Formula for dispersive power is (where symbols have their usual meanings)   
or
If the refractive indices of crown glass for red, yellow and violet colours are respectively  ${\mathrm{\mu }}_{\mathrm{r}}, {\mathrm{\mu }}_{\mathrm{y} }\text{ and }{\mathrm{\mu }}_{\mathrm{v}}$, then the dispersive power of this glass would be
 

• A

  $\frac{{\mathrm{\mu }}_{\mathrm{r}}-{\mathrm{\mu }}_{\mathrm{y}}}{{\mathrm{\mu }}_{\mathrm{r}}-1}$

• B

  $\frac{{\mathrm{\mu }}_v-{\mathrm{\mu }}_{\mathrm{r}}}{{\mathrm{\mu }}_{\mathrm{y}}-1}$

• C

  $\frac{{\mathrm{\mu }}_{\mathrm{v}}-{\mathrm{\mu }}_{\mathrm{y}}}{{\mathrm{\mu }}_{\mathrm{y}}-{\mathrm{\mu }}_{\mathrm{r}}}$

• D

  $\frac{{\mathrm{\mu }}_{\mathrm{v}}-{\mathrm{\mu }}_{\mathrm{r}}}{{\mathrm{\mu }}_{\mathrm{y}}}-1$