The Doppler Effect The sound of a police car siren changes as it moves past you. The pitch of the siren seems to change. Ahead of the car, the sound waves are piling up, so the pitch is higher. Behind the car the waves spread out, so the pitch is lower. This is known as the Doppler effect.

The Doppler effect The Doppler effect also happens with light, but there is a change in colour instead of a change in pitch. Atoms in the atmosphere of stars absorb some of the light of a galaxies spectrum, producing dark lines within it.

The Doppler Effect If the source of light is moving towards the Earth then the wavelength of all the emitted waves is compressed a little. This results in a shift of the spectral lines into the blue part of the spectrum, known as a blue shift. Conversely if the source of the light is moving away from the Earth then there is a shift of the lines into the red part of the spectrum known as a red shift.

The Doppler effect Edwin Hubble studied the positions of these lines and found out that most stars have a spectrum that is shifted towards the red, known as the red shift. It follows then, that most galaxies are moving away from Earth, supporting the Big Bang expanding Universe model.

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