refractive index

number that expresses the speed of light through a medium as a ratio between that speed and the speed of light in a vacuum

The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at times the speed in air or vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses and glasses. Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings.
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Noun
1. the ratio of the velocity of light in a vacuum to that in a medium
(synonym) index of refraction
(hypernym) ratio

Of a medium, the ratio of the velocity of propagation of an electromagnetic wave in vacuum to its velocity in the medium. (188 ) Synonym index of refraction .

INDICE DI RIFRAZIONE