Total internal reflection occurs when a ray of light in a medium with a higher index of refraction strikes the interface between that medium and one with a lower index of refraction. When the incident rays are at a small angle with respect to the perpendicular to the interface the rays pass through the interface being refracted to a larger angle. As the angle of incidence increases so does the refracted angle. At the “critical” angle the refracted angle is 90°, that is, the ray’s direction is along the interface. If the angle of incidence is increased any more there is only a reflected ray, the refracted ray no longer exists. Because all the light is reflected it is called total internal reflection.