Question

Define:
(i) Focus
(ii) Focal length, for a spherical mirror.
If the focal length of a convex mirror is $20\text{cm}$, find its radius of curvature.

Answer 1

Focus:
Focus of a concave mirror is a point on the principal axis at which the light rays incident parallel to the principal axis converge after reflection from the mirror.
Focus of a convex mirror is a point on the principal axis at which the light rays incident parallel to the principal axis, appear to diverge from after reflection from the mirror.
Focus is represented by letter '$\text{F}$'.

Focal length: The distance of the focus from the pole of the mirror is called the focal length of the mirror.
$\text{F}=\frac{\text{R}}{2}$

Given, for the convex mirror:
$\text{F}=20\text{cm}$
$\Rightarrow \text{R}=2\times \text{F}$
$\Rightarrow \text{R}=2\times 20=40\text{cm}$