According to the formula of a small lens, which of the following is correct is about the focal length of a lens?

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f is negative for a converging lens.

f is negative for a convex lens.

f is positive for a diverging lens.

f is negative for a concave lens.

answer is D.

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Detailed Solution

According to the Cartesian sign convention, all distances measured from the lens's optical centre to the right side are considered positive and to the left side are considered negative.
Since the focal point of a diverging or concave lens lies on the left side, its focal length is negative.
Since the focal point of a converging or convex lens lies on the right side, its focal length is positive.

Therefore, only the last statement is correct.

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