# A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 10 cm. What is the focal length and power of the combinat

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A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 10 cm. What is the focal length and power of the combination ?

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Here, f_(1) = + 20 cm, f_(2) -10 cm, F = ?, P = ?
From (1)/(f) = 1/(f_(1)) + 1/(f_(2)) = 1/20 - 1/10 = -1/20, F = -20 cm.
P = 100/F = 100/(-20) = -5 dioptre.

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