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  4. Two lens of focal lengths -20 cm and +10 cm are put in combination, find the power of the combination.

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Q. Two lens of focal lengths -20 cm and +10 cm are put in combination, find the power of the combination.

AIIMSAIIMS 2011Ray Optics and Optical Instruments

Solution:

Here $f_{1}= - 20 \,cm, f_{2}= + 10 \,cm$
Focal length of combination is.
$\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2}}=\frac{1}{-20}+\frac{1}{10}$
$\frac{1}{F}=\frac{1}{20}$
$\Rightarrow F = 20 \,cm $
Power of the combination,
$P=\frac{100}{F}$
$=\frac{100}{20}=+5D$