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Requirements for Total Internal Reflection (TIR):
Two requirements must be met for total internal reflection to occur. First, light must be in the more dense medium heading toward the boundary with the least dense medium. Second, light must be approaching the boundary with an angle of incidence that is greater than the value of the so-called critical angle.
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As mentioned in the Know the Law section above, TIR can only take place provided that the incident ray is traveling within the more dense medium. The optical density of a medium is related to the index of refraction of the material (see Know the Law section below). To be successful on this question, you will have to inspect each diagram carefully to determine if it satisfies the condition of light being in the more dense medium and heading towards the least dense medium.
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Optical Density and the Index of Refraction:
Every material is identified by a unique index of refraction value. The index of refraction value is an experimentally determined value that provides a relative measure of the optical density of that material. The higher the index of refraction value, the more optically dense a material is.
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