For an optical fibre ,the ratio of the refractive index of the core to the refractive index of the cladding is 1.25 . What is the critical angle at the core cladding interface ?

Options

A. `51^(@) 5'`
B. `52^(@) 12'`
C. `53^(@) 8'`
D. `54^(@) 5'`

Answer

C

Explanation

In an optical fibre , the R.I. of the core `n_(1) gt R.I`. Of the cladding `(n_(2))` and the ray suffers a large number of total internal reflections at the interface of core and cladding `(n_(1))/(n_(2))= 1.25 = (5)/(4) therefore (n_(2))/(n_(1)) = (4)/(5) = 0.8` and `(n_(2))/(n_(1)) = sin C ` where C is the critical angle .
`therefore sin C = 0.8 therefore C = 53^(@) 8'`