What is doppler effect ? Write its example and how its analysis can be done ?

Explanation

It is an everyday experience that the pitch (or frequency) of the whistle of a fast moving train decreases as it recedes away. When we approach a stationary source of sound with high speed, the pitch of the sound heard appears to be higher than that of the source. As the observer recedes away from the source, the observed pitch (or frequency) becomes lower than that of the source. This motion-related frequency change is called Doppler effect.
The Austrian physicist Johann Christian Doppler first proposed the effect in 1842.
Buys Ballot in Holland tested it experimentally in 1845.
Doppler effect is a wave phenomenon, it holds not only for sound waves but also for electromagnetic waves. However, here we shall consider only sound waves.
We shall analyses changes in frequency under three different situations : (1) Observer is stationary but the source is moving (2) Observer is moving but the source is stationary and (3) both the observer and the source are moving.