The incident ray, the refracted ray and the perpendicular to the interface between the media at the point of incidence of the ray lie in the same plane. The ratio of the sine of the angle of incidence to the sine of the angle of refraction for a given pair of media does not depend on the angle of incidence:
sin a / sin y = n21
where n21 is the relative refractive index of the second medium relative to the first
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