Doppler effect
Also called Doppler shift
The Doppler effect is the change in the frequency an observer measures when a wave source and the observer move relative to each other. Approaching raises the observed frequency, receding lowers it.
AP Physics 2 EK 14.5.A.1: the Doppler effect describes the relationship between the rest frequency of a wave source, the observed frequency of the source, and the relative velocity of the source and the observer.
Direction of the shift, which is what gets asked.
| Relative motion | Observed frequency |
|---|---|
| Source and observer at the same velocity (14.5.A.2.i) | Equal to the rest frequency |
| Source moving toward the observer (14.5.A.2.ii) | Greater than the rest frequency |
| Source moving away from the observer (14.5.A.2.iii) | Less than the rest frequency |
EK 14.5.A.2 adds the size rule: a greater relative velocity results in a greater measured difference between the observed and rest frequencies.
No formula, on purpose. The Topic 14.5 boundary statement reads: "Only qualitative treatments of the Doppler effect are required for AP Physics 2." The AP Physics 2 equation sheet prints no Doppler equation either. A shifted-frequency formula offered for this course is imported from a treatment the CED does not ask for.
It is relative motion that counts. A source and observer traveling together at 300 m/s measure no shift at all. And nothing about the wave itself changes: the source still emits at its rest frequency, and the medium still carries the wave at its own speed. Only the rate at which wavefronts reach the observer changes.