Here's why:
* Refractive power is a measure of how much a lens or curved surface bends light.
* Diopter is defined as the reciprocal of the focal length of a lens measured in meters.
* D = 1/f (where f is the focal length in meters)
So, a lens with a focal length of 1 meter has a refractive power of 1 diopter. A lens with a focal length of 0.5 meters (50 cm) has a refractive power of 2 diopters, and so on.
Important Note: Diopters are used for both converging (positive) and diverging (negative) lenses. A converging lens has a positive diopter value, while a diverging lens has a negative diopter value.
