In above shown figure, an object is kept 20 cm from the concave mirror and formed the real image at the distance of 5 cm from the mirror. Calculate the focal length of the mirror.

2.5 cm

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4 cm

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25 cm

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20 cm

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40 cm

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Solution

The correct option is B 4 cm
Given :
$u = - 20$ cm
$v = - 5$ cm

Using Mirror equation

$\frac{1}{u} + \frac{1}{v} = \frac{1}{f}$

$∴$ $\frac{1}{- 20} + \frac{1}{- 5} = \frac{1}{f}$ $⟹ f = - 4$ cm
Thus focal length of concave mirror is $4$ cm.

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In above shown figure, an object is kept 20 cm from the concave mirror and formed the real image at the distance of 5 cm from the mirror. Calculate the focal length of the mirror.

The correct option is B 4 cmGiven : u=−20 cm v=−5 cmUsing Mirror equation 1u+1v=1f∴ 1−20+1−5=1f ⟹f=−4 cmThus focal length of concave mirror is 4 cm.

The correct option is B 4 cm
Given :
$u = - 20$ cm
$v = - 5$ cm

Using Mirror equation

$\frac{1}{u} + \frac{1}{v} = \frac{1}{f}$

$∴$ $\frac{1}{- 20} + \frac{1}{- 5} = \frac{1}{f}$ $⟹ f = - 4$ cm
Thus focal length of concave mirror is $4$ cm.