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Refraction Through Prism

The refractive index of the material of a prism is given by

where A is the angle of the prism and D is the angle of minimum deviation. For a small angled prism

Where and δ refer to the angle of the prism and the angle of minimum deviation (for small values of , the angle δ is also small and the sines of the angles are taken equal to the angles).

fig. represents the angles of deviation δ_b and δ_r produced in the blue, mean yellow and red rays of light. The deviations δ_b, δ and δ_r can be written as:

The difference in deviation between any two colours is called angular dispersion. From (ii) and (iii)

Dividing (iv) by (i)

where and are the refractive indices for the blue and red rays of light and is the refractive index for the mean yellow rays of light. The expression is called the dispersive power of the material of the prism. It is constant for two colours (or wavelengths) chosen and is represented by ω.

The reciprocal of ω is called the constringence. It is also customary to represent correspond to the Fraunhoffer lines (dark lines) in the solar spectrum. The F, D and C lines lie in the blue, yellow and red regions of the spectrum and their wavelengths are 4861 Å, 5893 Å and 6563 Å respectively. (1 Å = 1 Angstrom unit = 10^-8 cm).