Physics
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.
Refraction Plane Surfaces
ICSE
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Answer
Below diagram shows the deviation of a ray of light through 90° by a total reflecting prism:
ABC is a total reflecting prism. A beam of light is incident normally at the face AB. It passes undeviated into the prism and strikes at the face AC making an angle of incidence equal to 45°.
For glass air interface, the critical angle is equal to 42°, therefore the beam of light suffers total internal reflection at the face AC because the angle of incidence is greater than the critical angle.
The reflected beam inside the prism then strikes the face BC, where it is incident normally and therefore passes undeviated. Hence, the incident beam gets deviated through 90°.
This action of prism is used in a periscope where a total reflecting prism is preferred over a plane mirror.
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