Physics HL · Chapter 16: The Doppler Effect
16.1 The Doppler Effect at Low Speeds
Use wavefront diagrams to explain observed-frequency shifts for moving observers and moving sources, then separate frequency shift from intensity change.
Estimated time: 36 minutes
Stationary Source, Moving Observer
If the source is stationary in still air, emitted wavefronts are equally spaced. A moving observer who runs toward the source meets those fronts more often, so the observed frequency rises. If the observer moves away, fronts are met less often and the observed frequency falls.
The key point is that observer motion in this case does not change spacing between wavefronts in the medium. The wavelength in air remains the source value. What changes is encounter rate, which is why the measured frequency shifts while wavefront spacing in the air does not.
With the chapter sign convention (positive to the right), negative observer velocity means moving toward a source located to the left, which increases f'.
Moving Source, Stationary Observer
If the source moves, each new wavefront is emitted from a new position. In front of an approaching source, wavefront centers shift toward the observer and spacing is compressed. Behind a receding source, spacing is stretched. This spatial bunching and spreading is the geometry behind the classic siren pitch jump.
lambda_{ ext{front}}= rac{v-u_s}{f},qquad lambda_{ ext{back}}= rac{v+u_s}{f}
Approaching-side wavefront spacing shrinks, receding-side spacing grows.
Because frequency is speed divided by wavelength, smaller front-side wavelength means larger observed frequency for an observer ahead of the source. Larger back-side wavelength means smaller observed frequency for an observer behind the source.
Frequency Shift Is Not Loudness Change
A common exam trap is to confuse pitch shift with loudness. If a source approaches at constant speed, observed frequency can remain essentially constant during the approach interval, while intensity still rises because distance is shrinking. Frequency and intensity are linked to different physical mechanisms.
Important
Before choosing equations, decide whether the source moves in the medium, the observer moves through wavefronts, or both. That classification determines the correct Doppler form.
Simulation: Wavefront Doppler Studio
Control source and observer velocities, inspect wavefront bunching/stretching, and compare geometry with computed observed frequency.
Doppler Effect Lab
Observed frequency
767.74 Hz
Wavefront spacing near observer
0.456 m
Source-frame wavelength
0.500 m
Wave speed relative to observer
350.00 m/s
Wavefront geometry (positive velocity points to the right)
Test Yourself
A source and observer both move to the right at the same speed in still air, keeping their separation fixed. What happens to observed frequency compared with emitted frequency?
Hint: Check relative motion along the source-observer line, not absolute motion in the lab.