when light enters from denser medium to rarer medium, it bends away from the normal.
more the angle of incidence more will be the bending away.
so for certain angle of incidence, the angle of refraction will be 900 . so the refracted ray will pass through the surface separating the two medium.this angle of incidence is called critical angle.
if the angle of incidense is ./critical angle, then the light ray does not refract at all. it will reflect back.this phenomena is called total internal reflection. here no reflection take place.
condition for tot int refl:
1. light shd pass from denser to rarer medium.
2. anglle of incidence should be > critical angle