Question

what is s a critical angle

Answer 1

The angle of incidence in which rays of light pass through a denser medium to the surface of a less dense medium aren't refracted anymore but completely reflected.

Answer 2

..CRITICAL ANGLE -WHEN A RAY OF LIGHT PASSES FROM A MEDIUM OF HIGHER OPTICAL DENSITY TO A MEDIUM OF LOWER OPTICAL DENSITY ,THE ANGLE OF INCIDENCE AT WHICH THE ANGLE OF REFRACTION BECOMES 90° IS THE CRITICAL ANGLE.