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  4. Two plano-convex lenses of focal lengths 20 cm and 30 cm are placed together to form a double convex lens. The final focal length will be

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Q. Two plano-convex lenses of focal lengths $20 \, cm$ and $30 \, cm$ are placed together to form a double convex lens. The final focal length will be

NTA AbhyasNTA Abhyas 2022

Solution:

Equivalent focal length
$\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2}}$
$ \, =\frac{1}{20}+\frac{1}{30}$
$\therefore F=\frac{20 \times 30}{20 + 30}$
$=\frac{600}{50}=12\text{ cm}$