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  4. An object has image thrice of its original size when kept at 8 cm and 16 cm from a convex lens. Focal length of the lens is

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Q. An object has image thrice of its original size when kept at $8\, cm$ and $16 \,cm$ from a convex lens. Focal length of the lens is

Ray Optics and Optical Instruments

Solution:

$m=\pm 3$, using $m=\frac{f}{f+u}$
For virtual image $3=\frac{f}{f-8}$ $\ldots $(i)
For real image $-3=\frac{f}{f-16}$ ....(ii)
Solving (i) and (ii) we get $f=12 \,cm$