In figure S is a sound source and O an observer at a horizontal distance D, the direct wave from S and the wave reflected from point A, at a horizontal level at altitude H are in same phase. When the layer rises a distance h and the wave is reflected from point B, no signal is detected at O. Given that the incident and the reflected rays make the same angle with the reflecting layer. Find an expression for the wavelength λ of the waves in terms of D, H and h.
