Snell's Law
This applet demonstrates refraction and reflection of light rays at the interface between two transparent media. Users can adjust the two refractive indices. Snell's law determines the angle of incidence. Light not refracted is reflected, with conservation of power in Maxwell's equations determining the relative intensity. All light is reflected past the "critical angle", with boundary marked by dotted line.
Adapted from "Snell's Law" by GeoGebra Tube Team.