Dispersive Power

According to Cauchy’s dispersion formula,



Differentiating equation (i)



dμ/dλ is the dispersive power of the medium. Therefore, dispersive power is inversely proportional to the cube of the wavelength of light. At λ = 4000 Å (violet) the dispersive power is about eight times the dispersive power at λ = 8000 Å (red). It means the spectral lines are more dispersed near the violet end of the spectrum that at the red end.

Example: Calculate the values of Cauchy’s constants A and B for crown glass. Given

= 1.514,                       = 1.524

λC = 6563 Å         and        λF = 4862 Å

According to Cauchy’s formula



Subtracting (i) from (ii)



B = 5.236 × 10^-11 cm2

Substituting the value of B in equation (i) and solving

A = 1.502

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