dispersion It is process of separation of different colors from a mixed light. This occurs when light enters a material medium from vacuum. The refractive index of any medium varies with the wavelength of light. Consequently light of different wavelengths will suffer different deviations and will be separated.
Dispersion can be observed using a prism, because the rays of different colors emerge at different angles producing spectrum.
The dispersive power of any material is defined as,
D.P. =(m F - m c)/( m D - 1)
where m F is the refractive index for blue F line of hydrogen, l = 486.1 nm, m D is refractive index for yellow line of sodium, l = 589.3 nm, and m c is the refractive index for red c line for hydrogen, l = 656.3 nm.
The reciprocal of dispersive power, designated by symbol n , is called n number. It lies between 30 and 60 for most optical glasses. n for crown > 55, and for flint < 50.
Almost all the transparent materials have refractive indices that are higher for shorter wavelengths and lower for longer wavelengths. In fig.d13 the variation of m with l is shown. An empirical relationship given by Cauchy is
m
= l + B/l 2 +C/l 2 (1)where A, B, C are constants found from experimental data. Usually C and subsequent terms are very small. Therefore third term onwards can be neglected. By differentiating (1)
dm /dl = -2B/l 3
which shows that dispersion varies inversely to the third power of wavelength.
The variation of m with l shows discontinuities (fig. d13). This behavior is called anomalous dispersion. Now it is understood that at these wavelengths absorption of energy of electromagnetic waves by the substance takes place. If the ultraviolet and the infrared wavelengths are included, most of the substances show some absorption.