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Fresnel lens
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| A Fresnel lens (left) compared with the solid lens
(right) to which it is optically equivalent. By reproducing the curved
surface of the solid lens as a series of rings separated by steps
on a thin lens, the Fresnel lens effects a great saving in weight,
allowing mechanically robust lenses of relatively short focal length
to be fitted to searchlights, spotlights, and automobile headlights.
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