What is the Doppler effect? Explain it in words for a moving source of sound, and give three of its applications.

1. 1 · Define. The apparent change in frequency heard when the source, the listener or both move relative to the medium.
2. 2 · Explain for an approaching source. The source moves forward after sending each wave, so the waves in front are crowded: a shorter wavelength reaches the listener, so a higher frequency is heard.
3. 3 · Explain for a receding source. Behind the source the waves are spread out: a longer wavelength and a lower frequency.
4. 4 · Draw the waves. [Source S moving to the right; circles crowded on the right and spread out on the left; a listener on each side marked higher pitch and lower pitch]
5. 5 · Give the formula. ν′ = ν(v − v₀)/(v − vₛ), with speeds positive from source to listener.
6. 6 · Give three applications.
   • Radar: the speed of aircraft and vehicles
   • Sonar: the speed of submarines
   • Red shift: distant stars are moving away
7. 7 · Real life. An ambulance siren drops in pitch as it passes you.

Scoring tip: Draw the crowded and spread-out wavefronts; one clear diagram explains the whole effect.