Question

Two thin equiconvex lenses each of focal length $0.2m$ are placed coaxially with their optic centres $0.5m$ apart. Then the focal length of the combination is

• A. - 0.4 m
• B. 0.4 m
• C. - 0.1 m
• D. 0.1 m

Answer 1

Answer: (A)

Equivalent focal length $(F)$ of two lenses separate by distance $d$ is given by
$\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1{f}_2}$
$=\frac{1}{0.2}+\frac{1}{0.2}-\frac{0.5}{(0.2)(0.2)}$
$=5+5-0.5\times 5\times 5$
$=10-12.5=-2.5$
$\therefore F=-\frac{1}{2.5}$
$=-0.4m$