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The dielectric slab is shown with the parallel boundaries. Two dashed vertical lines are drawn on mid of the dielectric slab. The index of refraction of slab is n sub 2. This is surrounded by medium with index of refraction labeled, n sub 1. The light beam in a form of a ray is passed from the medium n sub 1 to the dielectric media passing through the first vertical dashed line and crosses the second vertical dashed line to the medium 1 on bottom, labeled, n sub 3 equals n sub 1. Angle theta sub 1 is measured on medium n sub 1 on top, angle theta sub 2 is measured in dielectric slab, and angle theta sub 3 equals theta sub 1 is measured in bottom medium.