Question

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.

Solution
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Focal length of convex lenses, f1=30cm
Focal length of concave lenses, f2=20cm
1f=1f1+1f2
f=60cm. Therefore diverging lens .

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