Question

What is critical angle? Is it 90 degree? What happens when angle is more than critica angle?

Answer 1

The critical angle is not equal to 90 degrees.
Total internal reflection (TIR)is the phenomenon that involves the reflection of all the incident light of the boundary. TIR only takes place when both of the following two conditions are met:
1. a light ray is in the more dense medium and approaching the less dense medium.
2. the angle of incidence for the light ray is greater than the so-called critical angle.
For any angle of incidence greater than the critical angle, the light will undergo total internal reflection.

So the critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees. Make particular note that the critical angle is an angle of incidence value. For the water-air boundary, the critical angle is 48.6-degrees. For the crown glass-water boundary, the critical angle is 61.0-degrees. The actual value of the critical angle is dependent upon the combination of materials present on each side of the boundary.
When the angle of incidence is greater than critical angle than total internal reflection will occur and no refraction will occur as the whole of the light will be reflected back to the same medium.