What is a total internal reflection?

When a ray of light enters in a rarer medium through a denser medium, it bends away from the normal.

When the angle of incidence in the denser medium is such that the angle of refraction n the denser medium is 90 degree, such angle of incidence is called critical angle.

When a ray of light enters in a rarer medium by denser medium at an angle of incidence more than the critical angle then after refraction the ray comes back into the same denser medium. this phenomenon is called total internal refraction.

there are two necessary conditions for the total internal refraction:

(1) the angle of incidence in the denser medium should be more than the critical angle.

(ii) the refarction takes place from denser medium to rarer medium.