1. Refraction and Lenses

2. The path of light is bent.
These are not photographs, but rather computer generated graphics based on the artist’s understanding of the index of refraction.

3. The angle of incidence equals the angle of reflection
The angle of incidence equals the angle of reflection. Not so for refraction.

4. Refraction – when waves enter a new material, they change speed
Refraction – when waves enter a new material, they change speed. As a result, they change direction.

5. Refraction – the bending of light due to a medium change
Light does not travel at the speed of light when it is propagating through a medium. It travels much slower because it must be absorbed and re-emitted by all of the molecules and atoms.
The more tightly packed the molecules and atoms, the slower the light will propagate.

7. Solider/band analogy
The solider cannot walk as fast in the yellow sand as they can on concrete, so he slows down

8. What would happen to soldiers if they all hit the yellow sand at the same time?

9. Does this mean that if light doesn’t change speed, it doesn’t change direction?
If glass if clear, how can we see it?
What would happen if glass were surrounded by a liquid that carried light at the same speed? Would it “disappear”?

10. But – this is pretty hard to measure in the lab!
Index of Refraction of a material is the ratio of the speed of light in vacuum to the speed of light in that material.
Where:
“n” is the index of refraction
“c” is the speed light
“v” is the average speed of light in the material   
But – this is pretty hard to measure in the lab!

11. Snell’s Law
Because light slows down, it will also “bend” or change directions – this bending is known as refraction. How much bending depends on the angle of incidence and the index of refraction.

12. If the index of refraction of the new medium is less than the index of refraction of the old medium…
nv = c, velocity will increase.
And the light will bend away from the normal

13. How much the light bends depends on how much the light slows down.
Light slows down more in glass (2/3c) than it does in water (3/4c) and therefore bends more in glass than in water.

14. Materials that slow light down a lot are said to be “optically dense”.
Glass is more “optically dense” that water is. Both have a greater optical density that air.

15. Note:
The more “optically dense” the material, the slower the speed of light in that material. Thus n > 1 for all materials, and increases with increasing optical density.
n = 1 in vacuum (and pretty close to 1 for air).
The frequency of light does not change when it passes from one medium to another.

16. Snell’s Law

17. Applications and Optical Illusions

18. Refraction of the Sun

19. Measuring the index of refraction (n) in glass.
Make the entry line dark
“Sight” the exit line with a ruler
Connect the path through the glass block (dashed line)
Use the top “i” and “r” only for calculation

20. Instructions Each student must do their own drawings
Entry angles are 30 and 45 degrees
Make a separate drawing for each angle
Be very careful with the expensive glass blocks!

21. Index of refraction of glass
Snells’ Law Lab – Part 1
Index of refraction of glass

22. “Yesterday” you measured the index of refraction for glass.
Could you use the same process to measure the index of refraction for water?

23. Index of Refraction – Water -No Pin is Needed at Point B – There is already a “scratch mark” there instead. -Fill the dish 3/4 full with water.

24. Why is there no refraction as the light leaves the water dish?

25. You should end up with something like this.

26. You need:
Two Pins
A Pink Foam Sheet
Scratch Paper
Dish

27. Lenses Your book calls them concave and convex – but that is not really correct!
Converging
Diverging

28. Lens Diagrams for “Thin” Lenses Converging – Fat Middle (p495)
A ray traveling in parallel will go through the lens and converge through the far focal point
A ray traveling in through the near focal point will pass through the lens and travel parallel
A ray passing through the center of the lens will pass through unchanged

29. Lens Diagrams for “Thin” Lenses Diverging – Thin Middle (p498)
A ray traveling in parallel will go through the lens and diverge on a path that includes the near focal point
A ray traveling in through the far focal point will pass through the lens and travel parallel
A ray passing through the center of the lens will pass through unchanged

30. Formulas apply to both converging and diverging lenses.
f is the focal length of the lens
M is the magnification
di is the image distance
do is the object distance
ho is the height of the object
hi is the height of the image.

31. Conventions Converging Diverging “f” is positive
+di means “real” image
-di means “virtual” image
+hi means “upright” image
-hi means “inverted” image
Diverging
“f” is negative
di is always negative “virtual” image
hi is always positive “upright” image
Example Problems page 496

32. Pass out sample diagrams. We will practice 1 and 4.
Let’s Practice
Pass out sample diagrams.
We will practice 1 and 4.

39. Assignment 18L

40. Snell’s Law Calculations
Example Problems

41. Find the angle of refraction.

43. Find the index of refraction.

45. Find the angle of refraction.

47. When light travels from a fast material like air into a slow material like glass, Snell’s Law always works.

48. When light travels out of glass or water and back into air, something unusual happens as the angle of incidence gets larger.

50. What is the critical angle for water?

52. If the angle of incidence exceeds the critical angle, refraction no longer occurs. Instead the light is internally reflected!

54. Fish Tank TIR

55. Fish View of the World

56. Sparkling Diamond

57. Problem Set 18A

58. TIR and Fiber Optics

59. Fiber Optics

61. TIR and Prisms

62. Dispersion
Each color of light has a slightly different index of refraction, so each color bends a different amount.

63. Chromatic Aberration – A Lens Defect

64. Fixing Chromatic Aberration for Lenses

65. Good Lens and Cheap Lens

66. Structures of the Human Eye

67. Human Eye Diagram (p500) Refraction Structures Cornea (3/4) - fixed
Lens (1/4) - adjustable

68. Common Disorders Cataract – cloudy cornea, fixed with surgery.
Glaucoma – high pressure in eye fluid, fixed with medicine or surgery.
Macular Degeneration – leading cause of blindness over 50. Thinning and degeneration of the central part of the retina.

69. Corrective Lenses
Nearsightedness
Farsightedness
Astigmatism

71. Rainbows

72. Double Rainbow

73. Double Rainbow

74. Mirage

76. Superior Mirage

77. Superior Mirage

78. "Crocker Land", a mountain range that appeared on arctic maps from 1909 to 1916, was nothing more than a superior mirage.