What does refractive index mean?
The refractive number (often also referred to as the refractive index) is an optics term. It identifies the refracting of light when transmitted through a transparent material, and is the ratio between the phase speed of light c0 in a vacuum and its phase speed c in a respective medium:
c0
n = ------
c
The term “refractive index” comes from the term ‘refraction’ and its appearance in the Snell’s law of refraction. This physical quantity has no units, and is therefore expressed as a number. It can also be given as a complex number, and then also identifies optical density, mostly however only the real portion, through which the light speed is determined in a medium.
Refraction can also be defined by radiation optics. According to the above Snell’s law of refraction, n corresponds to the sine ratio of entry and exit angles. Thus the angle of the “light beam” which passes through the media interface, are related to the perpendicular to this.
Thus one must note that refraction through most materials depends upon wavelength, which is described as dispersion.
Material |
Refractive index |
Vacuum |
1.0 |
Air (near the ground) |
1.000292 |
Ice |
1.31 |
Water |
1.33 |
Ethanol |
1.37 |
Diamond |
2.47 |