Lecture 22

Summary

Plane and spherical mirrors
Refraction
Thin lenses

Today's joke

Written Quiz Ch. 25

Dispersion

Example #1

Rainbows
Interactive rainbow applet

[supplemental material] ice crystal display

halos and sun dogs

more on atmospheric optics

Green flash only rarely seen at sunset is caused by a competition between dispersive refraction and scattering in the atmosphere.

This photograph was taken from Torrey Pines, California on Jan. 7, 1996, by Andrew T. Young.

$Green flash only rarely seen at sunset is caused by a competition between dispersive refraction and scattering in the atmosphere$

More photos of halos
ice crystal display

This picture was taken at the south pole, and includes a great number of interesting optical effects created by pencil-shaped and plate-shaped ice crystals.

ice crystal diagram

These are the ice crystals responsible for some of the effects you see in the above photo

slight dispersion ice crystal display

This picture was taken in Alaska, and shows some color in the sun dogs and other effects that result from slight dispersion in ice.

Lecture learning outcomes
A student who masters the topics in this lecture will be able to:

describe the phenomenon of dispersion in terms of the index of refraction (or the wave speed) of the medium
describe the formation of a rainbow by water droplets

Practice:
Try these additional examples

Example #2

Example #3

Prepare:
Read textbook sections 27-1 through 27-2 before the next lecture

kw4
Blue light has a higher index of refraction in glass than does red light. A glass lens will thus have a blue focal length that is _____ its red focal length.
A. shorter than
B. longer than
C. the same as
D. depends on whether lens is converging or diverging
Answer

Walker5e 26.79
(Review from lecture 21) A small insect viewed through a convex lens is 1.8 cm from the lens and appears 2.5 times larger than its actual size. What is the focal length of the lens? insect viewed with magnifying glass

A. 0.33 cm
B. 1.8 cm
C. 3.0 cm
D. 4.5 cm
Answer

Walker5e 26.83
You take a picture of a rainbow with an infrared camera, and your friend takes a picture at the same time with visible light. The rainbow's radius in the infrared picture is _____ than its radius in the visible-light picture.
A. greater than
B. less than
C. the same as
Answer

A. shorter than
Snell's law predicts more ray bending by a large index of refraction than by a small index of refraction. Thus parallel blue rays that enter the lens will be bent more sharply than red rays, and the blue focal length will be shorter, both for converging and diverging lenses.

C. 3.0 cm
solution equation

A. greater than
As seen in Figure 26-52 (above), long-wavelength red light has a smaller index of refraction (higher wave speed) than violet light, and red light emerges from the rain drop at larger angle than violet light. That makes the outer radius of a rainbow red and the inner radius violet. Because infrared light has a longer wavelength than red light, we expect its radius to be outside the radius of the red portion of the rainbow. [This means the index of refraction for infrared light in water is even smaller (even higher wave speed) than the index of refraction for red light.] Likewise, if a photo were taken in the ultraviolet we expect the radius of the rainbow to be less than the radius of the rainbow in the visible-light picture.
rainbow formation diagram