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16.7 Dispersion of light , Angular Dispersion and Dispersive Power

It is well known that, when white light - sun light for example - passes through a prism, the emergent beam is made up of seven colors of rainbow ("VIBGYOR" spectrum). This phenomenon of separation of a given beam of light into its constituent colors is called dispersion of light. According to wave theory, dispersion is an obvious phenomena as the Refractive Indices of different colors (wave length or frequencies) are different and the deviation angle varies in the same manner as the Refractive Indices for the fixed angle of incidence. Therefore violet color bends by the greatest angle and red color bends by the least angle; hence, the order of the colors in the spectrum of white light from base to top of the prism, is in the order VIBGYOR ( and not "ROYGBIV" ).

Angular Dispersion : D & Dispersive Power : w

The capacity of the material of a prism, to disperse a given beam of light into of its constituent colors, is measured by the two quantities D & w; which are defined as

D = It is the difference between the angles of minimum deviations for a pair

of colors, usually the pair chosen is that of extreme colors,

i.e. Violet & Red in white light spectrum

D is obviously expressed in units of angle.

w = Angular Dispersion to mean angle of deviation for the chosen pair (usually extreme ones) of colors.



16.1 Laws of Reflection at plane surfaces
16.2 Mirrors
16.3 Laws of Refraction at plane surfaces, Refractive Index and Critical Angle
16.4 Refraction at curved spherical surfaces and Lens makerís formula
16.5 Lenses
16.6 Refraction in Prisms, Prism Formula
16.7 Dispersion of Light, Angular Dispersion and Dispersive Power
16.8 Optical Instruments and Magnifying Power

Chapter 17

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