What is Refractive Index?

In short

The refractive index of a material is the ratio of the speed of light in vacuum to the speed of light in the material. It tells you how much the material slows light and bends it.

In simple words

It is written $n = \dfrac{c}{v}$ , where $c = 3.00 \times 10^{8}\ \text{m/s}$ is the speed of light in vacuum and $v$ is its speed in the material. Because it is a ratio of two speeds, it has no unit. It is always at least 1. A larger refractive index means slower light and a larger bend when the light enters the material from air.

Example

The refractive index of glass is about $1.5$ , so light in glass travels at $v = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{1.5} = 2.0 \times 10^{8}\ \text{m/s}$ .

Also searched as: index of refraction, refraction index.