Ans. Doppler Effect in Light: According to Doppler Effect, whenever there is a relative motion between a source of light andobserver, the apparent frequency of light received by the observer is different from the true frequency of light emittedactually from the source of light.
The apparent frequency of light increases when the distance between source of light and observer is decreasing and the apparent frequency of light decreases if the distance between source of light and observer is increasing.
Blue shift: When source and observer approach each other, △ υ is +ve, i.e., apparent frequency increases or apparent wavelength decreases. This is called "Blue shift".
Red shift: When source and observer recede away from each other, △ υ is +ve, i.e., apparent frequency increases or apparent
decreases or apparent wavelength increases. This is called "Red shift‟.