A source emitting frequency f moves directly towards a large stationary wall with speed u. The source hears its own sound after reflection from the wall. If the speed of sound is v, which expression gives the echo frequency heard by the moving source?
Options

Question

A source emitting frequency f moves directly towards a large stationary wall with speed u. The source hears its own sound after reflection from the wall. If the speed of sound is v, which expression gives the echo frequency heard by the moving source?

Options

G1 Physics · Accepted Answer

First, the stationary wall receives sound from a source moving towards it. Hence the wall receives frequency f₁ = f×v/(v−u). The wall is stationary, so the reflected wave has the same frequency f₁ in the air. The original source now acts as an observer moving towards the reflected wave. Thus, fₑ = f₁(v+u)/v. Substitute f₁: fₑ = [f×v/(v−u)](v+u)/v = f×(v+u)/(v−u). Correct option: B.