Question

Speed of sound wave is v. If a reflector moves towards a stationary source emitting waves of frequency f with speed u, the wavelength of reflected waves will be
A. `(v-u)/(v+u)f`
B. `(v+u)/(v)f`
C. `(v+u)/(v-u)f`
D. `(v-u)/(v)f`

Answer 1

Correct Answer - C
Apparent frequency for reflector (which will act here as an observer) would be `f_1=((v+u)/(v))f`
Where f is the actual frequency of source. The reflector will now behave as a source. The apparent frequency will now become
`f_2=((v)/(v-u))f_1`
Substituting the value of `f_1` we get
`f_2=((v+u)/(v-u))f`