PHYSICS (IIT-JEE) Course: DISPERSION & SPECTRA LESSON 1.1 PREPARED BY, ASHISH BHAGCHANDANI
CONTENTS DISPERSION DISPERSIVE POWER
DISPERSION . As mentioned earlier, the refractive index of a material depends slightly on the wavelength of light. The relation between the two may be approximatelydescribed by the equation where A is a small positive constant known as Cauchy's constant. . The refractive index decreases as the wavelength increases. For visible light, it is maximum for the violet end and minimum for the red end.
1.70 Silicate Flint Class 1.60 orate Flint Glass Quartz Silicate Crown Glass Figure shows the variation of refractive index with wavelength for some transparent materials 1.50 Fused quartz Fluorite 1.40 400 500 600 700 Wavelength (nm)
Red Yellow Violet Because of the difference in refractive indices light of different colors bend through different angles on refraction. red rays deviate the least. This phenomenon of separation of different constituent colors of light while . If white light passes through a glass prism, the violet rays deviate the most and the . Thus, white light is separated into its various component colors. passing through a transparent medium is known as dispersion of light.
DISPERSIVE POWER Consider a prism of a transparent material. When a beam of white light is passed through the prism, light of different wavelengths are deviated by different amounts. The overall deviation of the light beam is measured by the deviation of the yellow light as this deviation is roughly the average of all deviations.
In figure. this deviation is shown by the symbol It is clear that if , and are the deviations for red and violet components, the angular divergence of the transmitted beam is -6, This divergence is called angular dispersion. Red Yellow Violet
The mean deviation depends on the average refractive index and the angular dispersion depends on the difference _H-It may be seen from figure (20.1) that if the average value of is small (fluorite), Ho-H-is also small and if the average value of is large (silicate flint glass), -ur is also large. Thus, larger the mean deviation, larger will be the angular dispersion.
The dispersive power of a material is defined as the ratio of angular dispersion to the average deviation when a light beam is transmitted through a thin prism placed in a position so that the mean ray (ray having the mean wavelength) passes symmetrically through it.