Total Internal Reflection, Superposition, and Interference

When light travels from a denser medium toward a less dense one, such as from glass or water into air, it bends away from the normal. As the angle of incidence increases, the refracted ray bends more and more until, at a particular angle, it travels exactly along the boundary. This special angle of incidence is called the critical angle. Beyond the critical angle, refraction can no longer occur and all the light is reflected back into the denser medium. This phenomenon is called total internal reflection.

The critical angle $\theta_c$ can be found from Snell's law by setting the angle of refraction to 90 degrees. For light passing from a medium of refractive index $n$ into a vacuum or air, $\sin\theta_c = 1/n$. A larger refractive index gives a smaller critical angle, making total internal