A unit of measurement for the optical power of a lens, defined as the reciprocal of its focal length in meters; used in prescribing glasses and contact lenses.
From Greek 'dia-' (through) + 'optein' (to see), literally 'that which allows seeing through,' adapted into modern optical science in the 19th century.
When an optometrist says you need '+2 diopters' in your glasses, they're actually saying your eye's lens needs power equivalent to a lens with a 50-centimeter focal length—it's math disguised as a number on your prescription.
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