In empty space light travels at $c = 3.0 \times 10^{8}\ \mathrm{m/s}$ — three hundred million metres per second, seven and a half laps of Earth in one second. Nothing beats it. But push light INTO glass or water and it genuinely slows down. Each material gets a report card for this: the index of refraction, $n = \dfrac{c}{v}$ — read aloud: n equals c over v, where $c$ is that vacuum speed and $v$ is the slower speed light manages inside the material, both in m/s. Water scores about 1.33, glass about 1.5: the bigger the $n$, the harder the medium leans on the light.

Two gifts come built in. First, $n$ is a speed divided by a speed, so it wears no units at all. Second, it is never less than 1 — light never speeds up inside stuff — so an answer like 0.7 is the formula's own way of telling you the fraction went in upside down. To keep the arithmetic kind, this lesson writes every speed as so-many $\times 10^{8}\ \mathrm{m/s}$: then $n$ is just 3 divided by a small friendly number.