6. Give an account of the relativistic theory of Doppler effect. 7. Compare and contrast Doppler effect observed in light and sound. 8. What is Doppler effect? Show that for a source receding along the line of sight of an observer, the Doppler shift is given by f = for(1 B) where fo is the natural frequency and f is the observed frequency. Prove also that a source observed at right angles to the motion shows a shift of f=forfo (1- B/2). 9. The phase of a light wave must be invariant under L.T. Use this fact to derive an expression for the Doppler shift in frequency of radiation emitted from a source moving with respect to an observer.