has changed how a lot of us eat.I could probably go on forever about the differences between the past and present, but those are definitely some of the biggest things I've noticed. From education, to entertainment, to food, to technology, the world has transformed so much in such a short period of time! Makes me really excited to see what kinds of new developments and changes will happen in the years to come. If the past few decades have brought this much change and progress, I can only imagine what life will be like in another 5, 10, or 20 years. While I'm sure some aspects of the past were nice and simpler, I'm really glad I was born in this modern era with all the conveniences, opportunities, and advancements we have access to. The future is looking brighter than ever!篇5My Life: Then and NowWow, it's crazy to think about how much my life has changed over the past few years! It feels like just yesterday I was a tiny little kid, running around the house in my Spider-Man pajamas and making a mess out of everything. Those were such carefree days, you know? All I had to worry about was which cartoon to watch or what game to play next.Back then, my biggest concern was probably trying to stay up past my bedtime. I remember begging my parents to let me watch just one more episode of "Phineas and Ferb" or "Spongebob Squarepants." They'd always give me that look, the one that said, "Nice try, kiddo, but it's time for bed." Of course, I'd whine and pout, but deep down, I knew they were right. After all, what did an eight-year-old need to stay up late for?Those days were filled with such simple joys, too. I'd spend hours upon hours running around the backyard, pretending to be a fearless explorer or a mighty superhero. The neighborhood kids and I would build elaborate forts out of cardboard boxes and fallen branches, defending our makeshift castles from imaginary monsters and evil villains. We'd play tag until our legs grew tired, or we'd have epic water balloon fights on hot summer days.Ah, summer – those were the best times of all! No school, no homework, just endless days of freedom. My friends and I would ride our bikes all over the neighborhood, stopping for popsicles or ice cream whenever we felt like it. We'd have sleepovers and stay up way too late, giggling about silly jokes and telling spooky stories. Those were the days when life felt like one big,never-ending adventure.But now, things are different. I'm a sixth-grader, which means I've entered the dreaded world of middle school. Gone are the days of carefree play and endless summers. Instead, my life is consumed by homework, tests, and extracurricular activities. It's like someone flipped a switch, and suddenly, I have all these expectations and responsibilities weighing down on me.Don't get me wrong, though – it's not all bad. In fact, there are plenty of things I love about being a middle schooler. For one, I finally get to have a bit more independence. My parents trust me to stay home alone for short periods of time, and I even get to walk to and from school by myself (which, let's be honest, makes me feel like a total badass).I also get to explore new interests and hobbies. In elementary school, I was kind of a one-trick pony – all I cared about was sports, sports, and more sports. But now, I'vediscovered a love for things like art, music, and even coding! Who knew I had such a knack for creating digital animations and games?And let's not forget about the social aspect of middle school. Sure, navigating the complex world of pre-teen relationships can be a total minefield, but it's also kind of exciting. There's something thrilling about trying to figure out who likes who and navigating the ever-changing dynamics of friend groups. It's like a real-life soap opera playing out right before my eyes!And then there are the social pressures. Suddenly, things like popularity, fashion, and fitting in have become way more important than they ever were in elementary school. It's like everyone is trying to find their place in the grand scheme of things, and the competition can be pretty intense.But you know what? As overwhelming as it all can be, I wouldn't trade this experience for the world. Sure, I miss the carefree days of my childhood, but I also feel like I'm growing and maturing in ways I never could have imagined. I'm learning to manage my time better, to think critically, and to navigate complex social situations. I'm discovering new passions and interests that will likely shape my future in some way.So, while I may look back on my younger self with a bit of nostalgia, I also feel proud of how far I've come. I'm no longer that wide-eyed, innocent kid who spent his days playingmake-believe. I'm a young adult in the making, slowly but surely figuring out who I am and what I want out of life.And you know what? That's pretty darn exciting.So, here's to growing up, to embracing change, and to all the adventures that lie ahead. Who knows what the future holds? All I know is that I'm ready to face it head-on, armed with the lessons and experiences I've gained along the way.After all, if a scrawny little kid like me could conquer the challenges of middle school, I'm pretty sure I can take on whatever life throws my way next. Bring it on, world – I'm ready for you!