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

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.