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According to the Cartesian sign convention used for lenses, the focal length of a convex lens and that of a concave lens carry which signs respectively?
APositive for both convex and concave
BNegative for convex, positive for concave
CPositive for convex, negative for concave
DNegative for both convex and concave
Answer & Solution
Correct answer: C. Positive for convex, negative for concave
1. By the sign convention, the focal length of a convex lens is taken positive.
2. The focal length of a concave lens is taken negative.
3. Therefore convex is positive and concave is negative, matching option A.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 7 "Lenses", p.94_
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