What wave propagation principle accounts for the apparent increase in frequency as a train whistle approaches and the apparent decrease in frequency as it moves away?

The Doppler effect is the wave propagation principle that describes the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. When a train approaches, the sound waves in front of it are compressed, leading to a higher frequency or pitch of the whistle as it gets closer. Conversely, as the train moves away, the sound waves are stretched, resulting in a lower frequency or pitch. This phenomenon can be observed with all types of waves, including sound, light, and electromagnetic waves, making it a fundamental concept in wave behavior.

Wave reflection pertains to the bouncing back of waves when they hit a barrier, which is not the case in this situation where movement is involved. Refraction involves the bending of waves as they pass through different media, which does not directly relate to the change in frequency based on motion. Interference describes how different waves can combine and affect amplitude but does not capture the frequency shift described in the question. Thus, the Doppler effect is the correct answer as it directly relates to the observed changes in frequency based on the relative motion of the source and the observer.