Question

11- A concave lens has focal length of 15cm. At what distance from the lens should the object
be placed, so that image is formed at 10cm from the lens.
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Answer 1

Question-:

A concave lens has focal length of 15cm. At what distance from the lens should the object be placed, so that image is formed at 10cm from the lens.

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Given-:

•Focal Length, f= $-15$

•Image distance, v= $10$

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To Find-:

•Object distance, u= $?$

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Answer-:

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

Putting all the values we get,

$\implies$ $\large\frac{1}{-10}$-$\frac{1}{u}$ =$\frac{1}{-15}$

$\implies$ $\large\frac{1}{u}$= $\frac{-1}{10}$+$\frac{1}{15}$

$\implies$ $\large\frac{1}{u}$ =$\frac{-3+2}{30}$

$\implies$ $\large\frac{1}{u}$ = $\frac{1}{-30}$

$\implies$ $\boxed{u=-30}$

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Final Answer-:

$\underline\color{purple}\boxed{u=-30}$

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Formula used-:

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