Whenever there is a relative motion between a source of light and the observer, the apparent frequency of light received by the observer is different from the true frequency of light emitted actually from the source of light.
If a source of light emits waves of frequency f and of wavelength l, then the velocity of light would be c = lf.
When the source and the observer approach each other with a velocity v along the direction of propagation of light, then in one second, the two come closer by a distance v.
\ Apparent frequency = number of light waves received per second by the observer.
f|= number of light waves emitted/sec by source + number of light waves contained in a distance v
Clearly f|>f
When the source and the observer move away from each other, then,
When source and the observer approach each other Df is +ve and the apparent frequency fl increases or apparent wavelength decreases. This is 'Blue Shift'.
When the source and the observer recede away from each other, Df is negative and the apparent wavelength increases. This is called the Red shift.
