A source emits an electromagnetic wave of frequency ????s towards a receiver, which has a relative receding

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Question:

A source emits an electromagnetic wave of frequency ????s towards a receiver, which has a relative receding velocity ???? with respect to the source. According to the Doppler formula for electromagnetic waves, the received frequency is ????r = √ ???? − ???? ???? + ???? ????s , where ???? is the speed of light. (a) Taylor-expand the Doppler formula for the case ???? ≪ ????. Express the answer up to the lowest non-trivial term of ????/????.