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Prisms
In optics, the word "prism" usually means a triangular piece of glass or plastic.
If we are thinking of another shape, we usually specify it in the name, for example the penta-prisms (five sides) used in many cameras.
If we refer to the refracting angle of a (triangular) prism, we mean the angle between the two sides by which the light enters and leaves the prism.
 
Deviation of Light by a Prism
The diagram below shows light being refracted by a prism of refracting angle A.
The direction of motion of the light has been changed.
We say that the light has been deviated and the angle of deviation is the angle d between the incident and refracted rays.
 
The angle of deviation depends on:
1.  the angle A of the prism
2.  the angle of incidence at the face of the prism
3.  the refractive index of the prism material
 
The angle of deviation has its minimum value when light passes through the prism symmetrically (as shown below).
The minimum angle of deviation, D, is related to the angle of the prism, A, and the refractive index of the prism material by the following formula
This relation is can be used as the basis for the measurement of refractive index of a material.
 
Dispersion of Light by a Prism
Different wavelengths of light travel at different speeds through glass (and other transparent materials).
This means that the refractive index is different for different wavelengths (or colours) of light.
A prism can therefore be used to separate the colours (wavelengths) of light from a source.
We say that the prism disperses the light and the angle q in the diagram above is called the angle of dispersion.
If db is deviation for blue light and dr is deviation for red light then the angle of dispersion q is simply the difference between these two angles
Equilateral prisms can be used to produce a spectrum of a source of light as in a spectroscope.
The colours present in the spectrum can give information about the materials in the source (see, for example, here).
If white light is passed through such a prism, the result is called a continuous spectrum as all the colours are visible.
 
 
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