A point source is 15 cm below the plane surface of a liquid of refractive index 1.667. What is the radius of the miniμm opaque disc placed concentrically on the surface that blocks all directly emerging rays?
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Question

A point source is 15 cm below the plane surface of a liquid of refractive index 1.667. What is the radius of the miniμm opaque disc placed concentrically on the surface that blocks all directly emerging rays?

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G1 Physics · Accepted Answer

The limiting ray is incident at the critical angle C, where sinC=1/μ. Hence disc radius=h tanC≈11.25 cm.