篇6Here's an essay of around 2,000 words about the changes from the past to the present, written from the perspective of a sixth-grade student in English:The World Around Me: Then and NowGosh, when I think about how different things were just a few years ago, it's kind of mind-boggling! The world seems to be changing at a rapid pace, and I can't help but marvel at the stark contrasts between the past and present. Let me take you on a journey through time and share some of the most significant transformations I've witnessed in my young life.Technology: The Ever-Evolving LandscapeRemember when we used to play outside with our friends until the streetlights came on? Nowadays, it's all about tablets, smartphones, and video games. Don't get me wrong, I love my gadgets, but sometimes I miss the good old days of jumping rope and playing hopscotch. Back then, we had to use our imagination to keep ourselves entertained. Now, it's all about the latest apps and online games.Speaking of technology, do you remember when we had to wait for our favorite TV shows to air at a specific time? Nowadays, we can stream movies and shows whenever we want, thanks to platforms like Netflix and Hulu. It's like having a never-ending supply of entertainment at our fingertips!Communication: Instant and EverywhereRemember when we had to use landlines to call our friends and family? Nowadays, we can video chat with people from all around the world using apps like FaceTime and Skype. It's like having a virtual sleepover with your best friend who lives across the country!And let's not forget about social media. In the past, we had to physically meet up with our friends to share stories and gossip. Now, we can keep up with each other's lives by scrolling through Instagram and Snapchat feeds. It's like having a front-row seat to everyone's daily adventures!Education: Learning in a Digital AgeRemember when we had to lug around heavy textbooks and notebooks? Nowadays, many schools provide tablets or laptops for students, making it easier to access digital resources and complete assignments. I have to admit, it's a lot more convenient than having to carry a backpack that feels like it weighs a ton!Speaking of school, do you remember when we had to wait for the teacher to hand out worksheets and assignments? Now, we can access our coursework online and submit our assignments digitally. It's like having a virtual classroom that's open 24/7!Environment: Eco-Friendly InitiativesRemember when we didn't think too much about recycling or reducing our carbon footprint? Nowadays, there's a lot more awareness about environmental issues, and many families are making conscious efforts to live more sustainably. From using reusable water bottles to switching to energy-efficient light bulbs, it's like we're all doing our part to protect the planet we call home.And let's not forget about the rise of electric cars and renewable energy sources like solar and wind power. In the past, we relied heavily on fossil fuels, but now there's a growing emphasis on finding cleaner and more eco-friendly alternatives.Health and Wellness: A Holistic ApproachRemember when we thought eating fast food every day was totally fine? Nowadays, there's a lot more emphasis on maintaining a balanced diet and leading an active lifestyle. From fitness trackers to healthy meal delivery services, it's like we're all on a collective journey towards better health and well-being.And let's not forget about the growing popularity of mindfulness practices like yoga and meditation. In the past, these activities were seen as。
小升初英语交通规则单选题60题
小升初英语交通规则单选题60题1.We must stop when we see a _____ sign.A.redB.yellowC.green答案：A。
红色标志通常表示停止，黄色标志表示警告，绿色标志表示通行。
所以看到红色标志时我们必须停下。
2.When the traffic light is _____, we can cross the road.A.redB.yellowC.green答案：C。
绿灯时我们可以过马路。
红灯表示停止，黄灯表示准备停止或警告。
3.What does a “No Parking” sign mean?A.We can park here.B.We can't park here.C.We can drive fast here.答案：B。
“No Parking”表示禁止停车，所以我们不能在这里停车。
选项 A 是可以停车，错误；选项C 是可以在这里快速行驶，与标志无关。
4.A “Speed Limit 40” sign means we can drive at most _____ kilometers per hour.A.30B.40C.50答案：B。
“Speed Limit 40”表示限速40 公里每小时。
5.What should we do when we see a “Pedestrian Crossing” sign?A.Drive fast.B.Slow down and be careful.C.Stop and wait.答案：B。
看到“人行横道”标志时，我们应该减速并小心。
