Lecture #2

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 peaksλ varies across visible spectrum • 400 nm • 700 nm

10. Frequency • Frequency of wave depends on wavelength and speed • c= λ*f f = c /λ • 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. 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 Speed, c • Speed of light in a vacuum (outer space) • 3 x 108 meters / second (299,792,458 m/s) • 6.7 x 108 miles per hour

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? • It will increase (move away from the normal) • It will stay the same • It will decrease (move towards the normal)

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

20. Snell’s law quantifies bending θ1 n1 sin θ1=n2 sin θ2 n1 n2 θ2

21. Snell’s law θ1 n1 and so light bends in n2 θ2

22. θ1 n1 n2 θ2 Snell’s law If go from low to high index - light bends in towards normal

23. Snell’s law - in reverse • If go from HI to LOW index • Light bends away from normal • Light path is reversible θ1 n1 n2 θ2

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 Which one can make a tighter turn?

32. Refraction differs with wavelength

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

34. Snell’s law θ1 Air n1=1.00 glass n2 =1.50917 n2 =1.51534 n2 =1.52136 θ2

35. Snell’s law θ1 Air n1=1.00 As n2 gets bigger… glass sin θ2 and θ2 get smaller n2 =1.50917 n2 =1.51534 n2 =1.52136 θ2

36. Snell’s law θ1 Air n 2θ2 n1=1.00 1.50917 27.94 1.51534 27.82 1.52136 27.70 glass n2 =1.50917 n2 =1.51534 n2 =1.52136 Shorter wavelength - θ2

37. Snell’s law θ1 Air n 2θ2 n1=1.00 1.50917 27.94 1.51534 27.82 1.52136 27.70 glass n2 =1.50917 n2 =1.51534 n2 =1.52136 Shorter wavelength – bends MORE θ2

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. θ r Constructive interferenceDistance two rays travel must differ by a multiple # of whole wavelengths D r = nλ D r

44. θ r Constructive interferenceDistance two rays travel must differ by a multiple # of whole wavelengths xn D L θ r = Dsinθ = nλ xn D θ r L Similar triangles sinθ = xn / L

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

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 λ

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

50. Diffraction depends on wavelength