An objective is kept at a distance of $ 16 \mathrm{~cm} $ from a thin lens and the image formed is real. If the object is kept at a distance of $ 6 \mathrm{~cm} $ from the same lens the image formed is virtual. If the size of the images formed are equal, then find the focal length of the lens?

Question

An objective is kept at a distance of $ 16 \mathrm{~cm} $ from a thin lens and the image formed is real. If the object is kept at a distance of $ 6 \mathrm{~cm} $ from the same lens the image formed is virtual. If the size of the images formed are equal, then find the focal length of the lens?, $ 11 \mathrm{~cm} $, $ -11 \mathrm{~cm} $, $ \sqrt{11} \mathrm{~cm} $, $ \sqrt{-11} \mathrm{~cm} $

MARKS App · Accepted Answer

m = h 2 h 1 = v u = f f + u for real image m is negative - m = f f - 1 6 ---- 1 for virtual image m is positive m = f f - 6 ---- 2 from (1) & (2) - f f - 6 = f f - 1 6 ⇒ 6 f - f 2 = - 16 f + f 2 ⇒ 2 f 2 = 2 2 f f = 11 cm

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