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The Clue
Unit of refractive power of a lens (7 letter answer)
The Answer
DIOPTER
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Notes
The answer DIOPTER is so rare that it appears less than once every 2500 puzzles.
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Definition
DIOPTER as a noun:
1. (diopter, dioptre) = a unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in meters; used by oculists