In a large room, a person receives direct sound waves froma 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 constuctively for wavelength of

Explanation

As shown in figure for reflection from the ceiling
Path SCP=SC+CP=2SC
`[" As " /_i=/_r, SC= CP]`
or Path `SCP= 2sqrt(60^(2)+25^(2))= 130` cm
So path difference between interfering waves along path SCP, and SP,
`trianglex= 130 - 120 = 10` m

Now for constuctive interfernece at P.
`trianglex= n lambda. i.e., 10 = n lambda`
or `lambda= (10)/(n)" with " n= 1,2,3..........`
i.e., `lambda = 10 m, 5 m, (10/3) m` and so on.