In a large room , a person receives direct sound waves from a source `120 m` away . He also receives waves from the same source which reach him after being reflected from the ` 5m` high ceiling at a point halfway between them . For which wavelengths will these two sound waves interfere constructively ?

Options

A. `10, 10//2, 10//3, 10//4`
B. `20, 20//3, 20//5, 20//7`………
C. `30, 20, 10`,……..
D. `10, 10//3, 10//5, 10//7`…….

Answer

A

Explanation

Path difference `(DeltaS)` between direct and reflected wave `= 130 - 120 = 10 m`
`lambda = (DeltaS)/(n) = (10)/(n) , (n = 1, 2, 3"……..")`
`lambda = 10, (10)/(2), (10)/(3), (10)/(4)`…..