A small object is so placed in front of a convex lens of 5 cm focal length that a virtual image is formed at a distance of 25 cm. Find the magnification.

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Solution

Given:
Focal length, f = 5 cm
Image distance, v = $-$ 25 cm (Image is virtual.)
Applying the lens formula, we get:
1/v $-$ 1/u = 1/f (u = Object distance)
1/( $-$ 25) $-$ 1/u = 1/5
or, 1/u =1/(-25)-1/5
or, 1/u =-6/25
or,
u = $-$ 25/6 cm
Now,
Magnification, m = v/u
= ( $-$ 25)/( $-$ 25/6)
= +6

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