The refractive index of a material medium
Refractometry

The speed of light in a vacuum is referred to as c. A constant for all electromagnetic radiation, its value, agreed at the 17th General Conference on Weights and Measures on October 20, 1983, is 299,792,458 m/s. This conventional value made it possible to redefine the metre as the length travelled by light in a vacuum in a time, in fractions of a second, equal to the inverse of the above number.

When propagating in a vacuum, light of frequency ν has wavelength λ₀ such that :

Passing through a homogeneous transparent material medium A, this light of frequency ν propagates at a speed V_A less than c . Its wavelength λ _A, for propagation...