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<a accesskey="3" href="page.php?w=plane_wave&amp;p=2">3.Next</a>
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<p>In <a href="page.php?w=physics">physics</a>, a <b>plane wave</b> is a special case of a <a href="page.php?w=wave_%28physics%29">wave</a> or <a href="page.php?w=field_%28physics%29">field</a>: a physical quantity whose value, at any given moment, is constant through any plane that is perpendicular to a fixed direction in space.</p>

<p>For any position  in space and any time , the value of such a field can be written as</p>

<p>where  is a <a href="page.php?w=unit_vector">unit-length vector</a>, and  is a function that gives the field's value</p><p>
<a accesskey="3" href="page.php?w=plane_wave&amp;p=2">3.Next</a>
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