The Doppler Effect

This lesson covers the Doppler effect for both sound and light, and its importance in astronomy, as required by the AQA GCSE Physics specification (4.8.2). This is a Physics-only topic. You need to understand what the Doppler effect is, how it applies to sound waves and electromagnetic waves, and how it provides evidence for the expansion of the universe through red-shift.

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What Is the Doppler Effect?

The Doppler effect is the change in the observed frequency (and wavelength) of a wave when the source of the wave is moving relative to the observer. It was first described by the Austrian physicist Christian Doppler in 1842.

The Doppler effect applies to all types of waves, including sound waves, light waves, and other electromagnetic waves.

Key Principle

• When a wave source moves towards an observer, the observed wavelength is shorter (compressed) and the observed frequency is higher.
• When a wave source moves away from an observer, the observed wavelength is longer (stretched) and the observed frequency is lower.
• The wave source itself does not change — it is only the observed frequency and wavelength that change due to the relative motion.