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  4. Two thin lenses of equal focal lengths but one convergent and the other divergent are in contact with each other. The combination would act as a

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Q. Two thin lenses of equal focal lengths but one convergent and the other divergent are in contact with each other. The combination would act as a

J & K CETJ & K CET 2016Ray Optics and Optical Instruments

Solution:

Let $f$ be the focal length of each lens
As the one lens is convergent and other is divergent, so the focal length of their combination is
$\frac{1}{f}=\frac{1}{f}+\frac{1}{\left(-f\right)}$
$=\frac{1}{f}-\frac{1}{f}=0$
or $F=\frac{1}{0}=\infty$
Since the focal length of plane glass plate is infinite, so the combination would act as a plane glass plate