Q1. Light from a laser is directed at a hemi-spherical glass. The light passes undeviated through the block and on to a screen, forming a spot as shown. The semi-circular block is rotated about an axis perpendicular to plane of paper through point P. The spot of light on the screen is seen to move downwards. When the spot reaches point B, it disappears. In a particular experiment, the distance PA is 120 cm and distance AB is 90 cm. Calculate the refractive index of the glass of the block.