Red-Shift and the Doppler Effect

This lesson covers the Doppler effect and red-shift of light from distant galaxies — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand what the Doppler effect is, how it applies to light from galaxies, and what red-shift tells us about the motion of the universe.

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The Doppler Effect

The Doppler effect is the apparent change in the wavelength (and frequency) of a wave when there is relative motion between the source and the observer.

The Doppler Effect with Sound

You have probably experienced the Doppler effect with sound:

• When an ambulance approaches you, its siren sounds higher pitched (higher frequency, shorter wavelength).
• When the ambulance moves away from you, its siren sounds lower pitched (lower frequency, longer wavelength).

Why does this happen?