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The focal length of a thin biconvex lens...

The focal length of a thin biconvex lens is `20 cm`. When an object is moved from a distance of `25 cm` in front of it to `50 cm`, the magni-fication of its image changes from `m_(25) to m_(50)`. The ratio `(m_(25))/(m_(50))` is.

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The correct Answer is:
6
`m = (f)/( f + 4) ` [ `f = 20 cm, u_(1) = - 25 cm u_(2) = - 50 cmm`]
Case- 1 ` m_(25) = (20)/( 20 - (-25)) = (20)/(-5) = - 4`
Case - 2 ` m_(50) = (20)/( 20 + (-50)) = (20)/(-30) = (-2)/( 3)`
According of given condition
`(m_(25))/( m_(50)) = (-4)/( -2//3) = 6 cm`
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