A prism is set for minimum deviation for light of wavelength  ${\lambda }_1$. The angle of minimum deviation D in this case is equal to the angle of the prism. When the prism is set for minimum deviation for another wavelength  ${\lambda }_2$, the angle of minimum deviation is 30°. The refractive index of the prism material for ${\lambda }_1$ is  $\sqrt{3}$. The refractive index of the prism material for ${\lambda }_2$ is