What is the Doppler effect?

• A. An observed change in the frequency due to relative motion between source and observer
• B. A change in the wave's speed in the medium
• C. A change in the amplitude due to distance
• D. The wavelength remains unchanged by motion

Answer: (A)

The Doppler effect is a change in the frequency of a wave that the observer detects because the source and observer are moving relative to each other along the line of sight. When they move toward one another, waves get squeezed together and the observer records a higher frequency (a higher pitch for sound); when they move apart, the waves spread out and the frequency drops. The wave’s speed in its medium stays the same for a given medium; what changes is how often the wave crests arrive, which is the frequency, and that in turn affects the wavelength since v = fλ. A helpful way to see it is that motion changes the number of crests hitting the observer per unit time, not the intrinsic speed of the wave itself.

The other descriptions don’t capture this effect: the Doppler effect isn’t about a change in the wave’s speed in the medium, nor about the amplitude changing with distance, and it isn’t saying the wavelength stays the same under motion.