State Snell's law of refraction.

(N/A) Snell's law states that for a given pair of media,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: $\frac{\sin i}{\sin r} = n_{21}$,where $n_{21}$ is the refractive index of the second medium with respect to the first medium.
This constant value is known as the refractive index of the second medium with respect to the first.