Total Internal Reflection

When light travels from denser medium to rarer medium it bends away from the normal. If we keep on increasing the angle of incidence, the refracted light will move more away from the normal. For an particular angle of incidence the refracted angle will be 90^o. That angle of incidence is called as critical angle (θ_C). If we further increase the angle of incidence, the light will be reflected back again to the denser medium. This phenomenon is called as Total Internal Reflection (TIR). We can derive an expression for critical angle by using Snell’s law

If the light is entering from air to other medium, for the condition of critical angle, i = θ_C; r = 90^o, µ₂ = 1 and µ₁ = µ

Hence we get,

Where θ_C is the critical angle.

Figure 1. Total internal reflection in an optical fiber