Hi there,

When light travels from denser to rarer medium angle r (angle of refraction) is always greater than angle I (angle of incidence). For a particular angle of incidence ,angle r in right angle is 90degree. The angle cc is called critical angle. The angle of denser medium w.r.t rarer.

When angle of incidence is greater than the critical angle after light travels from denser to rarer medium the ray of light use to reflect in the same medium. This phenomenon is called total internal reflection.

Hope it helps!