What is the Doppler effect? Explain the situations in which it occurs and give its applications.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define. The Doppler effect is the apparent change in the frequency of sound due to relative motion between the source and the listener.
2 · Give the everyday example. A train horn sounds higher as the train approaches and lower as it moves away.
3 · Write the situations with formulas.
Source towards listener: n′ = [v/(v − vS)] × n (higher).
Source away: n′ = [v/(v + vS)] × n (lower).
Listener towards source: n′ = [(v + vL)/v] × n (higher).
Listener away: n′ = [(v − vL)/v] × n (lower).
Both towards each other: n′ = [(v + vL)/(v − vS)] × n.
4 · Draw. [Diagram: a moving source with circular wavefronts crowded in front and spread out behind; listener A in front hears higher, listener B behind hears lower]
5 · State when it is not observed. Both at rest, same velocity in the same direction, perpendicular motion, or only the medium moves.
6 · Applications.
Speed guns and RADAR.
SONAR for submarines.
Satellite tracking.
Weather radar and blood-flow scans.
7 · Real-life link. The Doppler weather radar at Chennai tracks cyclones; ambulance sirens change pitch as they pass.
8 · Close. Thus relative motion, not any change in the source, changes the pitch we hear.
Scoring tip: Say whether the frequency heard is higher or lower in each case; examiners check this as well as the formula.
