A ray of light travels from glass of refractive index 1.5 into air. What is the critical angle for the glass-air interface?

The critical angle C is given by sin C = 1/n, where n is the refractive index of glass. sin C = 1/1.5 = 0.6667, so C = sin⁻¹(0.6667) ≈ 41.8°. At angles of incidence greater than this, total internal reflection occurs.