A crown glass prism with refracting angle 6⁰, is to be achromatised for red and blue light with a flint glass prism. Angle of the flint glass prism should be (Given Crown glass ${\mathrm{\mu }}_{\mathrm{r}}=1.513,{\mathrm{\mu }}_{\mathrm{b}}=1.523$ $\mathrm{Flint} \mathrm{glass} \left.{{\mathrm{\mu }}_r}^{\text{'}}=1.645,{{\mathrm{\mu }}_{\mathrm{b}}}^{\text{'}}=1.665\right)$