1. Refraction

2. Behavior of waves at boundaries When the energy in a wave hits a fixed boundary, the disturbance in the medium through which the energy is traveling (the wave) is inverted. When the wave hits a free boundary, the pulse is reflected but not inverted.

3. Behavior of waves at boundaries When the energy in a wave travels through media with different density (  ), part of the wave is reflected, part is transmitted. low  to high  high  to low 

4. Think about it… Make a rule summarizing the reflection and transmission characteristics of waves traveling through media with different densities. Include: Height of wave Width of wave Speed of wave Possible explanation? low  to high  high  to low 

5. Transmission of energy From less dense to more dense: Speed decreases Wavelength decreases From more dense to less dense Speed increases Wavelength increases  low to high   v,   high to low   v, 

6. Waves (and light) at boundaries Change in speed  change in direction When light passes from air (less dense) to glass (more dense), the speed and wavelength of light decrease.

7. Refraction Refraction: bending the path of light

8. The broken pencil Years of experience has trained us into thinking that light travels in straight lines from objects to our eyes. When the image forms somewhere unexpected, we have to fight that training to make sense of what we see.

9. The broken pencil, explained Light is reflected from pencil Light is refracted when it passes from water to glass and glass to air. Light travels straight to our eyes. In this example, the image appears to the left of the object’s location and appears broken.

10. Other examples Magnifying water Using water to start a fire Spearfishing Uncovering root beer deceit Best profile pics ever!

11. Marching soldiers 1.Stand shoulder to shoulder forming a straight line, connected with meter sticks. 2.A line of tape separates the room into two ‘media’ 3.When students approach the line, they use baby steps. When they pass the line, they abruptly change pace 4.Observe what happens to the direction of travel.

12. Conditions of refraction 1.Change in speed i.e., density change in media 2.Approach the boundary at an angle Otherwise, speed would change at the same time

13. Passing through… When light passes through material, it is actually absorbed and re-emitted and absorbed and re-emitted and absorbed and …. The speed at which light is transmitted is called optical density

14. Index of refraction MaterialIndex of Refraction Vacuum1.0000 Air1.0003 Ice1.31 Water1.333 Ethyl Alcohol1.36 Plexiglas1.51 Crown Glass1.52 Flint Glass1.66 Zircon1.923 Diamond2.417

15. Quantified n diamond = 2.42; c = 3.00 x 10 8 m/s v = ? What is the speed of light in a diamond? Try it first

16. Predicting the direction of bending Traveling from fast (less dense) to slow (more dense), bent TOWARDS the normal fast

17. Predicting the direction of bending Traveling from slow (more dense) to fast (less dense), bent AWAY the normal

18. Tractor analogy True if … sides are parallel medium on either side of transparent medium is the same.

19. Think about it… Copy this diagram onto a sheet of paper. What is the path of the tractor and light?

20. In real life…

21. Think about it… Draw the missing ray in each situation

22. Think about it… The fish is shown in its actual position. Where will the image of the fish appear to the person above the water? If the person wants to hit the fish, where should s/he throw the spear? Justify your answer.

23. Quantifying refraction, 1 of 3 The optical density makes a difference

24. Collect some (virtual) data Go to PHeT Bending Light simulator.PHeT Bending Light simulator. Set medium 1 to air. Set medium 2 to water. Turn on the ray. Measure and record the angle of incidence and angle of refraction. Repeat 10 or more times for angles between 0  and 90 

27. Mathematical model 1.33 is the index of refraction of water

28. Snell’s Law More? You want more?more

29. In real life…

30. Examples

32. Quantified n 2 = ? A jeweler wants to determine the index of refraction of a gem stone. She shines a laser through it so that the incident beam strikes the face at 45  from the normal. If the light travels through the gem at an angle of 17  from the normal, what is the index of refraction? Try it first

33. Quantified n water = 1.33;  water = 40  ; n air = 1.00  water = ? If your friend shone a laser from underwater to the air at an angle of 40  from the normal, at what angle would you expect it to travel? Try it first

34. Demo: triangular prism 1.Place a triangular prism upright on the table. 2.Place a pin (or friend’s eye or whatever) so that the light passes parallel to the normal into one face of the prism. 3.Look through for the object through the other face of the prism. Where is it? Why can’t you see it?

35. Critical Angle When light travels from an optically dense material (e.g., water) to less optically dense material (e.g., air), it bends away from the normal. At some incident angle, it will refract parallel to boundary of the substance, i.e., no light passes the boundary. This angle is called the critical angle.

36. Total Internal Reflection Beyond the critical angle, light is reflected back into the original substance. This is called total internal reflection

37. Total Internal Reflection

38. Total internal reflection From fun swimming… to fascinating demonstrations… to cool party tricks… to cool dorm room decorations… to optics… to revolutions in communications

39. Think about it… Is it possible for light to undergo total internal reflection as it travels from air into water? Explain your answer.

40. Collect some more (virtual) data Go to PHeT Bending Light simulator.PHeT Bending Light simulator. Set medium 1 to water. Set medium 2 to air. At what angle does light totally internally reflect at a water / air boundary? Repeat for glass / water boundary. Repeat for a glass / air boundary. What patterns do you notice?

41. Critical angle

42. Think about it Go to Physics Classroom Refraction simulator. http://www.physicsclassroom.com/PhysicsClassroom/media/interactive/Refraction/i ndex.html Turn on protractor. Randomize the top or bottom medium. Using the Snell’s, calculate its index of refraction. Predict its critical angle.

43. Summary of mathematical relationships

44. Deriving Snell’s Law

45. Law of refraction A wave or line of soldiers or whatever hits the surface at an angle  1. At time 0, A 2 reaches boundary. At some time t, A 2 has traveled some distance into new medium ( l 2 )and A 1 has reached boundary after traveling some distance ( l 1 ). The wave or soldiers or whatever has been deflected by angle  2.

46. ii rr

47. ii ii l1l1 a

48. rr l2l2 a rr

49. Law of refraction Clever!