A sound wave of wavelength ` lambda` travels towards the right horizontally with a velocity `V`. It strikes and reflects from a vertical plane surface , travelling at a speed `v` towards the left . The number of positive crests striking in a time interval of `3 s` on the wall is

Options

A. `3(V+v)//lambda`
B. `3(V-v)//lambda`
C. `(V+v)//3lambda`
D. `(V-v)//3lambda`

Explanation

The relative velocity of sound waves with respect to the walls is V + v. Hence the apparent frequency of the waves striking the surface of the wall is `(V+v)//lambda` . The number of positive crests striking per second is the same as frequency. In three seconds the number is `[3 (V+v)]//lambda`