Question

Object is $20\text{cm}$ away from the pole of a convex mirror of focal length $10\text{cm}$. Determine the position of image.

• A. $+\frac{20}{3}$
  
• B. $-\frac{20}{3}$
  
• C. $+\frac{10}{3}$
• D. $-\frac{10}{3}$

Answer 1

The correct option is A $+\frac{20}{3}$


Mirror formula for any given mirror,
$$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$$Given,
object distance, $u=$ $-20\text{cm}$
Focal length, $f$ = $+10\text{cm}$

Substituting the values,

$\Rightarrow \frac{1}{v}-\frac{1}{20}=\frac{1}{10}$

$\Rightarrow \frac{1}{v}=\frac{1}{10}+\frac{1}{20}$

$\Rightarrow v=+\frac{20}{3}\text{cm}$

Hence, option (a) is the correct answer.

Why this question: To understand the concept of mirror formula and sign convention.