law of refraction

noun

1. the principle that for a ray, radar pulse, or the like, that is incident on the interface of two media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the velocity of the ray in the first medium to the velocity in the second medium and the incident ray, refracted ray, and normal to the surface at the point of incidence all lie in the same plane.