Question

What is meant by the critical angle for total internal reflection? please write a definition supported by reasoning

Answer 1

critical angle

see here  

The critical angle is the angle of incidence where the angle of refraction is 90°°. The light must travel from an optically more dense medium to an optically less dense medium.

If the angle of incidence is bigger than this critical angle, the refracted ray will not emerge from the medium, but will be reflected back into the medium. This is called total internal reflection.

if incidence is bigger than this critical angle,

this is total internal reflection used in optical fibers to send data