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Dispersion - Introduction and Causes | Physics Grade XI

Introduction to Dispersion

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Refraction Through Prisms

Dispersion - Introduction and Causes | Physics Grade XI

Dispersion - Introduction and Causes

When a beam of white light is made to fall on one refracting face of the prism, it splits up into seven colors namely violet, indigo, blue, green,yellow, orange, red from the base. So, this phenomenon of splitting of light into its seven constituent colors is called dispersion of light.

DispersionCause of Dispersion
The cause of dispersion of light is due to the different wave lengths, which is maximum for red color and minimum for violet color, of different color which pass through different angle on a glass prism and deviate through different angles.This happens because according to Cauchy,
µ = a + b/λ2 + c/λ4 + ........
Where a,b and c are the constants for material of prism.

From the above relation, we find that refractive index is affected by the wavelength. So, here refractive index for red color (µr) is lesser than the refractive index of violet color (µv) i.e. µrv or µvr. Since the angle of deviation for the prism is (δ) = A (µ - 1), then δvr which says that the violet light deviates more than the red light and similarly each light deviate with different angle due to which dispersion occurs.

The Spectrometer: A spectrometer is an optical device used to produce the spectrum of light from a source and to measure the refractive index of the material on the form of prism for different colors. It essentially consists of three main parts: collimator, prism table and telescope as shown in figure;


  • Collimator: It consists of a tube of variable length with an adjustable slit at one end and a convex lens at the other. When a source of light is placed closed to slit, a parallel beam of light will emerge.
  • Prism table: It has horizontal circular base for keeping prism and can be rotated about a vertical axis passing through its center. The position of the table can be read with the help of two Vernier V1 and V2.
  • Telescope: An astronomical telescope is used to observe the spectrum emerging from the prism. The distance between eyepiece and objective can be changed by using a rack and pinion arrangement. A piece of cross-wire is kept in Infront of the eye-piece.

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