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plane wave[′plān ‚wāv]
a wave in which at every moment the displacements and velocities of the particles in a medium (for mechanical waves) or the strengths of the electric and magnetic fields (for electromagnetic waves) are the same at all points lying in any plane perpendicular to the direction of the wave’s propagation.
Strictly speaking, real plane waves do not exist, since a plane wave that propagates along the x-axis must span the entire region of space with y- and z-coordinates from — ∞ to + ∞. In many cases, however, a bounded (with respect to the y- and z-axes) portion of a wave can be found wherein the wave is nearly the same as a plane wave. This is primarily possible in free space at such great distances from the source that the source can be regarded as a point. Sometimes a wave that is propagating in a bounded region may coincide approximately with such a portion of a plane wave. An example is an elastic wave propagating in a rod.