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Summary

Ligh has a speed of 300,000 km/s in a vacuum, and lower average speeds in matter.

Light is energy that travels in elecromagnetic waves within a certain range of frequencies.

Light is produced by vibrating electric charges in atoms.

Light passes through materials whose atoms absorb the energy and immediately reemit it as light.

Light doesn't pass through a material when the energy is changed to random kinetic energy of the atoms.

Light waves are transverse, so they can be polarized (with vibrations all in the same direction).

Polarizing filters transmit components of incident nonpolarized light that are parallel to the polarization axis, and block components vibrating at right angles to the polarization axis. The result is the emergence of polarized light.

The following are questions or problems that are considered testable material in our physics curriculum. When you have chosen an answer for a question, check your answers with the solutions at the bottom of the page.

1. Electromagnetic waves are
a. transverse waves.
b. longitudinal waves.

2. Which of these electromagnetic waves has the shortest wavelength?
a. Radio waves
b. Infrared waves
c. X rays
d. Ultraviolet waves
e. Light waves

3. The source of all electromagnetic waves is
a. heat.
b. magnetic fields.
c. electric fields.
d. vibrating charges.
e. none of the above

4. Which of the following is fundamentally different from the others
a. Sound waves
b. X rays
c. Gamma rays
d. Light waves
e. Radio waves

5. If the sun were to disappear right now, we wouldn't know it for 8 minutes because it takes 8 minutes
a. for the sun to disappear.
b. to operate receiving equipment in the dark.
c. for light to travel from the sun to the earth.
d. all of the above
e. none of the above

6. Which of the following is NOT an electromagnetic wave?
a. Sound
b. Radio
c. Infrared
d. Light
e. X ray

7. Heat lamps give off mostly
a. radio waves.
b. microwaves.
c. infrared waves.
d. ultraviolet waves.
e. X rays.

8. Glass is transparent to visible light, but not to
a. infrared.
b. ultraviolet.
c. both
d. neither

9. The reason solar eclipses are seen less commonly than lunar eclipses is that
a. solar eclipses don't happen as often as lunar eclipses.
b. the sun is farther from the earth than is the moon.
c. the earth's shadow on the moon is larger than the moon's shadow on the earth.
d. the sun is so much larger than the moon.

10. A photographer wishes to use a safetly light in the darkroom that will emit low-energy
photons. The best color of this light is
a. violet.
b. blue.
c. green.
d. yellow.
e. red.

11. Which color carries the most energy?
a. Red
b. Orange
c. Yellow
d. Green
e. Blue

12. If an electron vibrates up and down 1000 times each second, it generates an electromagnetic
wave having a
a. period of 1000 s.
b. speed of 1000 m/s.
c. wavelength of 1000 m.
d. frequency of 1000 Hz.

13. What is the frequency of an electromagnetic wave having a wavelength of 300 000 km?
a. Less than 1 Hz
b. 1 Hz
c. More than 1 Hz

14. If two Polaroid® filters are held with their polarization axes at right angles to each other, the
amount of light transmitted compared to when their axes are parallel is
a. twice as much.
b. the same.
c. half as much.
d. zero.

15. According to Einstein, light consists of massless bundles of electromagnetic energy called
photons.
True
False

a. transverse waves.

2. c. X rays

3. d. vibrating charges.

4. a. Sound waves

5. c. for light to travel from the sun to the earth.

6. a. Sound

7. c. infrared waves.

8. c. both

9. c. the earth's shadow on the moon is larger than the moon's shadow on the earth.

10. e. red.

11. e. Blue

12. d. frequency of 1000 Hz.

13. b. 1 Hz

14. d. half as much.

15. True