It consists wave fronts. The lines normal to the wave fronts are called the light rays.
If the phase fronts are concentric spheres, the light is called a spherical wave, and if the phase fronts are parallel planes, the light is called a plane wave.
For a plane waves the rays are parallel whereas for the spherical wave, the emerge from the center of the spheres.
If the source is a a finite distance, the appropriate model is the spherical wave model, and if the source is assumed to be at infinite distance, the plane wave model is appropriate.
A plane wave can be represented by a wave function which is a composite function of space and time
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