The observed change in wavelength due to the Doppler effect occurs
A) whenever the light source is moving with respect to the observer (regardless of direction).
B) only when the light source has a radial velocity (toward or away from the observer).
C) only when the temperature of an object changes.
D) only when the light source has a proper motion (across the line of sight).

Question

Quizplus · Accepted Answer

The Doppler effect occurs when there is a relative radial motion between the light source and the observer, meaning the light source is moving either toward or away from the observer. It does not depend on the direction of the motion, only its radial nature. Temperature and proper motion do not have an effect on the Doppler effect.

The Observed Change in Wavelength Due to the Doppler Effect