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Whenever a light ray reaches the boundary with a transparent medium, a portion of the light energy is transmitted across the boundary and appears as a refracted ray; and a portion of the energy remains within the original medium and appears as a reflected ray. The path of the refracted ray follows Snell's law. The path of the reflected ray follows the law of reflection. The amount of light energy which is reflected and transmitted is dependent upon the angle of incidence. At certain angles, all of the light is reflected (none is transmitted) and remains inside the original medium. This is known as total internal reflection (TIR).