Question

b) Find the power of a convex lens which forms a real and inverted image of magnification -2
of an object placed at a distance of 20 cm from its optical centre.
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Answer 1

Answer:

$Given: Concave Lens$

            $m=-2$

            $u=-20cm$

$To find:$ $P=?$

$Sol^{n} :$

           $m=\frac{v}{u} \\-2=\frac{v}{-20} \\v=40cm\\\\\frac{1}{f}=\frac{1}{v} -\frac{1}{u} \\\frac{1}{f}=\frac{1}{40} -\frac{-1}{20} \\\frac{1}{f}=\frac{1+2}{40} \\\frac{1}{f}=\frac{3}{40}\\\\P=\frac{100}{f(cm)} \\P=\frac{100}{40/3} \\P=\frac{100*3}{40} \\P=7.5D$

∴ Power of a convex lens with magnification -2 when the object is at 20cm distance from the lens is 7.5 Diopters.