1. Lecture #2 Seeing the light 1/29/13

2. What happens to light when it interacts with matter? Reflects Absorbed Refracts Changes speed Polarized Diffracts

3. What happens to light when we see?

4. Today’s topics Learning styles Waves Refraction Diffraction / interference Light sources Intensity Homework on web site for next week

5. We can think about light in several ways Light as a wave: oscillating electromagnetic field

6. We can think about light in several ways Light as a wave: oscillating electromagnetic field Light as a ray: direction of wave

7. We can think about light in several ways Light as a wave: oscillating electromagnetic field Light as a ray: direction of wave Light as a photon: packet of energy which excites electrons

8. Light as a wave Wave characteristics Wavelength Frequency Speed Wavefront goes in one direction = ray Travels in straight line till it encounters different material

9. Wavelength – distance btn peaks λ varies across visible spectrum 400 nm 700 nm

10. Frequency Frequency of wave depends on wavelength and speed c= λ  f f = c /  Units make sense:

11. Frequency Typical frequency of visible light Huge number So we characterize light by wavelength

12. Visible light is small part of the electromagnetic spectrum

13. Different colors correspond to different wavelengths Wavelength is proportional to 1/ frequency

14. Speed, c Speed of light in a vacuum (outer space) 3 x 10 8 meters / second (299,792,458 m/s) 6.7 x 10 8 miles per hour Moon is 384,403 km away Takes 1.2 s for light go from moon to earth Sun is 149,600,000 km Takes light 8 min 19 s to get from sun to earth

15. Speed of light in other materials Light moves slower in matter Index of refraction = speed in vacuum speed in matter n depends on material More light interacts, the slower it goes

16. Speed of light in a material (v) versus index of refraction, n v = c / n water glass diamond silicon

17. What happens when light goes from one material into another?

18. What do you think will happen to the angle between the ray and the normal as it enters the water? a.It will increase (move away from the normal) b.It will stay the same c.It will decrease (move towards the normal)

19. What characteristics of the ray and/or the materials could be causing this? a. b. c. Possible answers?

20. Snell’s law quantifies bending   n 1 sin θ   n 2 sin θ  n1n1 n2n2

21. Snell’s law   n1n1 n2n2 and so light bends in

22. Snell’s law   n1n1 n2n2 If go from low to high index - light bends in towards normal

23. Snell’s law - in reverse   n1n1 n2n2 If go from HI to LOW index -Light bends away from normal -Light path is reversible

24. Can download simulator from PhET http://phet.colorado.edu/en/simulation/bending-light Part of homework#2 uses this simulator

25. Effect of changing angle and materials Can use tools to measure angles, light speed and light intensity

26. Outcome: Objects are not where they appear to be

27. Hemisphere of light above becomes a cone below

28. Archer fish make an adjustment

29. Snell’s window – see light above as a cone of light below the water Shanon Conway

30. How does refraction depend on wavelength?

31. Pink Floyd

32. Refraction differs with wavelength

33. Index of refraction depends on how much light interacts with material Glass

34. Snell’s law   n 1 =1.00 n 2 =1.50917 n 2 =1.51534 n 2 =1.52136 glass Air

35. Snell’s law   n 1 =1.00 n 2 =1.50917 n 2 =1.51534 n 2 =1.52136 glass Air As n 2 gets bigger… sin  2 and  2 get smaller

36. Snell’s law   n 1 =1.00 n 2 =1.50917 n 2 =1.51534 n 2 =1.52136 glass Air n 2 θ  1.50917 27.94 27.82 1.51534 27.82 1.52136 27.70 Shorter wavelength -

37. Snell’s law   n 1 =1.00 n 2 =1.50917 n 2 =1.51534 n 2 =1.52136 glass Air n 2 θ  1.50917 27.94 27.82 1.51534 27.82 1.52136 27.70 Shorter wavelength – bends MORE

38. PhET only does what you tell it Doesn’t have built in relationships of n and wavelength

39. Applications of Snell’s law Eye design Glasses design Seeing across interfaces Separating wavelengths of light

40. Another way to separate wavelengths – Diffractive interference Double slit – each slit becomes a point source of light

41. Interference

42. If waves are in phase – constructive interference; if they are out of phase – destructive interference Construct Destruct Construct Destruct

43. Constructive interference Distance two rays travel must differ by a multiple # of whole wavelengths r = n  r  D D r

44. Constructive interference Distance two rays travel must differ by a multiple # of whole wavelengths r = Dsin  = nλ r  D xnxn L D r  Similar triangles  sin   L xnxn

45. Constructive interference occurs at distance x, which is given by: D D = distance between two slits L = distance between slits and screen x = distance between bright spots L x

46. Diffraction What happens as slits get closer together? For more closely spaced slits, D is smaller and bright bands are further apart What happens as wavelength gets longer?

47. Two slit interference http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html

48. Diffraction Depends on wavelength Spots are further apart for longer 

49. Simulator http://www.walter-fendt.de/ph14e/doubleslit.htm

50. Diffraction depends on wavelength

52. Light sources

53. Light “source”?

54. Light source

55. Northern lights Phillipe Mousette, Quebec Canada

56. Biological light source Different species either make light through a luciferase reaction or have bacteria that make light and are symbionts.

57. Incandescent bulb California lawmaker proposes to ban the bulb http://sustainabledesignupdate.com/?p=115

58. Fluorescent bulb Electric discharge inside bulb causes high speed electrons to strike coating which fluoresces

59. Light emitting diode

60. Laser

61. Sun is a high temperature light bulb Temperature is around 5800 K This produces broad spectrum light just like an incandescent lightbulb

62. Solar spectrum peaks near 500 nm

63. Actual solar spectrum

64. Measuring spectral distribution Use computerized spectrometer Collects light Disperses with diffraction grating Sends to multipixel detector

65. Ocean Optics spectrometer 1. Fiber in 2. Slit 3. Filter 4. Collimating mirror 5. Diffraction grating 6. Mirror 7. Lens 8. Detector

66. Spectra of different light sources

67. Relative brightness of sun and moon Sun light comes directly to earth Moonlight - sun is scattered off moon and comes to earth

68. Light intensity Sun is about 10 5 - 10 6 brighter than a full moon We calculated sun to be 2 x 10 5 brighter 1candela/m 2 = 1 lux

69. Summary Waves Refraction - Snell’s law Interference - diffraction Light sources and spectral distribution Intensity Learning styles