Question

what is critical angle?

Answer 1

Hey!!!
Critical angle means the angle of incidence beyonds which  rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected.
I hope it helps you.....

Answer 2

When the angle of incidence in water reaches a certain critical value, the refracted ray lies along the boundary, having an angle of refraction of 90-degrees. This angle of incidence is known as the critical angle; it is the largest angle of incidence for which refraction can still occur.