Homework Helpers: Physics

8 Waves and Light

Answer Key

The actual answers will be shown in brackets, followed by an explanation. If you don”t understand an explanation that is given in this section, you may want to go back and review the lesson that the question came from.

Lesson 8–1 Review

1. [longitudinal wave]—Sound waves are examples of longitudinal waves.

2. [mechanical wave]—Sound, which consists of mechanical waves, can”t travel through a vacuum.

3. [rarefaction]—The rarefaction of a longitudinal wave is analogous to the crest of a transverse wave.

Lesson 8–2 Review

1. [Destructive interference]—Destructive interference is used in “noise reduction” technology.

2. [0.125 m]—Microwaves are electromagnetic waves, so we use the constant, C.

3. [1.0 × 10^-18 s]—Because the wave is electromagnetic, we use the constant, C.

Lesson 8–3 Review

1. [The Doppler effect]—We say the “apparent” change in frequency because the waves aren”t being generated faster, they are just reaching the listener faster, due to the motion of the source.

2. [amplitude]—A larger disturbance creates a louder sound.

3. [frequency]—A person with a high pitched voice generates waves faster than someone with a low pitched voice.

Lesson 8–5 Review

1. [virtual image]—Unlike real images, virtual images can”t be projected onto a screen.

2. [–19 cm and 2.0 × 10¹ cm]—Check the sign conventions for a concave mirror. All of our given values should remain positive.

Given: h_o = 8.5 cm d_o = 8.0 cm f = 14.0 cm

Find: d_i and h_i

The negative sign for our image distance indicates that it is formed behind the mirror, so it is virtual. This makes sense, because our object was placed at a distance less than the focal length.

Our image is enlarged and upright. Again, our answer makes sense. When a concave mirror forms a virtual image, it is enlarged and upright.

3. [–5.9 cm and 1.3 cm]—According to our sign conventions for convex mirrors, we need to make the focal length negative.

Given: h_o = 5.0 cm d_o = 22.0 cm f = -8.0 cm

Find: d_i and h_i

Here, the negative sign indicates that the image is formed behind the mirror.

Our answer for image height makes sense, indicating that the image is upright and reduced in size.

Lesson 8–6 Review

1.

2.

3. [–7.33 cm and 1.67 cm]—Checking the sign conventions for concave lenses reveals that our focal length should be given a negative sign.

Given: h_o = 5.00 cm d_o = 22.0 cm f = –11.0 cm

Find: d_i and h_i

As is always the case with concave lenses, the distance of the image is negative, placing the image on the same side of the lens as the object.

The height of the image is positive, because concave lenses always form upright images. The image is also reduced.

Chapter 8 Examination

1. [f. electromagnetic wave]—Electromagnetic waves, such as radio waves and visible light, can travel through space.

2. [j. compression]—This is where the molecules get pressed together.

3. [c. wavelength]—The wavelength is the length of one wave cycle.

4. [i. frequency]—Remember: Frequency is measured in hertz.

5. [d. amplitude]—The amplitude can be measured from the equilibrium (rest) position to either the crest or the trough.

6. [h. longitudinal wave]—A sound wave is a good example of a longitudinal wave.

7. [k. convex]—Some people remember that a concave mirror goes in, like the opening in a cave. Convex is the opposite type.

8. [e. mechanical wave]—Sound waves are mechanical waves, so they can”t travel through a vacuum.

9. [l. concave]—Don”t get confused by the fact that mirrors and lenses are opposites in one sense. Concave lenses and convex mirrors are diverging, whereas convex lenses and concave mirrors are converging.

10. [b. trough]—The trough of a wave is shaped like the troughs that horses drink out of.

11. [d. sound waves]—All electromagnetic waves, including gamma, visible light, and radio waves are transverse waves. Sound, on the other hand, is a longitudinal wave.

12. [a. amplitude]—The greater the amplitude of a sound wave, the more the particles of the medium are displaced, and the louder the sound.

13. [d. 35.0°]—The law of reflection (θ_i = θ_r) tells us that the angle of incidence is equal to the angle of reflection, both of which are measured from the normal to the medium boundary.

14. [c. interference]—The significance of Thomas Young”s double-slit experiment is that he observed a property, interference, that was not associated with macroscopic particles.

15.

16. [–10.9 cm and 3.21 cm]—According to our sign conventions for convex mirrors, we need to make the focal length negative.

Given: h_o = 10.0 cm d_o = 34.0 cm f = –16.0 cm

Find: d_i and h _i

Here, the negative sign indicates that the image is formed behind the mirror.

As with all images formed by convex mirrors, ours is reduced and upright.

17. [10.5 cm and –4.88 cm]—Check the sign conventions for a concave mirror first. You should see that you don”t need to make any of the given values negative.

Given: h_o = 6.50 cm d_o = 14.0 cm f = 6.00 cm

Find: d_i and h_i

The image is inverted and reduced in size.

18. [330 m/s]—Remember: The units hertz (Hz) = s^–1.

v = fλ = (15.000s^-1)(0.022m) = 330m/s

19. [250 m]—λ = vT = (380m/s)(0.65s) = 247 m or 250 m after rounding to 2 significant digits

20. [2.5 × 10⁸ m/s]—When the index of refraction is 1.0, like in outer space or our atmosphere, light travels at C, 3.00 × 10⁸ m/s. When it enters a medium with an index of refraction of 1.2 is slows down according to the formula ν = C/n = 3.00 × 10⁸ m/s / 1.2 = 2.5 × 10⁸ m/s.