Compute all parts of following problem:

Problem: When a light ray passes from a material of higher n to a material of lower n (for instance, a light ray passing from glass to air), the refracted angle is larger than the incident angle. In this case, the angle of the incident ray can exceed a critical angle, at which the angle of the refracted ray is 90o. For incident angles greater than the critical angle, there is no refracted ray, only a reflected ray. This phenomenon is called total internal reflection.

Part A: What is the critical angle for water with n = 1.33?

Part B: What is the critical angle for glass n = 1.50?

I know the correct answer for above problem but explain how to get this answer! Thanks.