Light Basics

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wavelength (nm)
2. Blackbody and Sunlight
The Sun is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields. It has a diameter of about 1,392,000 km, about 109 times that of Earth, and its mass (about 2×1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. Chemically, about three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. Less than 2% consists of heavier elements, including oxygen, carbon, neon, iron, and others. The Sun's stellar classificationis G2V, and is informally designated as a yellow dwarf, because its visible radiation is most intense in the yellow-green portion of the spectrum and although its color is white, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.In the spectral class label, G2 indicates its surface temperature of approximately 5778 K (5505 °C), and V indicates that the Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen each second. The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though the distance varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth by photosynthesis,[ and drives Earth's climate and weather.
Part of Spectrum of total range of electromagnetic radiation
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1.
What is light ?
1.
What is light ?
• Duality of Light: Wave or Particle • Light energy
• This equation says that the energy of a particle of light (E), called a photon, is proportional to its frequency (), by a constant factor (h). E的单位是焦耳(J)，h=6.626×10^(-34)，单位是焦耳·秒(Js)，Planck普朗克常数； ν是光子频率，单位赫兹(Hz)，c=3×(10^8)，单位是米每秒(m/s)，是真空中的 光速；λ是光子波长，单位是米(m)。
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1.
What is light ?
Light or visible light is the portion of electromagnetic radiation that is visible to the human eye, responsible for the sense of sight. Visible light has a wavelength in a range from about 380 to 780 nm. In physics, the term light often comprises the adjacent radiation regions of infrared (at lower frequencies) and ultraviolet (at higher), not visible to the human eye.
2. Blackbody and Sunlight
Spectral power distribution black body radiator
UV IR
UV relative spectral power
IR 7000 K 6000 K 5000 K 4000 K
3000 K
Black body line
• A blackbody is an idealized physical body that absorbs all incident electromagnetic radiation. • blackbody radiation is energy converted electrodynamically from the body's pool of internal thermal energy at any temperature greater than absolute zero. You're familiar with the fact that very hot objects glow
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3. How do human eyes react to light?
Pupil and lens Between the foremost chamber of the eye and the lens is a colored circular muscle, the iris. It has a variable opening which is called the pupil. The pupil opens or closes in a purely reflex action as a function of the amount of light falling on the eye.
2. Blackbody and Sunlight
3. How do human eyes react to light?
Cross Section of Eye
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3. How do human eyes react to light?
• The human eye is almost spherical in shape with an OD of about 25 mm. At the front is the cornea, a transparent bulge with a radius of 5-6 mm. • cornea, anterior chamber, iris, lens,vitreous chamber, retina, choroid, and sclera • the eye consists of a fixed lens of about 40 dioptres, formed by the cornea (n = 1.38), plus the fluid in the anterior chamber (n = 1.34), and an adjustable lens (n = 1.41) of 20-30 dioptres. • Defects in this optical system and the dimensions of the receptors in the retina set the limit for distinguishing details to about ½’ for young people and about 1’ for the elderly.
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3. How do human eyes react to light?
Retina contains two sorts of light-sensitive receptors, referred to on account of their shapes as rods and cones. • • • The rods are very light sensitive, but are not able to distinguish between colours. They play an important role in the detection of form and movement, and they determine the power of vision at night and in the twilight hours (scotopic range ≤ 0.035 cd/m2). The cones are less sensitive than the rods but, because they are divided up into red, green and blue-sensitive types, they are able to distinguish colors. They work mostly at luminances ≥ 3.5 cd/m2, the photopic range.