The unit of refractive power in optics is a diopter, which measures what?

• A. The reciprocal of focal length in meters
• B. The curvature of a surface
• C. The speed of light in a medium
• D. The index of refraction

Answer: (A)

Refractive power is about how strongly a lens bends light, which depends on its focal length. The diopter is defined as the inverse of the focal length measured in meters. So a lens with a focal length of 0.5 meters has a power of +2 diopters, and a lens with a focal length of −1/3 meter has −3 diopters. This means larger diopter values correspond to shorter focal lengths and stronger bending.

The other ideas aren’t the unit of refractive power: curvature describes the shape of a surface but isn’t the unit itself; the speed of light in a medium isn’t the unit of power; the index of refraction is a dimensionless number describing light’s speed change, not the reciprocal of focal length.