Question

What is doppler effect? Derive the formula for the change in frequency

(i) when the source is approaching and receding from the observer and
(ii) when the source is stationary and observer is moving towards and away from the source.

Answer 1

doppler effect is an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other. The effect causes the sudden change in pitch noticeable in a passing siren, as well as the red shift seen by astronomers.
                          λO = λS(c − vS)/(c − vO)