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A concave lens has a focal length of $20 cm$ . At what distance should an object be placed so that it forms an image $15 cm$ away from the lens?

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+ 50 cm

+ 40 cm

- 60 cm

+ 20 cm

answer is C.

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Detailed Solution

A concave lens has a focal length of

20 cm

. Distance of the object,

u = - 60 cm

when it forms an image

15 cm

away from the lens
Given,
The focal length of the concave lens,

f = - 20 cm

Distance of virtual image from the concave lens,

v = - 15 cm

As per the lens formula,

\frac{1}{f} = \frac{1}{v} - \frac{1}{u}

,
we have

\frac{1}{u} = \frac{1}{v} - \frac{1}{f}

= \frac{1}{- 15} - \frac{1}{- 20}

= \frac{- 4 + 3}{60}

= - \frac{1}{60}

\Rightarrow f = - 60 cm

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