Doppler effect
Summary
The Doppler effect is the change in observed wave frequency when a source and an observer move relative to each other. The observed frequency is higher when they approach and lower when they separate, while the wave speed in the medium stays the same.
Detail
Meaning: A moving source compresses the wavelength ahead of it and stretches it behind it. A moving detector meets wavefronts more or less often while the wavelength stays the same. Either way, the observed frequency rises when the two approach and falls when they separate (higher or lower pitch for sound). The medium sets the wave speed, so that speed does not change.
Equation: f' = f (v ± vD)/(v ∓ vS), where f' is the observed frequency (Hz), f is the source frequency (Hz), v is the wave speed in the medium (m/s), vD is the detector speed (m/s) and vS is the source speed (m/s). Use the upper signs when moving toward each other and the lower signs when moving apart.
On the MCAT: Typical setups are a passing siren, a moving reflector in ultrasound or radar, or a receding galaxy (redshift for light). You usually predict whether the observed frequency rises or falls, or compute f' from given speeds.
Common confusion: The source's emitted frequency and the wave speed do not change; the observed frequency is what shifts. To check signs, ask whether the separation is shrinking (f' higher) or growing (f' lower).
Sources
- Physics
- Fundamentals of Physics
- College Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.