Question

What is meant by critical angle and total internal reflection?

Answer 1

If the angle of incidence is bigger than this critical angle, the refracted ray will not emerge from the medium, but will be reflected back into the medium. This is called total internal reflection.
the critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees. 

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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.

TOTAL INTERNAL REFLECTION-WHEN A RAY OF LIGHT PASSES FROM A MEDIUM OF HIGHER OPTICAL DENSITY TO A MEDIUM OF LOWER OPTICAL DENSITY AT AN ANGLE OF INCIDENCE GREATER THAN THE CRITICAL ANGLE,THE RAY IS REFLECTED BACK TO THE SAME MEDIUM WITH OUT UNDERGOING REFRACTION.THIS KNOWN AS TOTAL INTERNAL REFLECTION.