Question

The dispersive powers of crown and flint glasses are $0.02\text{and}0.04$ respectively. In an achromatic combination of lenses the focal length of flint glass lens is $40cm$. The focal length of crown glass lens will be

• A. $+10cm$
• B. $-20cm$
• C. $+20cm$
• D. $-10cm$

Answer 1

The correct option is B $-20cm$

Applying condition of achromatic combination

$\frac{{\omega }_1}{f_1}+\frac{{\omega }_2}{f_2}=0$

Where $\omega =$ dispersive power,

$\frac{0.02}{f_1}+\frac{0.04}{40}=0$

$f_1=-20cm$

Hence option $\left(A\right)$ is the correct answer.