A point source S emitting light of wavelength 600 nm is placed at a very small height h above a flat reflecting surface AB [Fig.6.07].The intensity of the reflected light is 36% of the incident intensity.Interfrerence fringes are observed on a screen placed parallel to the reflecting surface at a very large distance D from it.

If the intensity t pint P corresponds to a maximum,calculate the minimum distance through which the reflecting surface AB sould be shifted so that the intensity at P again becomes maximum.