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refractive index, \(n\)

https://doi.org/10.1351/goldbook.R05240
The ratio of the speed of light in vacuum to that in a given medium.
Sources:
Green Book, 2nd ed., p. 30 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 989 (https://doi.org/10.1351/pac199668040957)