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Optics

Refraction of light by Prisms

The word "prism" usually means a triangular piece of glass or plastic.

Deviation by a Prism

a) The angle d is called the angle of deviation.
b) The magnitude of d depends on

i) the angle A of the prism

ii) the angle of incidence at the face of the prism

iii) the refractive index of the prism material
c) The angle of deviation has its minimum value when light passes through the prism symmetrically (as shown below).

 

 
d) 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

 

Dispersion by a Prism

Different wavelengths of light travel at different speeds through glass (and other transparent media). Hence, the refractive index is different for different wavelengths (or colours) of light.

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 is given by

= db - dr

Equilateral prisms can be used to produce a spectrum. If white light is passed through such a prism, the result is called a continuous spectrum as all the colours are visible.

 

© David Hoult 2008