Define dispersive power of the material of a prism. How can it be measured?

Explanation

The dispersive power of the material is its ability to disperse the constituent colours of incident light. The dispersive power of the material of the prism can be measured by the ratio of angular dispersion between any two colours to the mean of the angle of deviation of the two colours. Dispersive power has no unit. It is a dimensionless quantity.
`omega=(d_(v)-d_(R))/(d)=(n_(v)-n_(R))/(n-1)`