Answer:
As the ray is travelling from denser to rarer medium,
Therefore sin i / sin r = 1 / RI
= Sin 90 / Sin i = RI (Reciprocal)(Sin 90 because for critical angle Sin r= 90)
= 1 / Sin i = 3/2
= Sin i = 0.667
Now converting this value to degrees
It becomes Sin 42.
Therefore the critical angle is 42.
42 degrees = 0.733038286 radians