选项A 快速行驶错误；选项C 停止并等待不太准确，一般是有行人通过时才停止等待。
6.We should look _____ before crossing the road.A.left and rightB.only leftC.only right答案：A。
初三英语摄影技巧单选题30题(带答案)
初三英语摄影技巧单选题30题(带答案)1.Which one is used for taking photos in low light?A.Digital cameraB.Film cameraC.SmartphoneD.SLR camera答案：D。
单反相机（SLR camera）在低光环境下表现较好。
数码相机（（Digital camera）、胶片机（（Film camera）和智能手机（（Smartphone）在低光环境下的拍摄效果通常不如单反相机。
2.What can store more photos?A.Memory card with small capacityB.Memory card with large capacityC.Film roll with few exposuresD.Film roll with many exposures答案：B。
容量大的存储卡（Memory card with large capacity）可以存储更多照片。
容量小的存储卡存储照片少。
胶卷（Film roll）的曝光次数有限，不如大容量存储卡存储量大。
3.Which is not a feature of a professional camera?A.Small sizeB.High resolutionC.Fast shutter speedD.Variable aperture答案：A。
专业相机通常不会是小尺寸的。
专业相机具有高分辨率（（High resolution）、快的快门速度（（Fast shutter speed）和可变光圈（Variable aperture）等特点。
4.What is needed to transfer photos from a camera to a computer?B cableB.Bluetooth connectionC.Wi-Fi connectionD.All of the above答案：D。
以慢为美的英语作文怎么写
以慢为美的英语作文怎么写题目: Embracing the Beauty of Slowness。
In today's fast-paced world, where everything seems to move at the speed of light, the concept of embracing slowness may appear counterintuitive. However, it is precisely in the midst of this frenzy that the essence of life's beauty lies. "Slow living" is not merely a trend; it is a philosophy that advocates for a deliberate, mindful approach to life. In this essay, we will delve into the virtues of embracing slowness and explore how it can enrich our lives.To begin with, let us reflect on the frantic pace of modern life. From the moment we wake up to the incessant buzz of notifications on our smartphones to the rush-hour traffic jams and the relentless pursuit of career success, we find ourselves caught in a perpetual race against time. In this race, we often sacrifice our well-being, relationships, and even our sense of self. We become slavesto productivity, measuring our worth by our accomplishments rather than by the moments of joy and connection we experience.Contrary to this lifestyle, embracing slowness offers a sanctuary—a refuge from the chaos of the world. It encourages us to savor the simple pleasures of life, such as enjoying a leisurely walk in nature, sipping a cup of tea in solitude, or engaging in meaningful conversations with loved ones. When we slow down, we become more attuned to our surroundings, noticing the beauty of the world around us—the vibrant hues of a sunset, the delicate petals of a flower, or the soothing melody of birdsong.Moreover, slowness fosters mindfulness—the practice of being fully present in the moment. Instead of dwelling on past regrets or worrying about the future, we learn to immerse ourselves in the here and now. Mindfulness cultivates a deep sense of gratitude for the abundance in our lives, enabling us to find contentment in the simplest of pleasures. Whether it's relishing a home-cooked meal with family or appreciating the warmth of sunlight on ourskin, we discover that true happiness resides not in the pursuit of external goals but in the richness of our inner experiences.Furthermore, embracing slowness allows us to cultivate deeper connections—with ourselves, with others, and with the world at large. In a society that values efficiency and instant gratification, genuine relationships often take a backseat. We communicate through text messages and social media updates, but we seldom take the time to trulylisten—to empathize, to understand, to connect on a soul level. However, when we slow down, we create space for meaningful interactions, fostering bonds that transcend superficiality.In addition to enhancing our well-being and relationships, embracing slowness has profound implications for the environment. The relentless pursuit of economic growth and consumption has led to environmental degradation on an unprecedented scale—deforestation, pollution, and climate change threaten the very foundations of life on Earth. By adopting a slower, more sustainable way of living,we can reduce our ecological footprint and preserve the planet for future generations. Whether it's through consuming less, choosing renewable resources, or supporting local communities, every small step towards slowness contributes to the greater good.In conclusion, embracing the beauty of slowness is not about idleness or complacency; rather, it is a conscious choice to live with intention and presence. In a world that glorifies speed and productivity, slowness offers a path to fulfillment—a path where we can reconnect with ourselves, with others, and with the natural world. By slowing down, we rediscover the true essence of life—the moments of joy, the bonds of love, and the wonder of existence itself. So let us embrace slowness not as a luxury but as a necessity—a guiding principle that leads us towards a more meaningful and harmonious way of living.。
