Question

The change in pitch of a train's horn as it passes while you are standing still can be explained by

Answer 1

That change in pitch can be described by the math
of the Doppler effect. But that doesn't 'explain' it.

Answer 2

The correct answer is:

Doppler effect

Step-by-step explanation:

The Doppler effect is the variation in wavelength or wavelength of a wave in relative to an observer who is moving to correspond to the wave source. It is described after the Austrian physicist Christian Doppler, who explained the phenomenon in 1842.