PHYS 252 & PHYS 202

1. PHYS 252 & PHYS 202 • Polarization • Scattering • Absorption PHYS 252 / 202

2. 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

3. crest trough Polarization linearly polarized PHYS 252 / 202

4. Unpolarized Linearly Polarized Polarization PHYS 252 / 202

5. 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

6. 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

7. Polarization Two Polaroids The Law of Malus I = Iocos2 PHYS 252 / 202

8. Polarization Linear Polarization of Light by Reflection polarization angle PHYS 252 / 202

9. All of the reflected light is linearly polarized parallelto the reflecting surface when the incident angle equals the polarization angle. Polarization Angle tanp = 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

10. 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

11. extraordinary ray ordinary ray Polarization Linear Polarization of Light by Birefringent Crystal PHYS 252 / 202

12. 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. 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

15. 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

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 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

19. Absorption spectrum of red garnet. Absorption What color is this material? PHYS 252 / 202