The angle at which the refracted light ray lies along the boundary of the two mediums, this
is the definition of

Critical angle

Explanation:

The angle of incidence when the refracted angle is 90°; we have the critical angle at this point. At this point, the refracted ray lies between the boundary between the two medium, this is the angle just before total internal reflection occurs, as the refracted angle exceeds 90°, all rays are reflected. Hence, we can conclude that it is the largest angle at which reflection can still occur.

2b2t

explanation:

2b2t

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