1.

A real image, (2)/(3) rd of the size of an object, is formed by a convex lens when the object is at a distance of 12 cm from it. Find the focal length of the lens.

Answer»

SOLUTION :Here distance of the object from the lens u = - 12 cm and magnification of real image `m = - (2)/(3)`
As per relation `m = (v)/(u)`, we have v = mu = `(-(2)/(3)) xx (-12) = + 8` cm
So as per lens formula `(1)/(v) - (1)/(u) = (1)/(f)`, we have
`(1)/(f) = (1)/((+8)) - (1)/((-12)) = (1)/((8)) + (1)/((12)) = (5)/(24) RARR f = + (24)/(5) = + 4.8` cm


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