Refraction and total internal reflection. In this photograph a diode laser beam (635 nm wavelength) is incident on a tank of water from the left. First the beam refracts (bends) toward the normal to the surface. The angles of incidence and refraction are related through the Snell's law. Then the beam is totally internally reflected at the water-air interface. Total internal reflection occurs when light is incident on an interface between a more optically dense medium and a less optically dense medium and the angle of incidence exceeds a certain value, called critical angle. As the beam exits the tank to the right, it refracts again.