What best describes why the critical angle of an air-glass boundary is 39.3°?

A critical angle is described as the angle of incidence provided that the angle of refraction is equal to 90 degrees. The value of the critical angle depend on factors such as the materials present on different sides of the boundary. The critical angle of glass-water boundary is 39.3 degrees

It is because the angle of incidence in glass is 90 degree.