In the visible region the dispersive powers and the mean angular deviations for crown and flint glass prisms are $\mathrm{\omega },{\mathrm{\omega }}^{\mathrm{\prime }}\text{ and }\mathrm{d},{\mathrm{d}}^{\mathrm{\prime }}$ respectively. The condition for getting deviation without dispersion when the two prisms are combined is

• A

  $\sqrt{\mathrm{\omega d}}+\sqrt{{\mathrm{\omega }}^{\mathrm{\prime }}{\mathrm{d}}^{\mathrm{\prime }}}=0$

• B

  ${\mathrm{\omega }}^{\mathrm{\prime }}\mathrm{d}+{\mathrm{\omega d}}^{\mathrm{\prime }}=0$

• C

  $\mathrm{\omega d}+{\mathrm{\omega }}^{\mathrm{\prime }}{\mathrm{d}}^{\mathrm{\prime }}=0$

• D

  $(\mathrm{\omega d}{)}^2+{\left({\mathrm{\omega }}^{\mathrm{\prime }}{\mathrm{d}}^{\mathrm{\prime }}\right)}^2=0$