Parallel electromagnetic waves with a common point of origin, the same frequency and phase, and propagating through the same medium, form an advancing wavefront perpendicular to their direction of travel.

  • Point sources emitting light in all directions produce spherical wavefronts.
  • Lasers that form a pencil of light made of parallel rays produce waves with flat wavefronts.
  • An electromagnetic wave with a flat wavefront is called a plane wave.