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Rugate filter
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<p>A rugate filter, also known as a gradient-index filter, is an <a href="/facts/Optical_filter/WdCmGqaq">optical filter</a> based on a <a href="/facts/Dielectric_mirror/c9wsmYGp">dielectric mirror</a> that selectively reflects specific wavelength ranges of light. This effect is achieved by a periodic, continuous change of the <a href="/facts/Refractive_index/yjkzvgeE">refractive index</a> of the <a href="/facts/Dielectric_coating/c9wsmYGp">dielectric coating</a>. The word "rugate" is derived from corrugated structures found in nature, which also selectively reflect certain wavelength ranges of light, for example the wings of the Morpho butterfly. </p>