Which one of the following relations represents the dispersive power ω of a prism material in terms of the refractive indices for violet (μv), red (μr) and the mean colour (μ)?
a) (μv−μr)/(μ+1)
b) (μv+μr)/(μ−1)
c) (μv−μr)/(μ−1)
d) (μv−μr)/μ