Refractive index measures how much a medium slows down light

Refraction & Total Internal Reflection — AQA A-Level Physics

Key Definition

Refractive index — The ratio of the speed of light in a vacuum to the speed of light in the material. Symbol: n. It is dimensionless (no units).

$$n = \frac{c}{v}$$

n = \frac{c}{c_s}

$n$: refractive indexThe ratio of the speed of light in a vacuum to the speed of light in a medium. Determines how much light bends on entering the medium. of the material (no units)
$c$: speed of light in a vacuum ($3 \times 10^{8}$ m $s^{-1}$)
$c_s$: speed of light in the material (m $s^{-1}$)

The refractive indexThe ratio of the speed of light in a vacuum to the speed of light in a medium. Determines how much light bends on entering the medium. of air is $n = 1 (treat air$ as a vacuum for calculations).
Media more optically dense than air have n > 1.
The higher the refractive indexThe ratio of the speed of light in a vacuum to the speed of light in a medium. Determines how much light bends on entering the medium., the more optically dense the material and the slower light travels through it.
Typical values: water n = 1.33, glass n = 1.5, diamond $n = 2.42$.

Examiner Tips and Tricks

Always check your answer for refractive index is greater than 1.
If it is less than 1, something has gone wrong.
The refractive index of air is not given in questions -- always assume $n_{\text{air}} = 1$.