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  4. A concave lens of focal length 20 cm produces an image half the size of the real object. The distance of the real object is

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Q. A concave lens of focal length 20 cm produces an image half the size of the real object. The distance of the real object is

KEAMKEAM 2006Ray Optics and Optical Instruments

Solution:

From lens formula
$ \frac{1}{f}= \frac{1}{v}= \frac{1}{u}$
Given, $ v = \frac{u}{2}$ and $f=20\, cm$
$ \Rightarrow \frac{1}{20}= \frac{1}{u/2}= \frac{1}{u}= \frac{1}{u}$
$ \Rightarrow u = 20\, cm $