A source of frequency f moves through still air with speed uₛ

Question

A source of frequency f moves through still air with speed uₛ < v, where v is the speed of sound. Which pair correctly gives the wavelengths immediately in front of and behind the moving source?

G1 Physics · Accepted Answer

In one period 1/f, a wavefront travels distance v/f. The moving source travels distance uₛ/f. Ahead of the source, the source moves towards the previous wavefront: λ₁ = v/f−uₛ/f = (v−uₛ)/f. Behind the source, the spacing increases: λ₂ = v/f+uₛ/f = (v+uₛ)/f. Correct option: A.