An achromatic convergent doublet has power of +2D. If power of convex lens is + 5D, then ratio of the dispersive powers of a convergent and divergent lenses will be 

3 : 5                

$ P = P_1 + P_2 \Rightarrow +2D = 5D + P_2 $

$\Rightarrow P_2 = -3D$

$ \text{For Achromatic Combination } \frac{\omega_1}{f_1} +\frac{\omega_2}{ f_2}= 0$

$ \frac{\omega_1}{\omega_2} = -\frac{f_1}{f_2}  = -\frac{P_2}{P_1} = 3:5$