A diopter or dioptre is a unit of measurement used to express the optical power of a lens or curved mirror, which is a physical quantity that describes how much the lens or mirror converges or diverges light. It is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal meter, 1 dpt = 1 m⁻¹. Diopters are used to measure the focusing strength of glasses or contact lenses, and they measure how powerful a prescription is. A higher prescription will have larger numbers, whereas a milder prescription will have smaller numbers. Diopters are also used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams. The optical power of a healthy human eye is about 40 diopters, meaning the ability to focus on an object or image 1/40 of a meter from the eye. By age 45, the human eye is typically a mere 1 diopter due to natural weakening of eye muscles that focus vision and hardening of the eye lens. A diopter chart contains lines with letters of different sizes, and each line of letters has a reading strength in diopters next to it. A lower reading power is required when the smaller letters can be read, and a stronger reading power is recommended if the larger letters are the only ones that can still be read.