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Science and Society

Laser Bug
old a fully inflated balloon lightly The reflected beam, however, will have dif- hanging picture, or the glass front of a
Hbetween your fingertips and talk. ferent frequencies and amplitudes from china cabinet.
You will be able to feel the slight vibra- the windowpane vibrating in and out. The
tions from your voice. Likewise, the sound changes can be detected by a receiver and QUESTIONS TO DISCUSS
waves from your voice will cause a nearby converted into sound in a headphone.
1. Is it legal for someone to listen in on
window to vibrate slightly. If a laser beam is You cannot see an infrared laser beam
your private conversations?
bounced off the window, the reflection will because infrared is outside the frequen-
be changed by the vibrations. The incoming cies that humans can see. Any sound- 2. Should the sale of technology such as
laser beam is coherent; all the light has the sensitive target can be used by the laser bug, the laser bug be permitted? What are
same frequency and amplitude (see p. 191 ). including a windowpane, inflated balloon, the issues?

maker are also harmonic instruments. These include all the wind
instruments such as the clarinet, fl ute, trombone, trumpet, pipe
organ, and many others. The various wind instruments have dif-

v v
ferent ways of making a column of air vibrate. In the fl ute, air
vibrates as it moves over a sharp edge, while in the clarinet, saxo-
phone, and other reed instruments, it vibrates through fl uttering
thin reeds. The air column in brass instruments, on the other Sound

A wave fronts
hand, is vibrated by the tightly fluttering lips of the player.

The length of the air column determines the frequency, and
woodwind instruments have holes in the side of a tube that are (For motion
opened or closed to change the length of the air column. Th e toward observer)
resulting tone depends on the length of the air column and the
resonant overtones.
v o
SOUNDS FROM MOVING SOURCES

When the source of a sound is stationary, equally spaced sound (For motion away
waves expand from a source in all directions. But if the sound- B from observer)
ing source starts moving, then successive sound waves become
FIGURE 5.24 (A) Sound waves emitted by a stationary source
displaced in the direction of movement, and this changes the and observed by a stationary observer. (B) Sound waves emitted
pitch. For example, the siren of an approaching ambulance by a source in motion toward the right. An observer on the right
seems to change pitch when the ambulance passes you. Th e receives wavelengths that are shortened; an observer on the left
sound wave is “squashed” as the ambulance approaches you, receives wavelengths that are lengthened.
and you hear a higher-frequency siren than the people inside
the ambulance. When the ambulance passes you, the sound
waves are “stretched” and you hear a lower-frequency siren train approaching a crossing with a sounding bell thus hears a

(Figure 5.24). The overall effect of a higher pitch as a source high-pitched bell that shifts to a lower-pitched bell as the train

approaches and then a lower pitch as it moves away is called the whizzes by the crossing. The Doppler effect occurs for all waves,

Doppler eff ect. The Doppler effect is evident if you stand by including electromagnetic waves (see Figure 7.3 on p. 179 ).

a street and an approaching car sounds its horn as it drives by When an object moves through the air at the speed of sound,
you. You will hear a higher-pitched horn as the car approaches, it keeps up with its own sound waves. All the successive wave
which shifts to a lower-pitched horn as the waves go by you. Th e fronts pile up on one another, creating a large wave distur-

driver of the car, however, will hear the continual, true pitch of bance called a shock wave (Figure 5.25). The shock wave from
the horn because the driver is moving with the source. a supersonic airplane is a cone-shaped shock wave of intense

A Doppler shift is also noted if the observer is moving and the condensations trailing backward at an angle dependent on the
source of sound is stationary. When the observer moves toward speed of the aircraft. Wherever this cone of superimposed crests

the source, the wave fronts are encountered more frequently than passes, a sonic boom occurs. The many crests have been added
if the observer were standing still. As the observer moves away together, each contributing to the pressure increase. Th e human

from the source, the wave fronts are encountered less frequently ear cannot differentiate between such a pressure wave created by

than if the observer were not moving. An observer on a moving a supersonic aircraft and a pressure wave created by an explosion.
5-17 CHAPTER 5 Wave Motions and Sound 131