Question

A convex lens forms 19 times magnified image of an object on a screen placed 10 m from the lens. What is the focal length of the lens?

Answer 1

Given,

A convex lens having $u=-10m$

$Magnification=-19$

Since, $m=-\frac{v}{u}$

$19=\frac{v}{-10}$

$v=-190$

Now, we know that by the lens formula,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\frac{1}{f}=\frac{1}{-190}+\frac{1}{10}$

$\frac{10+1.9}{19}=\frac{11.9}{19}$

$f=\frac{190}{18}=10.55m$

Thus, $f=10.55m$