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PHYS 252 & PHYS 202. Polarization Scattering Absorption. The polarization of the light wave is determined by the oscillation axis of the electric field. polarization type description. Polarization. unpolarized axis changes direction randomly . linearly polarized axis is in one direction.
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PHYS 252 & PHYS 202 • Polarization • Scattering • Absorption PHYS 252 / 202
The polarization of the light wave is determined by the oscillation axis of the electric field. polarization type description Polarization unpolarized axis changes direction randomly linearly polarized axis is in one direction circularly polarized axis sweeps out circle over time elliptically polarized axis sweeps out ellipse over time PHYS 252 / 202
crest trough Polarization linearly polarized PHYS 252 / 202
Unpolarized Linearly Polarized Polarization PHYS 252 / 202
transmission axis incident electric field E This component gets through. The energy from this perpendicular component is absorbed. Polarization Linear Polarization of Light by Dichroic Polarizer (Polaroid) PHYS 252 / 202
E E E E Polarization Effect of a Single Polaroid on Linearly Polarized Light all light passes through less light passes through even less light passes through no light passes through PHYS 252 / 202
Polarization Two Polaroids The Law of Malus I = Iocos2 PHYS 252 / 202
Polarization Linear Polarization of Light by Reflection polarization angle PHYS 252 / 202
All of the reflected light is linearly polarized parallelto the reflecting surface when the incident angle equals the polarization angle. Polarization Angle tanp = n2 / n1 n1 n2 p air off water 1 1.33 53.1 air off glass 1 1.5 56.3 Examples Polarization Linear Polarization of Light by Reflection PHYS 252 / 202
optic axis Calcite 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 PHYS 252 / 202
extraordinary ray ordinary ray Polarization Linear Polarization of Light by Birefringent Crystal PHYS 252 / 202
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
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
Rayleigh Scattering size of scatterers is small compared to wavelength of light • amount of scattered light 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 Scattering • shorter wavelengths scatter much more than longer wavelengths PHYS 252 / 202
www.bigfoto.com/themes/nature/sky/ www.bigfoto.com/themes/nature/sky/ D ~ 820 km D ~ 50 km 6380 km daytime sunset Scattering Rayleigh Scattering PHYS 252 / 202
Scattering Linear Polarization by Scattering PHYS 252 / 202
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
Absorption 1.bacteriochlorophyll 2.chlorophyll A 3.chlorophyll B 4.phycoerythroblin 5.beta carotene Absorption Spectra of Some Organic Materials A careful look at the chlorophyll spectra will explain why most plants are green! PHYS 252 / 202
Absorption spectrum of red garnet. Absorption What color is this material? PHYS 252 / 202