**1. n. []**

The mathematical description of refraction, or the physical change in the direction of a wavefront as it travels from one medium to another with a change in velocity and partial conversion and reflection of a P-wave to an S-wave at the interface of the two media. Snell's law, one of two laws describing refraction, was formulated in the context of light waves, but is applicable to seismic waves. It is named for Willebrord Snel (1580 to 1626), a Dutch mathematician.

Snell's law can be written as:

where

*n*_{1} = refractive index of first medium

*n*_{2} = refractive index of second medium

sin *i* = sine of the angle of incidence

sin *r* = sine of the angle of refraction.

**See:**
angle of approach,
angle of incidence,
critical angle,
Fermat's principle,
least-time path,
normal incidence,
ray tracing,
reflection coefficient,
refractive index,
tomography,
wave