七年级英语交通规则单选题50题
七年级英语交通规则单选题50题1.You see a sign with a red circle and a diagonal line through it. What does it mean?A.You can park here.B.No parking.C.Slow down.D.Turn left.答案：B。
这个标志是禁止停车的标志。
A 选项是可以停车，错误；C 选项是减速，错误；D 选项是左转，错误。
本题考查交通标志的含义和英语词汇理解。
2.There is a sign with a picture of a person walking. What does it mean?A.No entry.B.Pedestrian crossing.C.One way.D.Speed limit.答案：B。
这个标志是人行横道的标志。
A 选项是禁止进入，错误；C 选项是单行道，错误；D 选项是限速，错误。
本题考查交通标志的含义和英语词汇理解。
3.A sign shows a bicycle. What does it mean?A.Bicycle lane.B.No bicycles.C.Parking for bicycles.D.Turn right for bicycles.答案：A。
这个标志是自行车道的标志。
B 选项是禁止自行车，错误；C 选项是自行车停车位，错误；D 选项是自行车右转，错误。
本题考查交通标志的含义和英语词汇理解。
4.The sign with a red triangle and an exclamation mark inside means?A.Danger.B.Yield.C.Stop.D.No passing.答案：A。
这个标志是危险的标志。
B 选项是让行，错误；C 选项是停止，错误；D 选项是禁止超车，错误。
本题考查交通标志的含义和英语词汇理解。
5.A sign with two arrows pointing in opposite directions means?A.One way.B.Divided highway.C.Two-way traffic.D.No U-turn.答案：C。
如何控制骑车速度英语作文
如何控制骑车速度英语作文1. Slow down! You don't want to be speeding on a bike. It's important to control your speed to ensure your safety and the safety of others around you. Take it easy and enjoy the ride at a comfortable pace.2. Keep an eye on your surroundings. Pay attention to the road conditions, traffic, and pedestrians. Adjust your speed accordingly to avoid any potential accidents or collisions. Stay alert and be prepared to slow down or stop when necessary.3. Use your brakes wisely. Don't slam on the brakes suddenly as it can cause you to lose control of the bike. Instead, apply gentle pressure on the brakes to gradually reduce your speed. Practice using your brakes in different situations to get a feel for how they respond.4. Be mindful of the terrain. Different terrains require different speeds. When going uphill, you may needto pedal harder and slow down to maintain control. On the other hand, when going downhill, be cautious and avoid excessive speed to prevent accidents.5. Maintain a safe distance from other vehicles. Give yourself enough space to react and maneuver if needed. Keeping a safe distance will give you more time to slow down or stop in case the vehicle in front of you suddenly brakes.6. Dress appropriately. Wearing proper cycling gear, such as a helmet and reflective clothing, not only protects you but also makes you more visible to others. This canhelp prevent accidents and ensure your safety on the road.7. Stay focused and avoid distractions. Don't use your phone, listen to music, or engage in any activities that may divert your attention from the road. By staying focused, you can better control your speed and react to any unexpected situations.8. Practice, practice, practice. The more you ride yourbike, the better you'll become at controlling your speed. Take the time to practice in different environments and conditions to gain confidence and improve your skills.Remember, controlling your bike's speed is crucial for your safety and the safety of others. Take the necessary precautions, stay alert, and enjoy your ride responsibly.。
time 和 light相关的俚语
time 和 light相关的俚语Time and Light: A Journey through Slang and Idioms Introduction:Time and light are two fundamental aspects of our lives that shape our experiences and perceptions. In this article, we will explore a collection of slang and idiomatic expressions related to time and light. These expressions not only add flavor to our language but also provide insights into how we perceive and value these concepts.1. "In the blink of an eye":This expression is used to describe something that happens very quickly, almost instantaneously. It signifies the speed at which light travels and how a moment can pass by in the blink of an eye.2. "Light at the end of the tunnel":This phrase is often used to provide hope and optimism in challenging situations. It refers to the idea that even in the darkest times, there is always a glimmer of light ahead, symbolizing a way out or a better future.3. "Time flies":We often say this when we are having fun or when we are engaged in something enjoyable. It expresses how quickly time seems to pass when we are fully absorbed in an activity, just like how light travels swiftly.4. "As slow as molasses":This phrase describes something or someone that moves or progresses very slowly. Molasses is a thick, sticky liquid, and the comparison suggests a sluggishness similar to the slow movement of the substance.5. "Daylight robbery":This expression refers to a situation where someone is charged an exorbitant price for something, often deemed unfair or excessive. It originated from the idea that a thief would usually commit a crime under the cover of darkness. In