Question

A small candle is 38 cm from a concave mirror having a radius of curvature of 24 cm. (a) what is the focal length of the mirror? (

Answer 1

For a concave mirror, the radius of curvature is twice the focal length of the mirror:
$r=2f$
where f, for a concave mirror, is taken to be positive.

Re-arranging the formula we get:
$f= \frac{r}{2}$
and since the radius of curvature of the mirror in the problem is 24 cm, the focal length is
$f= \frac{24 cm}{2} = 12 cm$