Refraction
REFRAC'TION, noun The deviation of a moving body, chiefly rays of light, from a direct course. This is occasioned by the different densities of the mediums through which light passes.
Refraction out of a rarer medium into a denser, is made towards the perpendicular.
Refraction may be caused by a body's falling obliquely out of one medium into another.
Refraction double, the separation of a ray of light into two separate parts, by passing through certain transparent mediums, as the Iceland crystal. All crystals, except those whose primitive form is either a cube or a regular octahedron, exhibit double refraction