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Lecture #11

Lecture #11. Polarization vision 2/28/13. Today. Polarized light What is it? Where is it? Can it be detected? What is it good for?. Light polarization. Orientation of oscillation of photon ’ s electric field. Can analyze polarization with polarizers. Polarization.

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Lecture #11

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  1. Lecture #11 Polarization vision 2/28/13

  2. Today • Polarized light • What is it? • Where is it? • Can it be detected? • What is it good for?

  3. Light polarization Orientation of oscillation of photon’s electric field

  4. Can analyze polarization with polarizers

  5. Polarization • Single polarizer lets through light polarized in one direction

  6. Polarization • Add a second polarizer at 90 degrees to 1st

  7. Black and white • What does it mean when something is white? • What does it mean when something is black?

  8. Light sources • Sun, flashlight - not polarized • Laser - partially polarized

  9. Reflection can cause polarization R|| R s-polarization (R ) is reflected more than p-polarization (R||)

  10. Reflected light n nair=1 θi This applies for light of normal incidence

  11. Transmitted light is refracted (Snell’s law) θt n nair=1 θi

  12. Two polarizations reflect differently R|| R s-polarization (R ) is reflected more than p-polarization (R||)

  13. Fresnel equations to calculate reflectance of either polarization θt θi What happens to light that is not reflected?

  14. Light polarization changes with angle more of light is polarized perpendicular Depends on angle of incidence

  15. Brewster’s angle is incident angle at which reflected light becomes totally polarized

  16. Reflectance

  17. Amount of reflected light changes with angle

  18. Flashlight demo – What is different? Reflect flashlight off mirror or off a water surface

  19. Polarization mystery • Put polarizers in white light beam of projector • What is happening??

  20. Possibility 1

  21. Possibility 2

  22. Simple equation works for unpolarized light at <45°

  23. At large angles, reflectance goes to 1 for both polarizations

  24. Light reflecting from surface is polarized

  25. Polarization off surfaces polarizer Fig 2.8

  26. Light from the sun is unpolarized Each photon has polarization - but group of them have random polarizations

  27. Sun light gets polarized scattering off atmosphere

  28. Polarized light patterns in sky

  29. Polarized light patterns in sky

  30. Polarized light field in sky

  31. Fly eye view of the sky N E S

  32. Underwater polarization field less well understood

  33. Drosophila eyes WT cinnabar sepia white 800 ommatidia per eye

  34. Drosophila compound eye

  35. Three kinds of ommatidia UV sensitive in dorsal rim

  36. Dorsal rim area (DRA) is sensitive to polarized light – UV pigment Microvilli are arranged in parallel fashion

  37. Bristles of rhabdomere orient visual pigment in membrane

  38. Absorption of light by retinalis greatest when E vector is along long axis of molecule

  39. Polarization field will be maximum in the UV N E S Bee’s orientation will determine DRA receptor absorption

  40. Polarization detection Labhart 2002, field crickets

  41. Why should polarization sensitive receptors work in UV • Rayleigh scattering • Air particles << wavelength of light • 10 nm << 500 nm • r << λ • Scattered intensity

  42. Light scattering off small particles is called Rayleigh scattering Shorter wavelengths scatter more

  43. Rayleigh scattering ≈1/λ4

  44. Rayleigh scattering ≈1/λ4 UV

  45. So if you can see in UV…

  46. the sky is UV

  47. Polarization field will be maximum in the UV because UV scatters most N E S

  48. Animals can use these patterns • Insects (bees) and fish • Uses • Navigation and local orientation • Time of day • Migration

  49. Bee communication http://www.youtube.com/watch?v=bFDGPgXtK-U

  50. How could we test if animals are sensitive to polarized light? • Behavioral test – create polarized light field for them to respond to and train to polarization • Test photoreceptors for polarization depended absorption • Ganglion cell or visual processing centers respond to polarized light

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