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A Closer Look

Doppler Radar
he Doppler effect was named after and weather radar that measures frequency show a higher frequency, and waves from
Tthe Austrian scientist Johann Doppler changes as a result of motion is called Dop- objects moving away from the antenna
(1803–1853), who first demonstrated the pler radar. show a lower frequency. These shifts of
effect using sound waves in 1842. The same Weather radar broadcasts short radio frequency are measured, then displayed as
principle applies to electromagnetic radia- waves from an antenna. When directed at the speed and direction of winds that move
tion as well as sound, but now the shifts are a storm, the waves are reflected back to the raindrops and other objects in the storm.
in the frequency of the radiation. A lower antenna by rain, snow, and hail. Reflected Weather forecasters can direct a Doppler
frequency is observed when a source of radar waves are electronically converted radar machine to measure different eleva-
light is moving away, and this is called a and displayed on a monitor, showing the tions of a storm system. This shows highly
redshift. Also, a blueshift toward a higher location and intensity of precipitation. A accurate information that can be used to
frequency occurs when a source of light is Doppler radar also measures frequency identify, for example, where and when a tor-
moving toward an observer. Radio waves shifts in the reflected radio waves. Waves nado might form or the intensity of storm
will also experience such shifts of frequency, from objects moving toward the antenna winds in a given area.

Does a sonic boom occur just when an airplane breaks the

sound barrier? The answer is no; an airplane traveling at or faster
than the speed of sound produces the shock wave continuously,
and a sonic boom will be heard everywhere the plane drags
its cone-shaped shock wave. In addition, high-speed airplanes

often produce two or more shock waves, which are associated
with the nose, tail, and other projections on the aircraft . Can
you find evidence of shock waves associated with projections on

the airplane pictured in Figure 5.26?

The Austrian physicist Ernst Mach (1838–1916) published
a paper in 1877 laying out the principles of supersonics. He also
came up with the idea of using a ratio of the velocity of an object
to the velocity of sound. Today, this ratio is called the Mach
number. A plane traveling at the speed of sound has a Mach
number of 1, a plane traveling at twice the speed of sound has
a Mach number of 2, and so on. Ernst Mach was also the fi rst
to describe what is happening to produce a sonic boom, and FIGURE 5.26 A cloud sometimes forms just as a plane
he observed the existence of a conical shock wave formed by a accelerates to break the sound barrier. Moist air is believed to form
projectile as it approached the speed of sound. the cloud water droplets as air pressure drops behind the shock wave.

SUMMARY

Elastic objects vibrate, or move back and forth, in a repeating motion 20 and 20,000 Hz are audible sounds that can be heard by humans. An

when disturbed by some external force. The maximum value of the audible sound of high frequency is perceived as a high-pitched sound,
displacement of a vibration is called the amplitude. Th e period is the and low frequencies are perceived as low-pitched sounds.
time required to complete one full vibration or cycle of the motion. Th e amplitude of a wave is the largest displacement from the equi-
Th e frequency is the number of vibrations that the object performs in a librium position that an element of the medium can experience when
second. The unit of frequency is called hertz. the wave is passing by. Th e period and frequency of a wave are the same

Traveling vibrations or disturbances of a medium are called waves. as the period and frequency of vibrations on one element of the medium
In a transverse wave, the elements of a medium vibrate in a direction when the wave is passing by. Wavelength is the distance the wave travels
perpendicular to the direction the wave is traveling. In a longitudinal during one period. All waves carry energy.
wave, the elements of a medium vibrate in a direction parallel to the When a traveling wave encounters a boundary between two
direction the wave is traveling. Sound is a longitudinal wave, where different media, it can be either reflected or refracted (or both) at
disturbances are periodic condensations (crests) and rarefactions the boundary. When two traveling waves meet, they interfere with
(troughs). Sound waves do not travel in a vacuum because a vacuum each other. At any point they meet, this interference may be either
does not have any medium. Sound waves of frequencies between constructive (“enhancement”) or destructive (“disappearance”). Beats

5-19 CHAPTER 5 Wave Motions and Sound 133