If ${\mathrm{f}}_{\mathrm{v}}$ and ${\mathrm{f}}_{\mathrm{R}}$ are the focal lengths of a convex lens for violet and red light respectively and${\mathrm{F}}_{\mathrm{v}}$ and ${\mathrm{F}}_{\mathrm{R}}$ are the focal lengths of concave lens for violet and red light respectively, then we have:

$\frac{1}{f}=(\mu -1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

According to Cauchy relation: 

$\mu =A+\frac{B}{{\lambda }^2}+\frac{C}{{\lambda }^4}\dots$ Hence $f$ $\propto \lambda .$

Hence, red light having maximum wavelength has maximum focal length. 

$\therefore {\mathrm{f}}_{\mathrm{v}}<{\mathrm{f}}_{\mathrm{R}}\text{ and also }{F}_{\mathrm{v}}>F_R$ as focal length is negative for a concave lens.