A real image formed by a concave mirror is $4.5$ times the size of the object. If the mirror is $20 c m$ from the object, its focal length is

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Solution

$m = - 4.5$ (negative because real image)
$u = - 20 c m$
$m = - \frac{v}{u}$
$\frac{- v}{- 20} = - 4.5$
$v = - 90 c m$
$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$
$= \frac{1}{- 20} + \frac{1}{- 90} = \frac{- 7}{180}$
$f = - 25.7 c m$

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A real image formed by a concave mirror is 4.5 times the size of the object. If the mirror is 20 cm from the object, its focal length is

m=−4.5(negative because real image)u=−20cmm=−vu−v−20=−4.5v=−90cm1f=1u+1v=1−20+1−90=−7180f=−25.7cm

A real image formed by a concave mirror is $4.5$ times the size of the object. If the mirror is $20 c m$ from the object, its focal length is

$m = - 4.5$ (negative because real image)
$u = - 20 c m$
$m = - \frac{v}{u}$
$\frac{- v}{- 20} = - 4.5$
$v = - 90 c m$
$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$
$= \frac{1}{- 20} + \frac{1}{- 90} = \frac{- 7}{180}$
$f = - 25.7 c m$