You have a layer of glass with a refractive index of 1.5 and the glass is surrounded by air. A light ray shines into the glass and Total Internal Reflection occurs at the glass-air boundary. What is the critical angle?

This question tests your knowledge of Snell's law, Total Internal Reflection (TIR) and the fact that the refractive index of air is 1 (which should be memorised as it is in the exam). The critical angle is the angle at which TIR occurs, which can be calculated with the equation from Snell's law that relates angle of incident and refraction with the two refractive indices. The trick here is to use the angle of refraction as 90 degrees to apply the condition of TIR.