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  4. Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with each other. The combination has a focal length equal to

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Q. Two thin lenses of focal lengths $20 \,cm$ and $-20 \,cm$ are placed in contact with each other. The combination has a focal length equal to

Ray Optics and Optical Instruments

Solution:

Powers: $ P_{1} =\frac{1}{0.2} $ and $ P_{2}=-\frac{1}{0.2} $
$ = 5 D $ and $-5 D$
Net power $=0$
$\therefore $ Focal length $=\frac{1}{0}=\infty$