Which one of the following spherical lenses does not exhibit dispersion? The radii of curvature of the surfaces of the lenses are as given in the diagrams.

$\frac{1}{f}=(μ-1)(\frac{1}{R_1}-\frac{1}{R_2})$  ... (1)

For no dispersion; $d(\frac{1}{f})=0$ 

Diff. equation (1);

$d(\frac{1}{f})=dμ(\frac{1}{R-1}-\frac{1}{R_2})=0$  $⇒R_1=R_2$