Critical angle
or Critical refraction angle

When a beam of light beam passes through a sample with at refractive index n₁ in a medium with index of refraction n₂ , such that n₂ < n₁, the angle q that the light beam makes with the perpendicular to the surface separating the two mediums is called the angle of refraction q.

If the condition n₁.sinq = n₂ is satisfied then the angle of refraction is called the critical angle or the critical refraction angle.

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