Critical Angle

The critical angle of an incident wave (when traveling from a denser medium to a less dense medium) is the smallest angle which causes the refracted wave to be 90° (i.e. the wave is totally reflected). Starting with Snell's law:

If we set θ₂ to be 90° and solve for θ₁ = θ_critical, we get:

Where n₂ < n₁. For a wave traveling in plexiglass (n=1.49), the critical angel is around 42° when the second medium is air.