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Q.
A lens forms a virtual image $4 \,cm$ away from it when an object is placed $10\, cm$ away from it. The lens is a .....lens of focal length .....
Ray Optics and Optical Instruments
Solution:
$\frac{1}{f}=\frac{1}{v}=\frac{1}{u}$
$\frac{1}{f}=\frac{1}{-4}-\frac{1}{-10}$
or $\frac{1}{f}=\frac{1}{10}-\frac{1}{4}$ or $\frac{1}{f}=\frac{2-5}{20}=-\frac{3}{20}$
or $f=-\frac{20}{3} \,cm =-6.67\,cm$
The negative sign indicates that the lens is concave.