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Study Notes: Prisms

A problem with normal glass-fronted mirrors is that part of the light is reflected from the glass front and the remaining refracted light is laterally displaced when it emerges back through the glass from the polished metal reflector.

This causes multiple images to appear. Normally we cannot see these multiple images because they are very close together. However, a sensitive scientific instrument may be able to detect the separate light beams and this can lead to a distortion of measured results.

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Special mirrors that have the silvered reflective material on the front, instead of the back, of the glass can be used to overcome this effect. With these mirrors there is no protective layer over the reflecting surface and as a result there is the problem that damage may occur during cleaning of the reflective surface. To overcome this, special dust free instrument compartments must be used. Another problem is that air will corrode the metallic surface and this will distort the specular reflection.

An easier approach is to use glass prisms to do the reflecting. A prism can be used to reflect light with little or no distortion, in two ways.

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The incident ray enters the surface of the prism at a 90° angle and at this angle it is not refracted but passes straight into the glass. When the light tries to emerge from the right hand side of the glass it cannot. This is because it strikes the inner surface of the glass at a 45° angle. This is larger than the critical angle (41.5° for the glass/air) so the light is totally internally reflected at a 45° angle. One reflection inside the prism changes the direction of the light beam by 90° while two reflections changes the direction by a total of 180°.

 

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