C.3.3—Refractive index

Define refractive index

The refractive index of a medium is $n=c/v$, where $c$ is the speed of light in vacuum and $v$ is its speed in the medium. A larger $n$ means a lower light speed in that medium.

Relate speed and wavelength

At a stationary boundary the frequency is unchanged. Since $v=f\lambda$, a lower speed means a shorter wavelength. For two media, $n_2/n_1=\lambda_1/\lambda_2$ when the frequency is common.

Use a graph or measurement

If a graph’s gradient represents $n$, state that interpretation before reading the value. For uncertainty, use the spread from suitable maximum and minimum lines or the specified uncertainty method.

Common trap

Do not use $n=v/c$, and do not assume wavelength stays fixed when light enters a different medium. Frequency is the quantity that remains fixed at a stationary boundary.