The intensity received at the focus of the lens is I when no glass slab has been placed in front of the slit. Both the slits are of the same dimension and the plane wavefront incident perpendicularly on them, has wavelength . On placing the glass slab, the intensity reduces to 3 I / 4 at the focus. Find out the minimum thickness of the glass slab (in Å ) if its refractive index is 3 / 2 . Given =6933 Å , =1.5 .