Question

The distance between the pole and the centre of curvature of a spherical lens is 15 cm. The focal length of the lens will be

• A. 30 cm
• B. 15 cm
• C. 7.5 cm
• D. 3.75 cm

Answer 1

The correct option is C 7.5 cm

The distance between the pole and centre of curvature of a spherical mirror is called radius of curvature.
Let $R$ = radius of curvature of the lens and
$f$ = focal length of the lens
Given: $R=15cm$
The relation between radius of curvature and focal length of a spherical mirror is $R$ = $2f$
or $f$ = $\frac{R}{2}$
So, focal length ($f$) = $\frac{R}{2}$ = $\frac{15}{2}$ = 7.5 cm