Which of the following is true about lenses?

convex mirror has negative focal lengths concave lens have positive focal length

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convex mirror has positive focal length and concave lens have positive focal length

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convex mirror has positive focal length and concave lens have negative focal length

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both convex mirror and convex lens have negative focal length.

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Solution

The correct option is C
convex mirror has positive focal length and concave lens have negative focal length

The focal length of the convex lens lies on the right side of the lens and for the concave lens, it lies on the left side of the lens as shown. Therefore, according to the convention, convex lens has a positive focal length and concave lens has a negative focal length.

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