In a Hali, a person recives direct sound waves from a source `120m` away. He also receives wave from the same source which reach him after being reflected from the `25m` high ceilling at a point half way between them. The two waves interface construtively for wave length (in meters).

Explanation

As shown in figure, for reflection from the ceiling :

Path SCP = SC + CP =2SC [as `angle I =angler, SC = CP`] or path `SCP = 2sqrt(60^2 +25^2) = 130m`
So path difference between interfering waves along paths SCP and SP,
`Deltax = 130 - 120 = 10 m`
Now for constructive intergerence at P, `Deltax = nlambda`,
`i.e., 10 = n lambda " or " lambda=10/n` with n =1,2,3,.......
`i.e., lambda =10m , 5m , (10//3)m` and so on.