A ray is incident at an angle of incidence i on one surface of a small angle prism (with angle of prism A) and emerges normally from the opposite surface. If the refractive index of the material of the prism is , then the angle of incidence is nearly equal to :
Light ray emerges normally, so . Thus . Since , we have . Applying Snell's law on the first surface: . For small angles, , so .
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