Question

The focal length of a convex lens is 20cm. Find the object and image distance if 4 times Magnified real image is formed.

Answer 1

$f= 20cm\\m=-2(red)\\v=2u$

SIGN CONVENTION

$u=-ve\\v= +ve\\ f=+ve$

USING LENS FORMULA

$\frac{1}{v} -\frac{ 1}{u} =\frac{ 1}{f} \\=>\frac{ 1}{ -2u} -\frac{1}{u} =\frac{1}{f} \\-> u=-\frac{3f}{2} =-\frac{ 3X 10}{2} \\=-15\\ Answer\\\\15$

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