A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of `15ms^-1`. Then the frequency of sound that the observer hears in the echo reflected from the cliff is (Take velocity of sound in air `=330ms^-1`)

Options

A. 800 Hz
B. 838 Hz
C. 885 Hz
D. 765 Hz

Answer

B

Explanation

v = 800 Hz
The frequency reflected by cliff
`v' = ((upsilon)/(upsilon - upsilon_(S)))xx 800`
`= ((330)/(330 - 15))800`
= 838.095 Hz
Cliff will be behaving as source, the frequency is heard by observer after echo from cliff. Both cliff and observer are at rest.
`therefore" "upsilon_(L) = 0, upsilon_(s) = 0`
`v'' = ((upsilon - upsilon_(L))/(upsilon - upsilon_(s)))v'`
`= ((upsilon - 0)/(upsilon - 0)) 838.095 rArr 838.095` Hz