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 by listening to 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 waves are “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 waves are “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: