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ASTRO 101; Doppler shift
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Understanding Doppler shift
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Next stop on our tour of the properties of light and sometimes sound, is an e ect rst explained byan Austrian mathematician and physicist, Christian Doppler in 1842. In a paper entitled On the
Colored Light of Binary Stars and Some Other Stars of the Heavens, Doppler presented his theory
that observed frequency of wave depends on both the emitted light and on the relative speed of the
source and the observer.
But what does this mean? Well, if we switch back to sound waves for a
moment, we can use a real world analogy to explore this. I'm sure many
of you have been present as emergency vehicles or a train pass by. When
this happens, what do you hear? Well, yes, you hear a siren. But what
happens to the siren as an ambulance drives by? Well, if you listen
closely, you can hear that the pitch of the siren changes over time. As the
ambulance moves towards us, you hear the siren at one pitch. But as it
passes by, it seems to drop in pitch. Let's listen to the sound of a
passing train to compare the e ect from a di erent source. There is a
clear drop in pitch. But what is the reason for this? Let's take the
example of trains and look at this in more detail to explore the physics of
the Doppler shift. As the train sets o , we begin to see things change. The sound waves in front of
the train begin to bunch up and are pushed together. When the wave fronts of the sound waves are
pushed closer together, the wavelength that a stationary person will hear will be smaller. Smaller
wavelengths correspond to higher frequencies and higher pitch.
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lower pitch qq.gg
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longer wavelength higher pitch
shorter wavelength
When a train or ambulance approaches, you hear a higher pitch sound. Now, if we look at the
waves behind the train, we can see that as the vehicle moves forward, the waves appear to be
more spread out. This apparent stretch results in longer wavelengths or lower frequencies, which
results in a lower pitch. As a train moves past us, the shift raises the pitch as it moves towards us
has no e ect as it's next to us and lowers the pitch as it moves away from us. Curtis has an
electronic sound device that emits a sound wave with just one pitch. As he twirls the sound emitter
in circles, we can hear the sound. waves change in pitch in higher and lower pitches as the emitter
moves towards us and away from us. This raising and lowering of pitch is known as the Doppler
e ect. As we hinted at earlier, it also applies to light. As light is emitted from a source, the waves
being emitted can be squashed or stretched along the direction of motion. So if a star is moving
towards us, the light waves moving ahead of the star can appear to be increased in frequency or
decreased in wavelength. This translates as shift towards the blue red of the electromagnetic
spectrum. Conversely, as the star moves away from us, the waves appear to be stretched resulting
in longer wavelengths and a wider look to the star.
no effect when it's next to us
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For light, the shifts caused by the Doppler e ect are known as blueshift and redshift, although we
should note that this is not con ned to the visible band of the electromagnetic spectrum. Both x-
rays and radio waves can also be blue and red shifted. The colors just speak to the direction of the
shift. While stars do not normally race surrounding cars, they are moving around in space. Many
stars that are observed in the night sky are actually in binary star systems. In such systems, we see
stars in orbit around one another. If we view a pair of stars from the side, it will appear as the one
star is moving towards us while the other star is moving away from us.
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This relative motion is detected as blueshifts and redshifts. In this binary star system, we see two
yellow stars moving in circles. An astronomer is watching the stars orbit from a location far away to
the left. The upper star is moving away from the observer at a speed v, and the observer will
measure a longer wavelength for the light emitted by the star. The lower star is moving towards the
observer at speed v, the observer will measure a shorter wavelength for the light emitted by the star.
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