The core of an optical fiber has an index of refraction of 1.35 , while the index of refraction of the cladding surrounding the core is 1.21 . What is the critical angle θc for total internal reflection at the core‑cladding interface?

The  critical angle is  

Explanation:

From the question we are told that

   The refractive index of the core is  

   The refractive index of the cladding  is

Generally according to Snell's law

Here for total internal reflection the refractive angle is   and  the critical angle is equal to the critical angle so  

substituting values