Basically, two waves diffracting at two narrow slits will interfere in an alternating constructive and destructive way ( there are many diagrams of this in double slit experiment . ) Light interferes with itself as a wave, but there is also light-to-light scattering ( Delbruck scattering ) which occurs when a virtual particle is exchanged between photons, similar to how electrons scatter off each other by exchanging a virtual photon.
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