Snell's law
![Refraction of light at the interface between two media of different refractive indices, with n2 > n1. Since the velocity is lower in the second medium (v2 < v1), the angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray in the higher-index medium is closer to the normal.](/Images/godic/202502/11/Snells_law2.svg5700.png")
![An 1837 view of the history of](/Images/godic/202502/11/Snell_Law_of_Sines_18375700.png")
![Christiaan Huygens' construction](/Images/godic/202502/11/Huygens_Refracted_Waves5700.png")
![](/Images/godic/202502/11/Snells_law_wavefronts5700.gif")
Snell's law (also known as the Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.