Question

A large glass slab (μ = 5/3) of thickness 8cm 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? 

(a) 6 cm 

(b) 7 cm 

(c) 8 cm 

(d) 9 cm 

Answer 1

Correct Option :-  (a) 6 cm 

Explanation :-

 (R/t) = tan θc              or                     R = t (tan θc)

But      sin θ_c = 1/μ = 3/5 

tan θc = ¾      so R = ¾ t = ¾ (8cm) = 6 cm 

Hence the answer is 6.