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A jar of height h is filled with a transparent liquid of refractive index µ. At the centre of the jar on the bottom surface is a dot. Find the minimum diameter of a disc, such that when placed on the top surface symmetrically about the centre, the dot is invisible.

tan ic d/2/h

ic = d/2h

d = 2h tan ic

d = 2h ×1/√μ2 – 1