2. Reflection and Refraction
Chapter 28
Chapter 28
Reflection and Refraction

3. 1. REFLECTION
Most objects we see reflect light.

4. 2. PRINCIPLE OF LEAST TIME
Fermat's principle:
Light travels in straight lines and will take the path of least time.

5. Fermat says take this path.

6. 3. LAW OF REFLECTION
Using Fermat's principle one can show the law of reflection. A B 1 5 2 4 3
Mirror

7. The law of reflection states that
the angle of incidence (qi) equals
the angle of reflection (qr).
This is true for specular reflection.
(Specular reflection is “mirror-like” reflection.)

8. Normal
Incident Ray
Reflected Ray qi qr
Specular Reflecting Surface
Included in the law of reflection is the fact that the incident ray, the normal, and the reflected ray all lie in the same plane.

9. Demo - Candle Burning Under Water
Video - Candle Burning Under Water
Video - Water Wave Reflections
Demo - Mirror Track Tracing – Take home exercise for PQ Points

10. Plane Mirror
Ray Diagramming
Plane Mirror

11. Plane Mirror
Ray Diagramming
Plane Mirror

12. Plane Mirrors Using ray diagramming one finds that the image is
1. Upright
2. Same size as the object
3. Virtual

13. Diffuse Reflection Video - Diffuse Reflection
If successive elevations of a surface are no more than l/8 apart, then
the surface is said to be polished at that wavelength.
Most objects are seen by diffuse reflection.

14. Diffuse Reflection Magnification of a surface
Colored lines are for the purpose of
distinguishing reflected rays from
incident rays.

15. Something that is barely polished for red light would not be polished for blue light.
Consider microwave satellite dishes.
They are polished for microwaves but not for visible.
The metal screen in a microwave oven serves what purpose?

16. Reflection from Curved Surfaces (Concave shown here)
Ray Diagramming f
Principal axis
Reflecting Surface
The law of specular reflection is still obeyed.

17. Reflection from Curved Surfaces (Concave shown here)
Ray Diagramming f
Principal axis
Reflecting Surface
The law of specular reflection is still obeyed.

18. Demo - Coin Mirage
Demo – Deep Concave Mirror
Demo - Make-up Mirror Images
Concave mirrors can produce real inverted images that are magnified, diminished, or the same size as the object.
Concave mirrors can produce virtual upright images that are magnified.

19. More on Mirrors
Convex mirrors always produce a diminished, upright, virtual image.
Demo - Convex Mirror
Demo - Two Perpendicular Mirrors
Demo - Three Perpendicular Mirrors
Demo – Road Reflectors
Question - Why is it hard to see at night after or during a rain?

20. Chapter 28 Review Questions

21. What type of mirror would you use to produce a magnified image of your face?
(a) flat
(b) concave
(c) convex
(d) you could use a concave or a convex mirror

22. What is (are) the purpose(s) of the wire screen in the door of a microwave oven?
(a) to absorb microwaves
(b) to allow you to see what's cooking
(c) to reflect microwaves
(d) all of the above
(e) only (b) and (c)

23. 4. REFRACTION
Fermat's principle can also explain refraction (remember the beach).
Refraction of light is the bending of light as it passes obliquely from one medium to another.
It is due to the different speeds of light in the two different media.

24. Index of Refraction Index of refraction of a material equals
the speed of light in a vacuum divided by
the speed of light in the material.

25. Slide - Twinkling Cartoon
Demo – Green Laser Refraction in an Aquarium Demo - Twinkling with Laser
Slide - Twinkling Cartoon

26. Video - Water Wave Refractions
Video - "Broken" Pencil Refraction

27. Because of atmospheric refraction,
we have lingering, elliptical sunsets.
Sun
Earth
Sun

28. Mirage
Cool air
Warm air
Surface of water?
Highway Mirage

29. Looming
Warm air
Cool air

30. Chapter 28 Review Question

31. An oar partially immersed in water appears "broken" because of
(a) refraction
(b) diffraction
(c) polarization
(d) interference
(e) absorption

32. 5. CAUSE OF REFRACTION
When light passes from one medium to another, its speed changes which in turn causes a bending of the light.
Examples:
Car running onto shoulder of road
Light passing from air into water

33. Wave fronts of light qi
Air
Water qR

34. This bending produces illusions.
Example: Objects in water appear closer and nearer to the surface.
Eye
Air
Water

35. Demo – Bow Fishing

36. Fish can see everything above the surface of water within a 960 cone.

37. 960

38. Demo - Disappearing Pyrex

39. Dispersion
Different frequencies are bent different amounts which causes a separation of white light into its constituent colors.
This is the basic principle behind the operation of a prism. We say that a prism disperses the light.
The higher frequencies interact most (slow down the most) and thus are bent the most.
Demo - Aquarium Prism

40. Dispersion in a Prism
Slit
White Light
Source
Prism

41. Rainbow

42. Picture - Rainbow
Individual drops act as dispersers.
The 42o cone
Demo – Rainbow Model
A single eye can only see a small range of colors from a single raindrop.

