Question

What is the maximum angle a light ray can make with the wall of the core if it is to remain inside the fiber?

Answer 1

Critical angle=
Arcsin(sqrt(eps2/eps1))
Where eps1 is relative permittivity of the fiber and eps2 is for the medium just outside the fiber (coating)
Note for this angle to exist
eps1>eps2 since the sin function is bound between-1 and 1. Thus the fiber is more optically dense than the jacket.