The Doppler effect
When an ambulance races towards you its siren sounds high, and the instant it passes the pitch drops — yet the siren never changed. This lesson explains that everyday mystery from the wavefronts up. We picture how a moving source bunches its wavefronts in front (shorter wavelength, higher frequency) and stretches them behind (longer wavelength, lower frequency), then meet the AS-level equation fₒ = fₛv/(v ± vₛ). We work through the sign convention carefully — minus for an approaching source, plus for a receding one — and do full calculations for both cases, including finding a source's speed from the shift. Finally we see that the Doppler effect works for all waves, so light from a receding galaxy is red-shifted, the key evidence that the universe is expanding.
By the end you should be able to (NSSCAS Physics (AS) 2.4):
- Explain that when a source of waves moves relative to a stationary observer there is a change in the observed frequency
- Explain, using the bunching and spreading of wavefronts, why an approaching source is heard at a higher frequency and a receding source at a lower frequency
- Use the expression fₒ = fₛv ÷ (v ± vₛ) for the observed frequency of a moving sound source, choosing the correct sign
- Calculate the observed frequency for both approaching and receding sources
- Determine the speed of a source from the observed frequency, source frequency and wave speed
- Explain that the Doppler shift is observed for all waves, including light (red shift), and give everyday examples