Question

For condition of achromatism,if two lenses are used is constant and one lens is convex then other should be -

• A. Convex
• B. Concave
• C. Phenoconvex
• D. None of these

Answer 1

The correct option is B Concave

We know the condition of achromatic is
$\frac{{\omega }_1}{f_1}+\frac{{\omega }_2}{f_2}=0$
As ${\omega }_1$ and ${\omega }_2$ are positive quantities $f_1$ and $f_2$ should have opposite signs, i.e. if one lens is convex the other must be concave.