1. Reflection and Refraction

2. Reflection When a wave reaches a boundary it is:
Partially reflected (bounces off surface)
Partially transmitted through surface.

3. Law of Reflection
Angle of incidence = Angle of reflection

4. Note
The angle of incidence is formed between the incident ray and the normal.
The angle of reflection is formed between the reflected ray and the normal.

5. Reflection
Regular reflection – occurs when parallel light waves strike a surface and reflect in the same parallel directions
“Smooth” surfaces
Diffuse Reflection – occurs when parallel light waves strike a rough uneven surface and reflect in many different directions.

6. Regular Reflection
A reflection produced by a smooth surface.

7. Diffuse Reflection
Reflection from a rough surface.

8. Diffuse Reflection
When the surface is rough, images are not formed, but the light still reflects

9. Regular
Regular

10. Diffuse Reflection
Bumps on the surface must be less than about 1/8 the wavelength of light in order to have a smooth surface
Big satellite dishes are made of wire mesh and appear smooth to the longer wavelength microwave signals

11. Satellite Dish

12. Satellite Dish

13. di=do
hi=ho

14. Type of Image
Real or Virtual
Enlarged or Reduced
Erect or Inverted

15. What is the minimum mirror height required for a person to see their full self in the mirror?

16. Hint: Compare AB to CD C A B D

17. Creating Images

18. Plane Mirrors We trace rays from the object
A plane mirror always produces a virtual image
Virtual image – copy of an object formed at location from which the light rays appear to come
The image is the same distance behind the mirror as the object is in front of it
The object and the image are the same size

19. Refraction
When a wave crosses a boundary between Medium 1 and Medium 2, the wave changes direction because it changes velocity.
Frequency remains constant.
Velocity changes as a result of wavelength change.
Simulations
Refraction
Phet Refraction

20. Refraction

21. Refraction

22. Example
Incident Ray
Less Dense Medium More Dense
Refracted Ray

23. Index of Refraction

24. Index of Refraction

25. Refraction
When light slows down at a boundary, it bends toward the normal (less dense to more dense)
When light speeds up at a boundary, it bends away from the normal (more dense to less dense)

26. Index of Refraction, n n>1 (Why?) n = c / v
c : the speed of light in a vacuum,
3 x 108  m/sec
v : speed of light in the medium.
n : medium's index of refraction
n>1 (Why?)

27. Indices of Refraction Vacuum 1.00 Air 1.0003 Water 1.33 Ethanol 1.36
Crown glass 1.52
Quartz 1.52
Diamond 2.42

28. Problem What is the speed of light in quartz? n = 1.54
Answer: 1.97 x m/s

29. Snell’s Law
n1 sin q1 = n2 sin q2

30. Problem
What is the angle of refraction when a ray from air with an angle of incidence of 25 o is incident to water?
Draw the ray diagram.
Answer: o

31. Total Internal Reflection
Can occur when ray goes from higher n to lower n.
Above a Critical angle (of incidence) the ray is reflected, not refracted
For problems, set the angle of refraction to 90, and solve for critical angle

32. Total Internal Reflection
This is used in fiber-optic cables to transmit data signals. The light inside the cable cannot escape so no energy (signal) is lost as it travels.

33. Why We Love Diamonds?

34. Total Internal Reflection in a Diamond

35. Problem
Find the critical angle for a light ray that is incident from water to air.
Answer: 48.8 o

36. Dispersion
The index of refraction of glass is different for the colors that make up white light because the speed of light is slightly different in glass for each frequency of light. (In vacuum all colors have speed c=3x108m/s.)

37. As white light passes through a prism, shorter wavelengths refract more than longer wavelengths, and the colors separate
Longer wavelength move faster as it enter the prism (red light) so it bends the least
Violet is bent the most

38. Rainbows

40. Problem
The index of refraction for crown glass for red light is What is the speed of red light in crown glass?
Answer: 1.98 x m/s

41. Refraction
This bending of light can play tricks on our minds

42. Mirages – false or distorted image

43. Mirage

44. Occurs because hot air has a lower n, than cold air.

45. Another Trick
Light travels faster at higher altitudes because the air is less dense.