At what angle does total internal reflection occur?

This question is to do with the refraction that occurs when a light ray hits a boundary between two media of different refractive index. We know that the relationship between the angle of incidence and angle of refraction is given by Snell's law. Using this, and the fact that total internal refraction occurs at an angle of 90 degrees to the normal of the boundary, we can calculate an angle of incidence that leads to total internal refraction for any material.