Index of Refraction

One reason why the wave nature of light was unknown for so long is that most optical phenomena only depend on the fact that light is a thing that travels in straight lines, whether as particles or waves it doesn't matter. This straight line is known as a ray. In wave terms, the ray of a light wave is a line which is perpendicular to the wavefronts and points in the direction the light is moving.

Because a ray is "a thing that moves", it has a speed. Light in vacuum travels at the fastest possible speed,

$$c=3\ten8\u{m/s}$$

which is known as the speed of light, although it would be more properly called the "speed of light in vacuum". When light enters other transparent or translucent materials, such as water or glass, it slows down; we will use the letter $v$ to refer to the light's speed in these media.

The amount by which light is slowed down in a material is known as that material's index of refraction, specified as $n$. It is defined as

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

Notice that the smaller the speed $v$ is, the larger the index is, and vice-versa. For instance,

Water has an index of refraction of $n=1.33$, so the speed of light in water is $v={3\ten8\over 1.33} = 2.25\ten8\u{m/s}$
Diamond has a higher index of refraction, $n=2.42$, so the speed of light in diamond is smaller: $v={3\ten8\over 2.42} = 1.24\ten8\u{m/s}$.

Note:

(This inverse relationship between $n$ and $v$ can be confusing to some students, and the name "index of refraction" doesn't give us any hints. It may be useful to think of the index as some sort of "resistance" or "viscosity" the material has for light: the higher the index, the harder it is for light to travel through it, and so the slower the light moves.)