Question

A convex lens of focal length 25 m is placed coaxially in contact with a concave lens of focal length 20 cm .Determine the power of the combination. Will the system be converging or diverging in nature?

Answer 1

The net focal length of the combination is

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$

$=\frac{1}{+25}+\frac{1}{-20}$

$=-\frac{1}{100}$

$⟹f=-100cm$

Power of combination=$P=P_1+P_2=\frac{1}{f_1}+\frac{1}{f_2}$

$P=\frac{1}{0.25}+\frac{1}{-0.20}=\frac{100}{25}-\frac{100}{20}=\frac{400-500}{100}=-\frac{100}{100}=-1$

$⟹P=-1D$

Since power is negative, the combination is diverging.