Page "Speckle pattern" Paragraph 5

When a surface is illuminated by a light wave, according to diffraction theory, each point on an illuminated surface acts as a source of secondary spherical waves.

The light at any point in the scattered light field is made up of waves which have been scattered from each point on the illuminated surface.

If the surface is rough enough to create path-length differences exceeding one wavelength, giving rise to phase changes greater than 2π, the amplitude, and hence the intensity, of the resultant light varies randomly.