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A lens of focal length 20 cm is used to produce a ten times magnified image of a film slide on a screen. How far must the slide be placed from the lens?
Given:
Magnification, $m$ = $-$10 ($\because$ only a convex lens can form image on a screen, means the image will be real and inverted, and it is possible only when the magnification has a negative sign)
Focal length, $f$ = 20 cm
To find: Object (slide) distance, $u$ from the lens.
Solution:
From the magnification formula, we know that-
$m=\frac {v}{u}$
Substituting the given values in the formula we get-
$-10=\frac {v}{u}$
$v=-10u$
Now,
According to the lens formula, we know that:
$\frac {1}{v}-\frac {1}{u}=\frac {1}{f}$
Substituting the value of $f$ and $v$ in the formula we get-
$\frac {1}{(-10u)}-\frac {1}{u}=\frac {1}{20}$
$-\frac {1}{10u}-\frac {1}{u}=\frac {1}{20}$
$\frac {1}{20}=\frac {-1-10}{10u}$
$\frac {1}{20}=\frac {-11}{10u}$
$10u=(-11)\times {20}$
$10u=-220$
$u=\frac {-220}{10}$
$u=-22cm$
Thus, the slide is placed at a distance of 22 cm from the poles of the lens.