Physics
What is a total reflecting prism? State three actions that it can produce. Draw a diagram to show one such action of the total reflecting prism.
Refraction Plane Surfaces
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Answer
A prism having an angle of 90° between its two refracting surfaces and the other two angles each equal to 45° is called a total reflecting prism. In such a prism, the light incident normally on any of its faces suffers total internal reflection inside the prism.
Due to this property, a total reflecting prism is used for the following three purposes —
- To deviate a ray of light through 90°.
- To deviate a ray of light through 180°.
- To erect the inverted image without producing deviation in its path.
Below diagram shows the deviation of a ray of light through 90° by a total reflecting prism:
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A ray of light enters a glass slab ABDC as shown in figure and strikes at the centre O of the circular part AC of the slab. The critical angle of glass is 42°. Complete the path of the ray till it emerges out from the slab. Mark the angles in the diagram wherever necessary.
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.
The refractive index of glass is 1.5. From a point P inside a glass slab, draw rays PA, PB and PC incident on the glass-air surface at an angle of incidence 30°, 42° and 60° respectively.
(a) In the diagram show the approximate direction of these rays as they emerge out of the slab.
(b) What is the angle of refraction for the ray PB?
(Take sin 42° = 2 / 3)
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