Question

The refractive index is equal to the reciprocal of sine of this angle.

Answer 1

critical angle is the answer

Answer 2

The above given statement is based on the Snell’s law.

Initially it strikes one medium making an angle θi due to incident and it bends towards or away from the normal making an angle θr because it got refracted.  

Refractive index can be expressed as the relation between the angle made by a light in the two media when the light passes through such that it will be Sinθ1/Sinθ2.

With this it can be said, it is the inverse proportional of sine of an angle called critical angle.