this case, it suggests that the price is so high that it feels like being robbed in broad daylight.6. "Night owl":A night owl is a person who prefers to stay awake and active during the night rather than during the day. This term is derived from the fact that owls are nocturnal creatures, most active during the night.7. "Lightning-fast":This phrase is used to describe something that happens or moves with incredible speed, just like the flash of lightning. It emphasizes the swiftness and suddenness of an action or event.8. "In the nick of time":This expression signifies that something happens or is completed just in time, right before it's too late. The term "nick" refers to a small cut or notch, suggesting that time was almost cut too close.9. "To shed light on":When we say we want to "shed light on" a topic or situation, we mean that we want to provide clarity or understanding. It is like illuminating a dark room with light, making things visible and easier to comprehend.10. "Time is money":This phrase emphasizes the value of time and how it is as precious as money. It suggests that wasting time is equivalent to losing potential earnings or opportunities. Conclusion:Slang and idiomatic expressions related to time and light offer a unique perspective on how we perceive and understand these concepts in our daily lives. These phrases not only add color to our language but also provide insights into the significance we attach to time and the influence of light on our experiences. So next time you come across one of these expressions, take a moment to appreciate the depth and richness they bring to our communication.。
速度术语对照表
速度术语对照表在日常生活和工作中，我们经常用到一些与速度相关的术语。
这些术语涉及到不同的领域，如运动、交通、科学等。
下面是一份速度术语对照表，让我们一起来了解一下吧。
1. 速度（Speed）：在单位时间内所走过的距离。
通常用米每秒（m/s）或千米每小时（km/h）来表示。
2. 加速度（Acceleration）：物体在单位时间内速度的变化率。
加速度的单位是米每秒平方（m/s²）。
3. 时速（Miles per hour）：速度的一种常用单位，表示每小时所行驶的英里数。
4. 光速（Speed of light）：光在真空中传播的速度，约为每秒299,792,458米（约300,000千米）。
5. 音速（Speed of sound）：声音在特定介质中传播的速度。
在空气中约为每秒343米。
6. 迎风速度（Headwind）：风向与物体运动方向相反的风速。
迎风速度会减慢物体的速度。
7. 顺风速度（Tailwind）：风向与物体运动方向相同的风速。
顺风速度会增加物体的速度。
8. 极速（Top speed）：物体能够达到的最高速度。
在汽车、飞机等交通工具中，极速往往是设计者所规定的极限速度。
9. 最大速度（Maximum speed）：物体能够持续达到的最大速度。
在运动竞技和机械设计中，最大速度是一个重要的指标。
10. 慢速（Slow speed）：速度较低的状态。
慢速可以用来形容运动员的表现、机器的运转等。
11. 快速（Fast speed）：速度较快的状态。
快速可以用来形容运动员的表现、机器的运转等。
12. 超速（Overspeed）：超过规定速度的行驶。
超速不仅危险，还可能导致交通违法行为。
13. 时差（Time difference）：在不同速度下，两个事件之间的时间间隔。
根据相对论的理论，时差会因速度的变化而产生。
14. 瞬间速度（Instantaneous speed）：物体在某一瞬间的速度。
我对慢节奏的看法英语作文
我对慢节奏的看法英语作文The Charms of a Slow PaceIn today's fast-paced world, where everything seems to move at the speed of light, the concept of a slow pace often seems foreign and even outdated. However, I firmly believe that embracing a slower lifestyle offers profound benefits that cannot be overstated.Firstly, a slow pace allows for deeper reflection and self-awareness. In the hustle and bustle of daily life, it's easy to get carried away by the never-ending stream of tasks and responsibilities. Taking things at a slower speed provides the necessary space to pause, breathe, and connect with one's inner thoughts and feelings. This inner dialogue is crucial for personal growth, as it helps us understand our wants, needs, and values.Secondly, a slower pace enhances the quality of our interactions with others. When we rush through our days, we often miss out on the subtle nuances of human communication. We fail to notice the small smiles, the glances, or the silent gestures that convey so much more than words. By slowing down, we give ourselves the chance to truly listen and engage with others, deepening our connections and fostering meaningful relationships.Moreover, a slower lifestyle promotes better health and well-being. The constant stress and anxiety caused by living at a breakneck speed can take a toll on our physical and mental health. By contrast, a slower pace allows us to relax, unwind, and recharge our batteries. It gives us the opportunity to engage in activities that promote well-being, such as exercise, meditation, or simply spending time in nature.In conclusion, while the world may be constantly urging us to speed up, I believe that embracing a slower pace is crucial for our happiness and fulfillment. It allows us to connect with ourselves, others, and the world in a deeper, more meaningful way. It's a choice that leads to a richer, more fulfilling life.。
- 1、下载文档前请自行甄别文档内容的完整性，平台不提供额外的编辑、内容补充、找答案等附加服务。
- 2、"仅部分预览"的文档,不可在线预览部分如存在完整性等问题,可反馈申请退款(可完整预览的文档不适用该条件!)。
- 3、如文档侵犯您的权益，请联系客服反馈,我们会尽快为您处理(人工客服工作时间：9:00-18:30)。
