All light propagates at a finite speed, a speed called c, in vacuum, and slower in other transparent media. The reduction of the speed of light in a denser material can be indicated by the refractive index, n,When a beam of light enters a medium from vacuum or another medium, it keeps the same frequency and changes its wavelength. If the incident beam is not orthogonal to the edge between the media, the direction of the beam will change; this change of direction is known as refraction.Refraction of light by lenses is used to focus light in magnifying glasses, spectacles and contact lenses, microscopes and refracting telescopes.
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