同济大学《大学物理》第十九章 电磁波
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Light waves
Agenda today
1. The nature of light waves 2. Reflection and refraction 3. Polarization
The spectrum of electromagnetic waves
Wavelength of visible light( in vacuum): 400--760 nm
Its application in reflection:
The law of reflection
A3B3 AB ut
ABB3 AA3B3
90o AB3 AB3 AA3B3 ABB3
1 2
incident angle equals to reflection angle
A3
i
1
A B1
B
i
2
B2 B3
i i
2、Snell’s law
http://www.walterfendt.de/ph14e/huygenspr.htm
BC u1t
AC sin i
AD u2t
AC sin
B
t
i i E1
A
E2
C
AC sin i u1t
D
AC sin u2t
sin i
sin
Solution:
The incident ray is normal to the water surface, thus the refraction angle must be the critical angle of water to the air
D=1.3m
Dispersion(色散）:
By Maxwell’s equations:
E c B
Which is called the intrinsic resistance of the medium（媒质的特征阻抗）
Determine the energy the waves transports per unit
time per unit area as the Poyting vector（坡印亭
The flowers appear differently in UV light.
What is light? Particle theory by I.Newton. Wave theory by C.Huygens The experiment done by T.Young. Photoelectric effect
Generation of Radio Waves
Accelerating charges produce electromagnetic waves
+
+
+
+ E
-
-
-
-
-
-
-
-
+
+
+
+
Why are cell phone antennas so short?
Electromagnetic Radiation
Huygens’s principle(惠更斯原理）:
All point on a wavefront serve as point soures of spherical secondary wavelets. After a time t, the new position of the wavefront will be that of a surface tangent to these secondary wavelet.
u1 Fra Baidu bibliotek2
n21
Total internal reflection c: the critical angle
n: the index of refraction
Example: A swimmer is in a deep pool with her eyes a horizontal distance R=1.5m from the edge. How far below the surface are her eyes if she is just able to see the full height of a lifeguard who is standing on the pool’s edge? The index of refraction of water is 1.33.
• Interrelated electric and magnetic fields traveling through space
• Electromagnetic waves travel through empty space All electromagnetic radiation travels at c=3 x 108 m/s in vacuum – the cosmic speed limit!
向量）.
S
1
EB EH
0
Which is w*c, the intensity of the waves
Radiation Pressure
F p t
E IAt p E
c PF/A
P I C
Optical levitation of a drop of glycerol at the center of the chamber by a vertical green laser
For plane electromagnetic waves travel in x direction:
By Maxwell’s equations:
Compared with the wave equation of mechanical waves
Let:
Energy of electromagnetic waves
The index of refraction depends on the wavelength of the light being transmitted.
Can you explain why the rainbow is so colorful and its likeness to a bow ?
Scattering （散射）
The rays are scattered into various direction when light pass through matter consisted of many small particles
Agenda today
1. The nature of light waves 2. Reflection and refraction 3. Polarization
The spectrum of electromagnetic waves
Wavelength of visible light( in vacuum): 400--760 nm
Its application in reflection:
The law of reflection
A3B3 AB ut
ABB3 AA3B3
90o AB3 AB3 AA3B3 ABB3
1 2
incident angle equals to reflection angle
A3
i
1
A B1
B
i
2
B2 B3
i i
2、Snell’s law
http://www.walterfendt.de/ph14e/huygenspr.htm
BC u1t
AC sin i
AD u2t
AC sin
B
t
i i E1
A
E2
C
AC sin i u1t
D
AC sin u2t
sin i
sin
Solution:
The incident ray is normal to the water surface, thus the refraction angle must be the critical angle of water to the air
D=1.3m
Dispersion(色散）:
By Maxwell’s equations:
E c B
Which is called the intrinsic resistance of the medium（媒质的特征阻抗）
Determine the energy the waves transports per unit
time per unit area as the Poyting vector（坡印亭
The flowers appear differently in UV light.
What is light? Particle theory by I.Newton. Wave theory by C.Huygens The experiment done by T.Young. Photoelectric effect
Generation of Radio Waves
Accelerating charges produce electromagnetic waves
+
+
+
+ E
-
-
-
-
-
-
-
-
+
+
+
+
Why are cell phone antennas so short?
Electromagnetic Radiation
Huygens’s principle(惠更斯原理）:
All point on a wavefront serve as point soures of spherical secondary wavelets. After a time t, the new position of the wavefront will be that of a surface tangent to these secondary wavelet.
u1 Fra Baidu bibliotek2
n21
Total internal reflection c: the critical angle
n: the index of refraction
Example: A swimmer is in a deep pool with her eyes a horizontal distance R=1.5m from the edge. How far below the surface are her eyes if she is just able to see the full height of a lifeguard who is standing on the pool’s edge? The index of refraction of water is 1.33.
• Interrelated electric and magnetic fields traveling through space
• Electromagnetic waves travel through empty space All electromagnetic radiation travels at c=3 x 108 m/s in vacuum – the cosmic speed limit!
向量）.
S
1
EB EH
0
Which is w*c, the intensity of the waves
Radiation Pressure
F p t
E IAt p E
c PF/A
P I C
Optical levitation of a drop of glycerol at the center of the chamber by a vertical green laser
For plane electromagnetic waves travel in x direction:
By Maxwell’s equations:
Compared with the wave equation of mechanical waves
Let:
Energy of electromagnetic waves
The index of refraction depends on the wavelength of the light being transmitted.
Can you explain why the rainbow is so colorful and its likeness to a bow ?
Scattering （散射）
The rays are scattered into various direction when light pass through matter consisted of many small particles