Alexander Fufaev
My name is Alexander FufaeV and here I write about:

What is the Optical Power of a Lens?

Important Formula

Formula: Refractive power of a lens
What do the formula symbols mean?

Refractive power

The refractive power of a lens is the reciprocal of the focal length and is usually specified in dioptres (\( \mathrm{dpt} ~=~ \frac{1}{\mathrm{m}} \)).

Focal length

Focal length is the distance between the lens and the focal point of the lens. The focal point is the point at which the lens focuses all light rays.
Thin lens - object width, image width, focal length

The optical power (formula symbol: \( D \)) indicates how strongly the light rays are refracted towards the focal point of a lens. The optical power is defined as the reciprocal of the focal length \( f \):

Thin lens - object width, image width and focal length are shown.

The unit of optical power is usually given in diopters (\( \mathrm{dpt} ~=~ \frac{1}{\mathrm{m}} \)).

Example: Eye

An average eye has approximately \( 22 \, \mathrm{dpt} \). So, when you hear something like: I have \( +2 \, \mathrm{dpt} \) at my left eye, you know that the focal length of their left eye is approximately \( f ~=~ \frac{1}{22 \, \mathrm{dpt} ~+~ 2 \, \mathrm{dpt}} ~\approx~ 41 \, \mathrm{mm} \). This person is farsighted!