A lamp is 5.0 m from a wall. Find the focal length of a concave mirror which will form a four times magnified image of the lamp on the wall.

4 m

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2 m

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20 m

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10 m

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Solution

The correct option is A 4 m
$u = 5 m$
$m = \frac{v}{u} = 4$
$\Rightarrow v = 4 u$
Mirror Formulae:
$\frac{1}{f} = \frac{- 1}{4 u} - \frac{1}{u} = \frac{- 1 - 4}{4 u} = \frac{- 5}{4 u}$
or, $f = \frac{- 4 u}{5} = \frac{- 4 \times 5}{5} = - 4 m$
$∴$ focal length of a concave mirror is 4 m.

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A lamp is 5.0 m from a wall. Find the focal length of a concave mirror which will form a four times magnified image of the lamp on the wall.

The correct option is A 4 mu=5 mm=vu=4⇒v=4uMirror Formulae:1f=−14u−1u=−1−44u=−54uor, f=−4u5=−4×55=−4 m∴ focal length of a concave mirror is 4 m.

The correct option is A 4 m
$u = 5 m$
$m = \frac{v}{u} = 4$
$\Rightarrow v = 4 u$
Mirror Formulae:
$\frac{1}{f} = \frac{- 1}{4 u} - \frac{1}{u} = \frac{- 1 - 4}{4 u} = \frac{- 5}{4 u}$
or, $f = \frac{- 4 u}{5} = \frac{- 4 \times 5}{5} = - 4 m$
$∴$ focal length of a concave mirror is 4 m.