Question

Focal length of a lens is half of the radius of curvature of the lens.

• A. True
• B. False

Answer 1

The correct option is B False

Lens has two curved surfaces, hence two radii of curvature and in general they are not equal.
According to lens maker formula,
$\frac{\text{1}}{\text{f}}$ =$\text{(n-1)(}$$\frac{\text{1}}{R_1}$-$\frac{\text{1}}{R_2}$$\text{)}$
where, $f$ is the focal length of the lens in the medium where it is kept and $n$ is the refractive index of the lens with respect to the medium. $R_1$ and $R_2$ is the radii of curvature of the two refracting surface of the lens.
So, in general focal length is not half of the radius of curvature. So the above statement is false. (Note that the above statement is always true for spherical mirrors but not for spherical lenses)