The bending of light rays as they pass from one medium to another, such as from air to water or glass. As light travels into a denser medium (such as into glass from air, or from the vacuum of space to the Earth's atmosphere), it will be bent toward a line perpendicular to the interface (the border of the two media). Additionally, light rays of shorter wavelengths—such as the blue end of the visible spectrum—will be bent to a greater extent than rays of longer wavelengths. It is this principle on which a prism is based.
Printing inks are also based on the principle of refraction. The size and dispersion of the particles of pigment in an ink vehicle determine the index of refraction of that ink; the greater this index, the more opaque the ink film will appear when printed. Refraction is also the principle upon which a camera lens is based; light passing through a suitably shaped piece of glass can focus the rays on a single point, allowing a clear (or less clear) image.