wave optics a knowledge of the properties of light allows us to understand the blue color of the sky...
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Wave Optics
A knowledge of the properties of light allows us to understand the blue color of the sky and the design of optical devices…
Polarization Interference Diffraction
Dual Nature
In Newton’s time: light consisted of streams of
particles. In 1873, Maxwell predicted the light is a sort of
electromagnetic waves. In 1887, Hertz showed light is an electromagnetic
wave. Thomas young also prove the wave properties of light.
Particle Model :
The straight line traveling of light,
reflection and refraction, emission
and absorption of light.
Wave Model :
polarization, interference and
diffraction of light.
polaroid polaroid reflectionreflection birefringencebirefringence
Producing of polarized lightProducing of polarized lightphenomenaphenomena
Polarization of lightPolarization of light
applicationapplication
Chap 34 Polarization of light
Chap 34 The nature and propagation of light
Key Terms:Optics Physical opticsWave front RayReflection RefractionSpecular/diffuse reflectionIndex of refraction Critical angleTotal internal reflection Linear polarizationPolarizing filter DichroismPolarizing axis Malus’s lawBrewster’s lawBirefringence
transverse waveLongitudinal wave
1. polarization
1.1 What is polarization?
Polarization is characteristic of all transverse waves
1.2 polarization status of light
Unpolarized light: natural light
Polarized light:
Linearly polarized light
partial polarized light
circular polarized light
elliptic polarized light
E
When a wave has only y-displacements (or z), we say it is linearly polarized in this direction.
y
z x
v
y
z x
v
E
E
Linearly polarized light
• The waves emitted by any one molecule may be linearly polarized, but any actual light source contains a tremendous number of molecules with random orientations, so the emitted light is a random mixture of waves linearly polarized in all possible transverse directions.
yz
v
x
natural light (unpolarized light)
y
z
partially polarized light
• Linearly polarized light “+” natural light
circular and elliptical polarized light
E
E
2. How to get polarized light2. How to get polarized light
Polarization by polaroid
polarization by reflection
polarization by birefringence
Malus’s law
If the intensity of incident polarized light is I0, and the polarizing axis makes an angle with the displacement of the incident light, the intensity of the light transmitted through the polarizer is: I=I
0 cos 2
I=(Eocos )2 = Iocos2
Eo
Io= Eo2
Eocos
• Polarizing filter can absorb one of the polarized components, and transmit the components that is polarized parallel to a certain axis ,called the polarizing axis. And the transmitted intensity : I=Io cos 2 .(for natural light: I=Io /2 )
2
II 0 2
0 cosII
2
I)A( 0
4
I)B( 0
8
I3)C( 0
16
I3)D( 0
8
I330cos)
2
I(I 0020
Example: The natural light has intensity I0. Find the intensity transmitted by the second polarizer if the angle between the axes of the two filters is 300.
030
Solution:
0I 2I0
1I
2I
221 cosII
2
1
)(cos)cos2
1( 22
21 III
Example: The incident light is a mixture of natural light with intensity I1 and linear polarized light with intensity I2 , find the transmitted intensity by the second polarizer.
Solution:
2.2 polarization by reflection
pp
b
an
bn
bn
anpp
b
For most angles of incidence:
Reflected light: partially polarized (>//)
Refracted light: partially polarized (//>)
At one angle of incidence, called the polarizing angle:
Reflected light: completely polarized ()
Refracted light: partially polarized (//>)
: reflected and refracted
//: only refracted
Brewster’s law
bn
anpp
b
Brewster discovered that when p equals the polarizing angle, the reflected ray and the refracted ray are perpendicular to each other.
From the polarizing angle: sin b= cos p
From the law of refraction: sinp / sinb=nb / na
Then: tanp =nb / na
The natural light is refracted again and again by a stack of glasses, with incidence angle equals the polarizing angle.
Discussion:
2) tell the difference of critical angle and brester angle
total internal reflection.
1) Reflected light: completely polarized ()
Refracted light: partially polarized (//>)
reflected ray is perpendicular to refracted ray
1
2sinn
n
2
1
n
ntg brester angle
critical angle
draw the reflected light and refracted light
n1
n2
i i
n1
n2
n1
n2
n1
n2
i
i0 i0 i0
,ii 01
20 n
narctgi
n1
n2
n1
n2
2.3 polarization by birefringence
• Birefringence: the material has different indexed of refraction for different directions of polarization. Here are some examples:
Extraordinary light
Ordinary light
Calcite crystal (方解石晶体 ):