Question

which of these would be best explained by the Doppler effect? A) the change in frequency as you shorten a guitar string. B) the change in pitch heard as an ambulance approaches you. C) the change in volume as a train pulls away from the station. D) the change in seed of a sound wave as it travels from air into water.

Answer 1

The correct answer is B) the change in pitch heard as the ambulance approaches.
In fact, doppler effect occurs when the source of a wave is moving with respect to the observer. In this case, the source is the ambulance and the wave is a sound wave. The result is that there is a shift of the frequency (and therefore of the pitch) as seen by the observer, with respect to the original frequency of the source.