Editing Wavelength (section)

=== Standing waves ===
A [[standing wave]] is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called [[node (physics)|nodes]], and the wavelength is twice the distance between nodes.

The upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box (an example of [[boundary conditions]]), thus determining the allowed wavelengths. For example, for an electromagnetic wave, if the box has ideal conductive walls, the condition for nodes at the walls results because the conductive walls cannot support a tangential electric field, forcing the wave to have zero amplitude at the wall.

The stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities.<ref>
{{cite book
 | title = The World of Physics
 | author = John Avison
 | publisher = Nelson Thornes
 | year = 1999
 | isbn = 978-0-17-438733-6
 | page = 460
 | url = https://books.google.com/books?id=DojwZzKAvN8C&q=%22standing+wave%22+wavelength&pg=PA460
 }}</ref> Consequently, wavelength, period, and wave velocity are related just as for a traveling wave. For example, the [[Speed of light#Cavity resonance|speed of light]] can be determined from observation of standing waves in a metal box containing an ideal vacuum.