Optical PhenomenaInterference, diffraction, and polarization are three of the most obvious and fundamental phenomena of physical optics (the branch of optics that takes the wave nature of light into account). Interference and diffraction really should be treated simultaneously as they are both effects due to the combining of waves by to the principle of superposition. However, the term interference is usually reserved for situations in which only a few waves overlap, while diffraction considers the interference effects due to the overlapping waves from many point sources. Although interference effects might seem like fairly substantial evidence in favor of a wave theory of light, it is important to bear in mind that it can be given a full treatment from the point of view of Quantum Electrodynamics (QED), which is a particle theory of light. Fenyman makes this point crystal clear: I want to emphasize that light comes in this form--particles. It is very important to know that
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