Question

In a large room, a person receives direct sound waves from a source 120 metres away from him. He also receives waves from the same source which reach him, being reflected from the 25 metre high ceiling at a point halfway between them. The two waves interfere constructively for wavelength of

Answer 1

The path difference should be $n\lambda$
$\Rightarrow OA+AB-OB=n\lambda \Rightarrow 2OA-OB=n\lambda \Rightarrow 2{\sqrt{60}}^2+{25}^2)-120=n\lambda \Rightarrow 2\times 65-120=n\lambda \Rightarrow n\lambda =10$
So, the values are $10,20,\mathrm{30...}$.