2 changes from slower-than-c to faster-than-c. We show our main results based on numerical calculation. Using the definition of group velocity vg ≡ dω/dk and the relation between index of refraction n and √ 1 + χ, and ignoring the the susceptibility χ: n = absorption or enhancement we get 1 1 = vg c d(Reχ) ω . 1 + Reχ + √ 2 1 + Reχ dω The index of group velocity is defined as ng ≡ c/vg . Equation (1) indicates that (ng − 1) ∝ d(Reχ)/dω when Reχ ≪ 1, so in the following we concentrate on d[Reχ(ωp )]/dωp , where χ(ωp ) is the susceptibility of the system for the probe field. We solve the master equation for the atomic density operator to get the χ(ωp ). Ignoring the optical pump, the effective Hamiltonian of this system can be written as[9]:
Control of light speed: From slow light to superluminal light
Qun-Feng Chen, Yong-Sheng Zhang, Bao-Sen Shi, and Guang-Can Guo
Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, Pepple’s Republic of China (Dated: July 1, 2008) A scheme for controlling light speed from slower-than-c to faster-than-c in an atomic system is presented in this paper. The scheme is based on far detuning Raman effect. Two far detuning coupling fields with small frequency difference will produce two absorptive peaks for the probe field in a Λ structure, and an optical pump between the two ground states can change the absorptive peaks into enhanced peaks, which makes the normal dispersion between the two peaks change into anomalous dispersion, so the probe field can change from slow light to superluminal propagation.
(Ec1 and Ec2 ) with a small frequency difference of 2∆ are used. The center frequency of them is set to the center of the |2 → |3 and |2 → |4 transitions. The probe field Ep is detuning from the |1 → |3 transition by ∆p . These three fields form a far detuning Λ structure. The two detuning coupling fields produce two Raman absorptive peaks for the probe field. Therefore the probe field will encounter normal dispersion and the group velocity will be slowed down when its frequency lies between the two Raman absorptive peaks. In order to change the group velocity of the probe field, an optical pump from |1 to |2 is applied to the system (using the level |5 ). As the strength of the optical pump is increased gradually, the Raman absorptive peaks will become lower and finally change into enhanced peaks, at the same time the full width at half maximum (FWHM) of the two Raman peaks are almost unchanged[19]. Therefore the dispersion between the two Raman peaks changes from positive to negative, and the group velocity of the probe field
arXiv:0807.0139v1 [quant-ph] 1 Jul 2008
PACS numbers: 42.50.Gy, 42.50.Nn, 42.65.-k
Control of light speed has attracted much attention in the past years. The slow light has been demonstrated in both atomic vapor [1, 2, 3, 4, 5] and solid systems [6, 7], most of them [1, 2, 3, 4, 5, 6] are based on the electromagnetically induced transparency (EIT)[8, 9]. The first superluminal light propagation without large absorption or reshaping using far detuning Raman enhancement was reported in Ref. [10]. Recently, experiments in solid systems show that both superluminal and subluminal propagations can be obtained in the same system [11, 12, 13, 14]. In addition, some theoretical proposals [15, 16, 17] show the possibility of changing light propagation from subluminal to superluminal in an atomic system. However, there are some drawbacks in these schemes, in which either coupling of the dipole transition forbidden levels[16, 17] or some very special levels structure is required[15], which makes these schemes difficult to be realized. Furthermore, they do not show a good relation between the controlling and the dispersion of the system either. In this paper we give a scheme which can be used to change the group velocity of probe field from slower-thanc to faster-than-c continuously by controlling the strength of an optical pump. The dispersion of the system can be changed from positive to negative with the increment of the pumping rate. At the same time the probe field will not encounter large absorption or reshaping in large range of dispersion change. This scheme is based on far detuning Raman effect, so it is easy to be realized in many atomic systems, e.g. in cold or hot rubidium system. The atomic level structure used in this scheme is a five-level structure, as shown in Fig. 1. Four of the five levels (levels |1 to |4 ) are used to create a far detuning symmetric Λ structure. The electronic dipole moments between the ground states (levels |1 and |2 ) and the excited states (levels |3 and |4 ) satisfy the following relation: three of the dipole moments have the same phase while the phase of the forth is opposite to them. This is the requirement to get far detuning Raman absorptive peak when the coupling field lies at the center of the |2 → |3 and |2 → |4 transitions. This requirement can be fulfilled in many systems, for example rubidium 87, sodium and cesium systems[18]. In this scheme two coupling fields