Rami Arieli: "The Laser Adventure"

Rami Arieli: "The Laser Adventure" Chapter 1.1.2 page 1

1.1.2 Electromagnetic Radiation in Matter

(We shall use the words "electromagnetic radiation" and "light" as synonyms)

Light Velocity in Matter
When electromagnetic radiation passes through matter with index of refraction n, its velocity (v) is less than the velocity of light in vacuum (c), and given by the equation:

v = c / n

This equation is used as a definition of the index of refraction (n):

n = (speed of light in vacuum)/(speed of light in matter) = c/v

Gases, including air, are usually considered as having index of refraction equal to vacuum n₀=1.
The values of the index of refraction of most materials transparent in the visible spectrum is between 1.4-1.8, while those of materials transparent in the Infra-Red (IR) spectrum are higher, and are 2.0-4.0.