question_answer 15)

For sound waves, the Doppler formula forfrequency shift differs slightly between thetwo situations :(i) source at rest; observermoving, and (ii) source moving; observer atrest. The exact Doppler formulae for the caseof light waves in vacuum are, however, strictlyidentical for these situations. Explain why thisshould be so. Would you expect the formulaeto be strictly identical for the two situationsin case of light travelling in a medium ? 

Answer:

Sound requires material medium forpropagation. Light when passing throughmaterial medium is also governed bydifferent Doppler formulae for (i) source at rest; observer moving and (ii) sourcemoving ; observer at rest. But when light passes through vacuumthe formulae become exactly same for thetwo different situations because speed oflight and frequency/wavelength of lightremain unchanged in vacuum.