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Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by snell's law.

where θ₁ is the angle between the ray and the surface normal in the first medium, θ₂ is the angle between the ray and the surface normal in the second medium, and n₁ and n₂ are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.

When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as reracetion.