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Question
Show with the help of a diagram how a total reflecting prism can be used to turn a ray of light through 90°. Name one instrument in which such a prism is used.
Solution
As shown in the diagram, a beam of light is incident on face AB of the prism normally, so it passes undeviated and strikes face AC, where it makes an angle of 45° with the normal to AC. Because here the incident angle is greater than the critical angle, rays suffer total internal reflection and reflect at an angle of 45°. The beam then strikes face BC, where it is incident normally and so passes undeviated. As a result, the incident beam deviates by 90°. Such a prism is used in periscopes.
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