# Review Questions: Geometric Optics

1. Light will almost always travel from one place to another along a path of least

a. distance.
b. time.
c. effort.
d. expense.
e. complication.

Reasoning: This is the Fermoat's principle. See page 533 of the text.

2. The law of reflection holds for

a. plane mirrors.
b. curved mirrors.
c. both of these.
d. neither of these.

3. The amount of light reflected from the front surface of a common windowpane is about

a. 4 percent.
b. 8 percent.
c. 40 percent.
d. 92 percent.
e. 96 percent

4. Diffuse reflection occurs when the size of surface irregularities is

a. small compared to the wavelength of the light used.
b. large compared to the wavelength of the light used.

5. A surface that is considered rough for infrared waves may be polished for

b. light waves.
c. both of these.
d. none of these.

6. When light reflects from a surface, there is a change in its

a. frequency.
b. wavelength.
c. speed.
d. all of these.
e. none of these.

7. The shortest plane mirror in which you can see your entire image is

e. dependent on your distance from the mirror.

8. A yellow-white candle flame reflected from a piece of red glass shows two images; one from each surface.

a. Both images are yellow-white.
b. One image is yellow-white and the other red.
c. Both images are red.
d. Both images are reddish yellow.

9. When you look at yourself in a pocket mirror, and then hold the mirror farther away, you see

a. more of yourself.
b. less of yourself.
c. the same amount of yourself.

10. It is difficult to see the roadway in front of you when you are driving on a rainy night mainly because

a. light scatters from raindrops and cuts down the light to reach your eyes.
b. of added condensation on the inner surface of the windshield.
c. the film of water on your windshield provides reflecting surface.
d. the film of water on the roadway makes the road less diffuse.
e. none of these.

11. Ninety-five percent of light incident on a mirror is reflected. How much light is reflected when three of these mirrors are arranged so light reflects from one after the other?

a. 81%
b. 85%
c. 86%
d. 90%
e. 95%

Reasoning: Each mirror reflects 95%, thus for three mirrors we have, .95 x .95 x .95 = .86 or 86%.

12. A pair of plane mirrors are at right angles to each other. A coin placed near the mirrors has at most

a. 2 images.
b. 3 images.
c. 4 images.
d. more than 4 images.

13. The bending of light as it refracts actually

a. shortens the time of travel.
b. lengthens the time of travel.
c. has nothing to do with the time of travel.

Reasoning: Like reflections, this is also in accordance to the Fermat's principle.

14. Light travels fastest in

a. warm air.
b. cool air.
c. a vacuum.

15. Atmospheric refraction makes the daylight hours a bit

a. longer.
b. shorter.
c. longer in summer but shorter in winter.

16. A mirage is a result of atmospheric

a. reflection.
b. refraction.
c. scattering.
d. dispersion.
e. aberrations.

17. The twinkling of the stars is a result of atmospheric

a. reflection.
b. refraction.
c. scattering.
d. dispersion.
e. aberrations.

18. Refraction results from differences in light's

a. frequency.
b. incident angles.
c. speed.
d. all of these.
e. none of these.

19. Light refracts when traveling from air into glass because light

a. has greater intensity in air than in glass.
b. has greater intensity in glass than in air.
c. has greater frequency in air than in glass.
d. has greater frequency in glass than in air.
e. travels slower in glass than in air.

20. When a light beam emerges from water into air, the average light speed

a. increases.
b. decreases.
c. remains the same.

21. Refraction causes the bottom of a swimming pool to appear

a. farther down than it actually is.
b. closer to the surface than it actually is.
c. neither.

22. The average speed of light is greatest in

a. red glass.
b. orange glass.
c. green glass.
d. blue glass.
e. is the same in all of these.

Reasoning: This is called dispersion (see page 544 of the text). Lights of different frequencies have different speeds in glass, water, etc. A red glass, absorbs all frequencies except red. And since red bends the least as light travels through a piece of glass (see Figure 28.29), it means that the light slows down the least for red and the most for violet.

23. Different colors are dispersed by a prism because different colors in the prism have different

a. frequencies.
b. speeds.
c. directions.
d. energies.
e. none of these.

24. Rainbows are not usually seen as complete circles because

a. the ground is usually in the way.
b. they are actually elliptical.
c. they have no bottom part.
d. raindrops are not perfectly round.
e. rainbows are actually arched shaped.

25. In an optical fiber, light actually

a. curves in a direction parallel to the central axis of the fiber.
b. travels in straight-line segments.
c. travels along the outer surface of the fiber.
d. all of these, depending on the type of fiber.
e. none of these.

Reasoning: See Figure 28.42 of the text.

26. The type of lens that spreads parallel light is a

a. converging lens.
b. diverging lens.
c. combination converging-diverging lens.

27. A "burning glass" used to concentrate sunlight in a tiny spot is a

a. converging lens.
b. diverging lens.
c. either.
d. neither.

a. dilated.
b. constricted.
c. same either way.

29. Chromatic aberration is a consequence of different colors in a lens having different

a. aberrations.
b. frequencies.
c. energies.
d. critical angles.
e. speeds.

30. Fermat's principle of least time applies to

a. reflection.
b. refraction.
c. both of these.
d. neither of these.

31. Ninety percent of light incident on a certain piece of glass passes through it. How much light passes through two pieces of this glass?

a. 80%
b. 81%
c. 85%
d. 89%
e. 90%