In Young's double slit experiment set-up 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 is 2 m. The slits are of equal intensity. When a sheet of glass of refractive index 1.5 (which permits only a fraction η of the incident light to pass through) and thickness 8000 Å is placed in front of the lower slit, it is observed that intensity at a point, P, 0.15 mm above the central maxima does not change. Find the value of η .