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What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.

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Focal length of convex lenses, $f_{1} = 30 c m$
Focal length of concave lenses, $f_{2} = - 20 c m$

$\Rightarrow \frac{1}{f} = \frac{1}{f _{1}} + \frac{1}{f _{2}}$
$f = - 60 c m$ . Therefore diverging lens .

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