A convex lens of focal length f1 and a concave lens of focal length f2 are placed in contact. Using the sign convention, what is the focal length of the combination?
Correct answer A. f1f2/(f2 + f1)
Explanation
For thin lenses in contact the powers add, 1/F = 1/f1 + 1/f2, with the concave lens focal length taken as negative, giving F = f1f2/(f1 + f2).
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