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Question:

A large glass slab (μ=5/3) of thickness 8 cm is placed over a point source of light on a plane surface. It is seen that light emerges out of the top surface of the slab from a circular area of radius R cm. What is the value of R?

Solution:

In case of total internal refraction,
μ = sin θc / sin θc = 3/5
tan θc = 3/4 = R/8
∴ R = 6 cm.