Firstly we must remind ourselves of what is meant by critical angle. The critical angle is the angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected (easily represented on a diagram).
So in our equation for refraction we can substitute in n2 = 1 (because air has a refractive index of 1) and π1 = 41. We also know that π2 = 90 degrees because it is the critical angle which is the point at which the angle of refraction is 90 degrees. sin90 = 1 so π1 sin π1 = π2 sin π2 becomes n1 x sin41 = 1. So the refractive index of the glass (n1) = 1/sin41 = 1.524. Because we have only been given information to two significant figures in the question, we leave our answer as 1.5.