In Young's experiment, the upper slit is covered by a thin glass plate of refractive index 1.4 while the lower slit is covered by another glass plate, having the same thickness as the first one but having refractive index 1.7. Interference pattern is observed using light of wavelength 5400A˚ . It is found that the point P on the screen where the central maximum ( n=0 ) falls before the glass plates were inserted now has 3/4 the original intensity. It is further observed that what used to be the fifth maximum earlier, lies below the point P while the sixth minimum lies above P . Calculate the thickness of the glass plate. (Absorption of light by glass plate may be neglected.)