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When transmitted from a dense medium to a sparse medium, the direction of light is refracted away from the surface normal in accordance with Snell's Law. As the angle of incidence increases, the angle of refraction increases at a faster rate. Eventually, the refracted direction lies in the tangent plane of the surface. The incident angle at this point is called the critical angle, at which the refracted direction has reached a limit, unable to bend further from the surface normal. If the angle of incidence is increased beyond the critical angle, the incident light is reflected off the interface of the media in accordance with Newton's Law. This effect is called total internal reflection.
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