Question

The magnification of an object placed it front of a convex lens of focal length $20cm$ is $+2$. To obtain a magnification of $-2$, the object will has to be moved a distance equal to

• A. 10 cm
• B. 20 cm
• C. 30 cm
• D. 40 cm

Answer 1

Answer: (B)

When magnification is $+2$.
$u=-x$
$v=-2x$
$f=+20$
Using $\frac{1}{-2x}+\frac{1}{x}=\frac{1}{20}$
or $x=10cm$
To have a magnification of $-2$
$u=-y$
$v=+2y$
and $f=+20$
$\therefore \frac{1}{2y}+\frac{1}{y}=\frac{1}{20}$
or $y=30cm$
$\therefore y-x=20cm$