Question

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be

a) 10cm

b) 20cm

c) – 10cm

d) – 20cm
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Answer 1

The power of a lens is the reciprocal of it's focal length. F = 1/f. It is 1/f if the focal length is in meters, 100/f in centimeters and 1000/f in millimeters. The answer to your question would be F = 100/10 = +10 diopters of power.

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