138 PART 2^ |^ THE STARS
When astronomers talk about the Doppler eff ect, they are
talking about a shift in the wavelength of electromagnetic radia-
tion. But the Doppler shift can occur in any form of wave phe-
nomena, including sound waves, so you probably hear the
Doppler eff ect every day without noticing.
Th e pitch of a sound is determined by its wavelength.
Sounds with long wavelengths have low pitches, and sounds with
short wavelengths have higher pitches. You hear a Doppler shift
every time a car or truck passes you and the pitch of its engine
noise drops. Its sound is shifted to shorter wavelengths and
higher pitches while it is approaching and is shifted to longer
wavelengths and lower pitches after it passes.
To see why the sound waves are shifted in wavelength, con-
sider a fi re truck approaching you with a bell clanging once a
second. When the bell clangs, the sound travels ahead of the
truck to reach your ears. One second later, the bell clangs again,
but, during that one second, the fi re truck has moved closer to
you, so the bell is closer at its second clang. Now the sound has
a shorter distance to travel and reaches your ears a little sooner
than it would have if the fi re truck were not approaching. If you
timed the clangs, you would fi nd that you heard them slightly
less than one second apart. After the fi re truck passes you and is
moving away, you hear the clangs sounding slightly more than
one second apart because now each successive clang of the bell
occurs farther from you and the sound travels farther to reach
your ears.
■ Figure 7-11a shows a fi re truck moving toward one
observer and away from another observer. Th e position of the
bell at each clang is shown by a small black bell. Th e sound of the
clangs spreading outward is represented by black circles. You can
see how the clangs are squeezed together ahead of the fi re truck
and stretched apart behind. If the fi re truck has a siren instead of
a bell, the sound coming from the siren will be a wave with a
series of compressions and un-compressions. If the truck and
siren are moving toward an observer, the compressions and un-
compressions of the siren’s sound wave will arrive more often—at
a higher frequency—than if the truck were not moving, and the
observer will hear the siren at a higher pitch than the same siren
when it is stationary.
Now you can substitute a source of light for the clanging bell
or wailing siren (Figure 7-11b). Imagine the light source emitting
waves continuously as it approaches you. Each time the source
emits the peak of a wave, it will be slightly closer to you than
when it emitted the peak of the previous wave. From your
vantage point, the successive peaks of the wave will seem closer
together in the same way that the clangs of the bell seemed closer
together. Th e light will appear to have a shorter wavelength,
making it slightly bluer. Because the light is shifted slightly
toward the blue end of the spectrum, this is called a blueshift.
After the light source has passed you and is moving away, the
peaks of successive waves seem farther apart, so the light has a
longer wavelength and is redder. Th is is a redshift. Th e shifts are
■ Figure 7-11
The Doppler effect. (a) The clanging bell on a moving fi re truck produces
sounds that move outward (black circles). An observer ahead of the truck
hears the clangs closer together, while an observer behind the truck hears
them farther apart. Similarly, the sound waves from a siren on an approach-
ing truck will be received more often, and thus be heard with a higher tone,
than a stationary truck, and the siren will have a lower tone if it is going
away. (b) A moving source of light emits waves that move outward (black
circles). An observer toward whom the light source is moving observes a
shorter wavelength (a blueshift), and an observer for whom the light source
is moving away observes a longer wavelength (a redshift). (c) Absorption
lines in the spectrum of the bright star Arcturus are shifted to the blue in
winter, when Earth’s orbital motion carries it toward the star, and to the red
in summer when Earth moves away from the star.bc657 658
Wavelength (nm)Balmer alpha line in the spectrum of Arcturus655 656When Earth’s orbital
motion carries it
toward Arcturus, you
see a blueshift.When Earth’s orbital
motion carries it away
from Arcturus, you
see a redshift.Laboratory wavelenth λ 0Redshift
Positions of clanging bell
aBlueshift