Refractive index:

The index of refraction is the ratio of the constant velocity of light in vacuum (c) to the variable velocity of light in a transparent medium (v).

n = c / v

This property is also known as optical density and expressed as n_D²⁰. The number 20 represent the Celsius temperature of the sample while D represents the monochromatic D line of the sodium spectrum (wavelength: 589.3 nm).

The consequence of this differing velocity in different materials results in varying amount of bending of light in different substances. Snells's law illustrates a well-known relationship between the angle of bending and the refractive index:

n₁sin₁ = n₂sin₂

The value for the index of refraction of several materials is given below:

Material	Refractive Index
Diamond	2.4173
Polycarbonate polymer	1.6
Flint glass	1.575
Quartz	1.553
Rock salt (NaCl)	1.544
Hexane	1.3750
Ethanol	1.3611
Acetone	1.3588
Water	1.3328
Methanol	1.3288
Air	1.003

The refractive index of a substance changes if the temperature changes or if the color of the light used changes. Refractive index measurements can be used to determine the concentration of a solution or ascertain purity and identify a substance.

The n_D^value decreases as the temperature rises. To compare experimental results with those listed in standard tables, set at 20 degrees Celsius, the correction is as follows:

n_D²⁰ = n_D^T + (T - 20)(.00045)

The temperature T is in degrees Celsius.