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

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

Ray Optics and Optical Instruments

Solution:

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