43. Secondary Rainbow
Double rainbows are dimmer, higher, and have colors reversed. Link to picture.

44. Green Flash Sun Earth Dispersion occurs causing
multiple images of the sun.
The last to set would be blue,
but most of the blue has been
scattered which leaves green.

45. 6. TOTAL INTERNAL REFLECTION
Video - Laser Under Water
Critical angle is the angle where total internal reflection (TIR) begins.
TIR is possible only when light is entering a medium of lesser index of refraction.
Binoculars make use of TIR.

46. Two Prisms in a Monocular
Flex Cam – Monocular

47. Fiber optic devices make use of TIR.
Demo - Laser and Light Pipe
Video - Woman at Edge of Pool
Flex Cam – Aquarium and Finger Dip in Water

48. Chapter 28 Review Questions

49. A lingering sunset is (a) a looming effect
(b) caused by an elliptical (oval) sun
(c) due to atmospheric refraction
(d) caused by reflections from the upper atmosphere

50. When a beam of light emerges at a nonzero angle from water to air, the beam
(a) bends away from the normal
(b) continues in the same direction
(c) bends toward the normal
(d) changes frequency
(e) slows down

51. 7. LENSES
Lenses use refraction to form images.
Demo - Fresnel Lens

52. A Convex Lens Converges Light Raysf

53. Imaging with a Convex Lens
and passes through a point
called the focal point.
is bent upon entering the lens.
Arrow as
Object
Upon exiting the lens it is bent again
A ray parallel to the principal axis
sees an
image here. f
Principal Axis
An eye placed here
Convex Lens
A ray passing through the center of the lens is basically undeflected.
This arrangement produces an inverted, real, diminished image.

54. More Imaging With a Convex Lens
is bent upon entering the lens.
Upon exiting the lens it is bent again
Arrow as
Object
and passes through a point
called the focal point.
A ray parallel to the principal axis f
Principal Axis
sees an
image here.
Convex Lens
An eye placed here
A ray passing through the center of the lens is basically undeflected.
This arrangement produces an upright, virtual, magnified image. It is a simple magnifying glass.
Farsighted people use lenses similar to these.

55. A Concave Lens Diverges Light Raysf

56. Imaging with a Concave Lens
is bent upon entering the lens.
Arrow as
Object
Upon exiting the lens it is bent again
A ray parallel to the principal axis
such that is appears to have come
from a point called the focal point. f
An eye placed here
Principal Axis
sees an
image here.
Concave Lens
A ray passing through the center of the lens is basically undeflected.
This arrangement produces an upright, virtual, diminished image.
Nearsighted people use lenses similar to these.

57. Types of Lenses Convex lenses are positive converging lenses.
Plano
Convex
Plano
Concave
Double
Convex
Double
Concave
Convex Meniscus
Concave Meniscus
Convex lenses are positive converging lenses.
Concave lenses are negative diverging lenses.
Nearsighted people use lenses similar to these.
Farsighted people use lenses similar to these.

58. 8. LENS DEFECTS Spherical Aberration Cure – Diaphragm
Convex Lens
Cure
– Diaphragm
or lens combination

59. Cure – Diaphragm or lens combinations (achromatic lens)
Chromatic Aberration
Convex Lens
Cure – Diaphragm or lens combinations
(achromatic lens)
Demo – Overhead Chromatic Aberration
Astigmatism – due to barrel-shaped cornea

60. Eyeglasses
Picture - Farsightedness
Picture - Nearsightedness

61. Pinhole

62. Chapter 28 Review Questions

63. One way to reduce the problem of spherical aberration in a positive lens is to join a negative lens to it to form a compound lens.
(a) True
(b) False

64. If you wish to take a picture of your image while standing 5 ft in front of a plane mirror, for what distance should you set your camera to provide the sharpest focus?
(a) 10 ft
(b) 5 ft
(c) 2.5 ft
(d) it can't be done