Question

what is critical angle and its two conditions

Answer 1

The critical angle allows the light ray to be reflected back into the first medium (reflects internally), and does not refract into the second medium.
The critical angle occurs when the incident angle causes the angle of refraction to be 90 degrees from the normal.

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.