Why does light slow down?

1. Why does light slow down?

2. Teacher Example The absorption and emission of a photon in an atom takes 1 X 10^-12 seconds. Determine the number of atoms that a photon collides with if the light travels 3,000 m in 10.1 microseconds.

3. Refraction The bending of a light ray as it enters a different media The speed of light is not constant; it is only constant for a vacuum

4. Examples of Refraction • Prisms • Glasses • Spearfishing

5. Light Slows down when it changes media • This slowing of light leads to the index of refraction N = c / v

6. Teacher Example Determine the index of refraction for water if the speed of light in water is 2.25 X 108 m/s

7. Bending of Light • Light slows down and it bends but how much? • Can be calculated with Snell’s Law n1sinθ1 = n2sinθ2

8. Teacher Example Determine the refracted angle when light moves from air (n = 1) to water (n = 1.33) and the incident light ray is 45 degrees.

9. Teacher Example Determine the index of refraction for an unknown object when it bends from a vacuum to the material resulting in moving from 30 degrees to 22 degrees.

12. Critical Angle • At certain angles light no longer leaves to a substance of lower index of refraction but instead undergoes total internal reflection • If TIR than you no longer see the object

13. Critical Angle Can be Calculated from Snell’s Law • Snell’s Law n1sinθ1 = n2sinθ2 n1 >n2 And θ2 = 90 Becomes θ1 = θCA = sin-1 (n2 / n1)

14. Teacher Example • Determine the critical angle for air (n=1.00027) and water (n = 1.33).

15. Teacher Example Determine the critical angle for air (n=1.00027) and a vacuum (n = 1).

16. Clicker Question #1 Where should you aim when spear fishing if trapped on a desert island? • Higher than what you see • Lower than what you see • At what you see • Fish have feelings (I would become a vegetarian)

17. Clicker Question #2 What is an example of total internal reflection • Mirrors • Lenses • Fiber optic cables • Speaker wire • All of these

18. Student Example #3 – 60 s • Determine the velocity of light in a diamond if the index of refraction is 2.417. Round to 3 significant digits (ex: 578,000)

19. Student Example #4 – 90 s Determine the incident angle of a light ray that moves from air (n = 1) to a diamond (n = 2.417) resulting in a refracted angle of 10 degrees. Round to 1 decimal place

20. Student Example #5 – 90 s Determine the critical angle between air (n = 1.00027) and a diamond (n = 2.417). Round to a whole number answer.

21. Apparent Depth • Whenever you reach into a pool to grab an object more often than not you miss and grab only water • Because light rays are bent so are images • The apparent depth can be calculated with: d’ = d(n2 / n1)

22. Teacher Example Calculate the apparent depth of a coin if it is actually 3.00 m deep, and the index of refraction for air is 1.00 and water is 1.33

23. Student Example #4 – 90 s Calculate the apparent depth of a coin if it is actually 2.00 m deep, and the index of refraction for air is 1.00 and corn oil 1.47

24. Lenses • Lenses are just used to refract light and bend it to a certain angle.

25. Converging Lens • Converge light on the focal point • Positive focal length

26. Diverging Lens Diverge Light from Focal Point Negative Focal length

27. Ray Diagrams 3 rules for lens ray diagrams: • Any ray traveling thru the center of the lens will not be bent • Any ray traveling parallel to principal axis will refract thru the focal point

28. Ray Diagram Converging Lens

29. Draw the refracted rays using the rules

30. Image Formation • The image is formed at the intersection of the light rays

31. Converging Lenses Focal Length is Positive • Distance Object Positive Distance Object Negative • Distance Image Negative Distance Image Positive • Real Image Virtual Image

32. Diverging Lens Ray Diagrams

33. Diverging Lenses Focal Length is Negative • Distance Object Positive Distance Object Negative • Distance Image Negative Distance Image Positive • Real Image Virtual Image

34. The Lens Equation • The lens equation can be used for converging or diverging mirrors: 1 = 1 + 1 f do di

35. Teacher Example What type of image is produced by a converging lens with a focal length of 5 cm when an object is placed 10 cm in front of the lens

36. Student Example #1 – 90 s What type of image is produced by a diverging lens with a focal length of 5 cm when an object is placed 10 cm in front of the lens • Real, bigger • Real, smaller • Virtual, bigger • Virtual, smaller

37. Teacher Example Determine the focal length of a diverging lens that produces an image 10 cm from the lens when an object is place 8 cm in front of the lens

38. Student Example #2 – 90 s Determine the image location of a converging lens with a focal length of 10 cm when an object is placed 30 cm in front of the lens?

39. Magnification Equation • Represented by: M = hi = -di ho do • |M| < 1; image is larger • |M| > 1; image is smaller • M is ‘+’

40. Teacher Example What is the magnification of a converging lens with a focal length of 10 cm when an object is placed 30 cm in front of the lens?

41. Student Example #3 – 60 s What is the magnification of your iphone when the letter increases in height from 0.15 cm to 0.8 cm?

42. Teacher Example To have the same effect as a lens assuming that the object distance doesn’t change then what will the focal length of the new “lens” be?

43. Multiple Lenses • Treat each lens individually • The image for the first lens becomes the object of the second lens

44. Teacher Example What type of image is produced when a convex and concave lens are used in combination?

45. Diffraction • Divergence of light away from its original direction of travel