Question

a light transmits through the interface of medium1(n1) and medium2(n2) at incident angle of theta. what will the incident angle be, if the reflected angle is 90degree?

Answer 1

This is a critical angle question if the 90 degrees referred to is to the normal. Use Snell's law and one of the sines will be sin 90 = 1. That should give the critical angle at which TOTAL INTERNAL REFLECTION occurs.
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Answer 2

The attached may help. It's done for air to glass and glass to air, though.

Answer 3

thank you