When the light in a denser medium strikes an interface separating the denser medium and rarer medium at an angle of incidence greater than the critical angle ,total light is reflected without any refraction through the rarer medium .This phenomenon is called "Total Internal Reflection " OR TIR.
when the angle of incident from the denser medium to a rarer medium is greater than the critical angle of that denser medium, the ray of light totally reflects back without any refraction in the rarer medium. This phenomenon is known as total internal refraction.
Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. The critical angle is the angle of incidence above which the total internal reflection occurs. This is particularly common as an optical phenomenon, where light waves are involved, but it occurs with many types of waves, such as electromagnetic waves in general or sound waves.