What is the Doppler effect? Write its example and explain how its analysis can be performed.

(N/A) The Doppler effect is the phenomenon where the observed frequency of a wave changes due to the relative motion between the source of the wave and the observer.
Example: The pitch (or frequency) of the whistle of a fast-moving train appears to decrease as it recedes away from an observer. Conversely,when an observer approaches a stationary source of sound at high speed,the pitch of the sound heard appears to be higher than the actual frequency of the source.
Analysis: The Doppler effect is a wave phenomenon applicable to both sound and electromagnetic waves. We analyze the frequency changes under three primary conditions:
$(1)$ The observer is stationary,but the source is moving.
$(2)$ The observer is moving,but the source is stationary.
$(3)$ Both the observer and the source are moving relative to the medium.