In Young’s double-slit experiment setup with light of wavelength λ = 6000 Å, distance…

In Young’s double-slit experiment setup with light of wavelength λ = 6000 Å, distance between the two slits is          2 mm and distance between the plane of slits and the screen in 2 m. The slits are of equal intensity. When a sheet of glass of refractive index 1.5 (which permits only a fraction of the incident light to pass through) and thickness 8000 Å is placed in front of the lower slit, it is observed that the intensity at a point P, 0.15 mm above the central maxima, does not change.    
 
 Intensity at point P is