Thank you for reporting, we will resolve it shortly
Q.
Two thin equiconvex lenses each of focal length $0.2\, m$ are placed coaxially with their optic centres $0.5\, m$ apart. Then the focal length of the combination is
JIPMERJIPMER 2012Ray Optics and Optical Instruments
Solution:
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)} $
$=5+5-0.5 \times 5 \times 5$
$=10-12.5=-2.5$
$\therefore F =-\frac{1}{2.5}$
$=-0.4 \,m$