The angle of incidence for an incident ray in an optically denser medium for which the angle of refraction is 90 degrees is called critical angle.
BEFORE talking about the critical angle, let's understand total internal reflection.
CASE 1. A ray of light originates from water at an angle
CASE 2. A ray originates at an angle
CASE 3. When the angle of incidence increases to such a value that it is greater than the critical angle, the light reflects into the same denser medium. Hence reflection with refraction occurs.
For water, the critical angle is
Remember that for total internal reflection, the angle of incidence should be greater than the critical angle.