A luminous point object S is placed in front of a screen at a distance a = 225 cm from the screen.

A convex lens of focal length f = 20 cm is broken along a diameter into two equal parts. These two parts are placed between source and screen at a distance b = 30 cm from the source. One of these parts is displaced by d1 = 0.25 mm from line SC which is normal to the screen and the other part is displaced by d2 = 0.50 mm as shown in figure. The gap between two parts of the lens is filled by an opaque material. Calculate refractive index µ of a transparent sheet of thickness t = 1.25 µm to be placed in path of rays emerging from one of the parts so that a white spot is formed at point C.