Total internal reflection occurs when a ray of light travels from a denser medium to a less dense medium (e.g. glass to air) and the angle of incident in the dense medium just exceeds the critical angle of the dense medium.
Phenomenon / Application of total internal reflection. Total internal reflection occurs when
(i) The ray of light passes from a denser to less dense medium and
(ii) The angle of incidence in the denser medium is greater than the critical angle.