Doppler Effect
This is the apparent change of wavelength (and
frequency) of a wave from a source that is moving relative to an
observer.
You
may have heard the Doppler effect in sound as a police car with its siren on. As the
car comes towards you the pitch of the siren goes up, only to fall as it passes you
and goes away.
Stationary source
The waves are
equally spaced in all directions around the source.
Source moving towards
you
If the source is moving towards the observer the wave is "squashed
up" so the wavelength is made smaller. The faster the source moves the more the
waves are squashed.
With sound this means that the pitch of the sound that
you hear is increased and with light the colour of the light that you see is shifted
towards the blue end of the spectrum.
Source moving away from
you
If the source is moving away from the observer the wave is "stretched
out" and so the wavelength is made larger. The faster the source moves the more the
waves are stretched. With sound this means that the pitch of the sound that you hear
is decreased and with light the colour of the light that you see is shifted towards the
red end of the spectrum.
The change in frequency is much more noticeable
in sound than it is with light. This is because the speed of light is so much greater
than the speed of sound.
Red Shift
In astronomy the
Doppler effect is used to measure the motion of the galaxies. The faster a galaxy
moves the greater will be the shift in the colour of the light that you receive from it. If
the galaxy is moving away from us then the light from it will be shift towards the red
— this is known as Galactic Red Shift.
Since the Universe started with a Big
Bang most of the galaxies that you can see are moving away from us at high speed.
These speeds may be many tens of thousands of kilometres a second!
Uses and
effects of the Doppler Effect:
Radar speed trap
Red shift of the
galaxies
Siren of a train passing through a station
Change of pitch of a car
horn as it moves towards you and then away
You might find these links
useful:
Doppler 1Doppler 2Doppler 3
