Question

The radius of curvature of a spherical mirror is $30\text{cm}$. What is its focal length if it always forms a virtual image?

• A. $+10\text{cm}$
• B. $+15\text{cm}$
• C. $-10\text{cm}$
• D. $-15\text{cm}$

Answer 1

The correct option is B $+15\text{cm}$

We know,

$\text{Focal length}=\frac{\text{Radius of curvature}}{2}$

$\Rightarrow f=\frac{R}{2}=\frac{30}{2}$

$\Rightarrow f=15\text{cm}$

Also, convex mirror always forms a virtual image.
Therefore, the focal length of the mirror is $+15\text{cm}$.

Why this question? Concept: Convex mirror always forms a virtual image.