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Standard X

Physics

Lens Formula

An object 20 ...

Question

An object 20 cm from a convex mirror gives rise to a virtual image 15 cm behind the mirror. The focal length of the mirror is

8.5 cm

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30 cm

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90 cm

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60 cm

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Solution

The correct option is D
60 cm

Taking the new cartesian sign conventions $\frac{1}{u} + \frac{1}{v} = \frac{1}{f} \Rightarrow - \frac{1}{20} + \frac{1}{15} = \frac{1}{f}$
$\Rightarrow \frac{- 3 + 4}{60} = \frac{1}{f} \Rightarrow f = 60 c m$

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An object 20 cm from a convex mirror gives rise to a virtual image 15 cm behind the mirror. The focal length of the mirror is

The correct option is D 60 cm Taking the new cartesian sign conventions 1u+1v=1f⇒−120+115=1f ⇒−3+460=1f⇒f=60 cm

The correct option is D
60 cm

Taking the new cartesian sign conventions $\frac{1}{u} + \frac{1}{v} = \frac{1}{f} \Rightarrow - \frac{1}{20} + \frac{1}{15} = \frac{1}{f}$
$\Rightarrow \frac{- 3 + 4}{60} = \frac{1}{f} \Rightarrow f = 60 c m$