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 phenomenon that explains 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 the train are compressed, leading to a higher frequency, which is perceived as a higher pitch. Conversely, as the train moves away, the sound waves are stretched, resulting in a lower frequency and a lower pitch. This effect is observable in sound waves, as well as in light waves, and has practical implications in various fields, including radar and astronomy.

The other principles mentioned have different applications and do not describe the change in frequency based on relative motion. Wave reflection involves waves bouncing off surfaces, refraction pertains to the bending of waves as they pass through different mediums, and interference refers to the interaction of waves resulting in constructive or destructive patterns. These concepts do not account for the frequency change as an object moves toward or away from an observer.