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.
Answer & explanation
Correct answer: option 3
$\frac{1}{f}=(μ-1)(\frac{1}{R_1}-\frac{1}{R_2})$ ... (1)
For no dispersion; $d(\frac{1}{f})=0$
Diff. equation (1);
$d(\frac{1}{f})=dμ(\frac{1}{R-1}-\frac{1}{R_2})=0$ $⇒R_1=R_2$