From the geometry of the prism, the angle of the prism is .

The incident ray is parallel to the base, so the angle of incidence at the first surface is equal to the base angle . Thus, .

Applying Snell's law at the first surface: .
Given , we have .
At the second surface, the emergent ray grazes the surface, so the angle of emergence .
Applying Snell's law at the second surface: .
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The relation between prism angle and internal angles is .
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Substituting in the first equation: .
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