When light goes from denser medium to rarer medium then on increasing the angle of incidence angle of refraction also increasesand then at a certain value of angle of incidence refracted ray slides on the the surface that is now the angle of refraction is 90 degrees this angle of incidence is the critical angle

"when light goes from denser medium to rarer medium then the angle of incidence at which the angle of refraction is 90 degrees ,is called thecritical angle."

The critical angle

when a ray of light move from denser to rearer medium, then at a certain angle of incidence, refracted ray become perpendicular to the normal i.e. angle of refraction become 90^·. that angle of incidence is known as critical angle.