State Snell's law of refraction.

Snell's law states that for a given pair of media and for light of a given wavelength,the ratio of the sine of the angle of incidence $(i)$ to the sine of the angle of refraction $(r)$ is constant.
Mathematically,it is expressed as:
$\frac{\sin i}{\sin r} = n_{21} = \frac{n_2}{n_1}$
Where:
$i$ is the angle of incidence.
$r$ is the angle of refraction.
$n_1$ is the refractive index of the first medium.
$n_2$ is the refractive index of the second medium.
$n_{21}$ is the refractive index of the second medium with respect to the first medium.