Question

For unit magnification, the distance of an object from a concave mirror of focal length $20cm$ will be

• A. $20cm$
• B. $10cm$
• C. $40cm$
• D. $60cm$

Answer 1

The correct option is C $40cm$

Given,

The focal length $f=-20cm$

The magnification of the concave mirror is $m=1$

Hence,

The magnification is given as

$m=\frac{-v}{u}\Rightarrow 1=\frac{-v}{u}\Rightarrow v=-u$

Now according to mirror formula

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\Rightarrow \frac{1}{-20}=\frac{1}{-u}+\frac{1}{-u}$

$\frac{1}{-20}=\frac{-2}{u}\Rightarrow u=40cm$

Hence the object distance of the object from mirror is $40cm$.