The focal length of a lens is (i) positive, (ii) negative. In each case, state the kind of lens.
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(i) When the focal length of a lens is positive, then the lens is a convex lens.
(ii) When the focal length of a lens is negative, then the lens is a concave lens.
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The power of a concave lens of focal length 20 cm is :
What information about the nature of the image (i) real or virtual, (ii) erect or inverted, do you get from the sign of magnification + or - ?
How is the power of a lens related to its focal length ?