question_answer

A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of \[\text{15m}{{\text{s}}^{-1}}\]. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is:                                                                                                                                                                                 (Take velocity of sound in air\[=330\text{m}{{\text{s}}^{-1}}\])

A)  765 Hz                 

B)  800 Hz

C)  838 Hz                 

D)   885 Hz

Correct Answer: C

Solution :

  \[n'=\frac{v}{v-{{v}_{s}}}{{n}_{0}}\] \[n'=\frac{330}{330-15}(800)=\frac{330\times 800}{315}=383Hz\]