A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters.
[Obsolete diopter, an instrument for measuring angles, from Latin dioptra, from Greek dioptrā : dia-, dia- + optos, visible; see okw- in the Appendix of Indo-European roots.]
di·optral (-trəl) adj.
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Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
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