75. Let θbe the angle of incidence and 2be the angle of refraction at the left face of the plate. Let nbe the index of refraction of the glass. Then, the law of refraction yields sin = nsin 2. The angle of incidence at the right face is also 2. If 3is the angle of emergence there, then nsin 2= sin 3. Thus sin 3= sin and 3= . The emerging ray is parallel to the incident ray. We wish to derive an expression for xin terms of . If Dis the length of the ray in the glass, then Dcos 2= tand D = t/cos 2. The angle αin the diagram equals – 2and x = D
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