Definition:
Dispersive power is defined as the power of a transparent medium to separate different colors of light by refraction as measured by the difference in refractivity for two specified widely differing wavelengths divided by the refractivity at some specified intermediate wavelength.
Dispersive Power of a Grating:
The dispersive power of a grating is defined the ratio of the difference in angle of diffraction of any two neighbouring spectral lines to the diffraction in the wavelength between the two spectral lines. It is expressed as dθ/dλ
We know (a+b) sinθ = nλ
Differentiate with respect to λ
[(a+b)cosθ] dθ/dλ=n
dθ/dλ= n / [(a+b)cosθ] is dispersive power of grating.
