The Doppler Effect
- Usually, when an object emits waves, the wavefronts spread out symmetrically
- If the wave source moves, the waves can become squashed together or stretched out
- Therefore, when a wave source moves relative to an observer there will be a change in the observed frequency and wavelength
Wavefronts are even in a stationary object but are squashed in the direction of the moving wave source
- A moving object will cause the wavelength, λ, (and frequency) of the waves to change:
- The wavelength of the waves in front of the source decreases (λ – Δλ) and the frequency increases
- The wavelength behind the source increases (λ + Δλ) and the frequency decreases
- This effect is known as the Doppler effect
- Note: Δλ means 'change in wavelength'
Examiner Tip
Although you will not be expected to do any calculations with the Doppler effect, it is important you remember the relationship between wavelength and frequency (wavelength increases, frequency decreases and vice versa)
