Refractive index.

Loosely speaking, a number describing the ability of a transparent material such as glass, plastic, liquid or crystal, to bend (refract) light. More technically, the ratio of the velocity of light in a vacuum compared to the velocity of light in a given medium.

The refractive index of a vacuum is 1 (by definition) and the refractive index of air is very close to 1. Ordinary glass, however, has a refractive index around 1.5. Water has a refractive index of about 1.3.

Different types of glass and crystal have differing refractive indices, which is useful for lens designers, who can use different types of glass to correct different forms of aberration. ED and UD glass and fluorite crystals are common examples of this.

cf. ED, element, fluorite, refractive index, lens, optical glass, refraction, Snell’s law, transparent.

Entry last updated 2002-05-09. Term 1022 of 1487.

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