PHYS 252 & PHYS 202
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Transcript of PHYS 252 & PHYS 202
PHYS 252 / 202 1
PHYS 252 & PHYS 202
• Polarization• Scattering• Absorption
PHYS 252 / 202 2
Polarization
The polarization of the light wave is determined by the oscillation axis of the electric field.
polarization type description
linearly polarized axis is in one direction
elliptically polarized axis sweeps out ellipse over time
circularly polarized axis sweeps out circle over time
unpolarized axis changes direction randomly
PHYS 252 / 202 3
Polarization
crest
trough
linearly polarized
PHYS 252 / 202 4
Polarization
Unpolarized Linearly Polarized
PHYS 252 / 202 5
Polarization
Linear Polarization of Light by Dichroic Polarizer (Polaroid)
transmission axis
E
incident electric field
The energy from this perpendicular component is absorbed.
This component gets through.
PHYS 252 / 202 6
Polarization
Effect of a Single Polaroid on Linearly Polarized Light
E E E E
all light passes through
less light passes through
even less light passes through
no light passes through
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Polarization
Two Polaroids
I = Io cos2
The Law of Malus
PHYS 252 / 202 8
Polarization
Linear Polarization of Light by Reflection
polarization angle
PHYS 252 / 202 9
Polarization
Linear Polarization of Light by Reflection
n1 n2 p
air off water 1 1.33 53.1
air off glass 1 1.5 56.3
Examples
tanp = n2 / n1
Polarization Angle
All of the reflected light is linearly polarized parallel to the reflecting surface when the incident angle equals the polarization angle.
PHYS 252 / 202 10
Polarization
Birefringent Material
Birefrigence
• material exhibits two values of refractive index for each frequency of light
• the value depends on the orientation of the electric field with the optic axis of the material
Calciteoptic axis
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Polarization
Linear Polarization of Light by Birefringent Crystal
extraordinary ray
ordinary ray
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Polarization
Rotation of Polarization Axis by Birefringent Material
Amount of rotation depends on
• thickness of material
• wavelength of light
• values of refractive index
PHYS 252 / 202 13
Scattering
Atoms, molecules, and small particles scatter light by absorbing it and re-emitting it in random directions.
Amount of scattering depends on
• size of scatterers
• wavelength of light
PHYS 252 / 202 14
Scattering
• shorter wavelengths scatter much more than longer wavelengths
Rayleigh Scattering
size of scatterers is small compared to wavelength of light
• amount of scattered light 4
1
Example: Scattering of sunlight by oxygen & nitrogen molecules in Earth’s atmosphere
Daytime: sky looks blue, Sun looks yellow/white
Sunrise/Sunset: Sun looks orange/red
PHYS 252 / 202 15
Scattering
sunset
D ~ 820 km
Rayleigh Scattering
D ~ 50 km
daytime
6380 km
www.bigfoto.com/themes/nature/sky/ www.bigfoto.com/themes/nature/sky/
PHYS 252 / 202 16
Scattering
Linear Polarization by Scattering
PHYS 252 / 202 17
Absorption
Atoms and molecules will absorb light energy of certain frequencies (and wavelengths) much better than others.
A plot of the amount of absorption versus frequency (or wavelength) is called a material’s absorption spectrum.
Some example absorption spectra appear on the next slides.
PHYS 252 / 202 18
Absorption
Absorption Spectra of Some Organic Materials
1.bacteriochlorophyll 2.chlorophyll A 3.chlorophyll B 4.phycoerythroblin 5.beta carotene
A careful look at the chlorophyll spectra will explain why most plants are green!
PHYS 252 / 202 19
Absorption
What color is this material?
Absorption spectrum of red garnet.