A sound wave of period T from the transducer in a Doppler ultrasonography diagnostic reflects off
the blood flowing in an artery. How does the period of the reflected wave compare to that of the original
wave, if the blood flows primarily away from the transducer?
(a) The two periods are exactly the same.
(b) The received period is larger than the original period T .
(c) The received period is smaller than the original period T.
answer is (b) The received period is larger than the original period T. but I dont understand why!