The magnification of an object placed in front of a convex lens of focal length $20 c m$ is $+ 2$ . The image distance is________.

10 c m

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20 c m

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- 30 c m

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- 20 c m

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Solution

The correct option is D $- 20 c m$
Given:
Magnification, $m = \frac{v}{u} = + 2$
$∴$ Object distance $u = - x$
Image distance $v = - 2 x$
Focal length $f = + 20 c m$

Using Lens formula $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$\frac{1}{- 2 x} - \frac{1}{- x} = \frac{1}{20}$
or $x = 10 c m$
Hence,
Object distance, $u = - 10 c m$
Image distance, $v = - 20 c m$

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The magnification of an object placed in front of a convex lens of focal length 20 cm is +2. The image distance is________.

The correct option is D −20 cmGiven: Magnification, m=vu=+2 ∴ Object distance u = −x Image distance v = −2x Focal length f=+20 cm Using Lens formula 1v−1u=1f 1−2x−1−x=120 or x=10 cm Hence, Object distance, u = −10 cm Image distance, v = −20 cm

The magnification of an object placed in front of a convex lens of focal length $20 c m$ is $+ 2$ . The image distance is________.