The Double Slit Diffraction Experiment Those of you who took physics in school remember the double slit diffraction experiment. It is an obvious infraction of the law of cause and effect for a single photon to pass through two slits at once. So there must be at least two photons passing through the two slits at once in order for the diffraction pattern to appear. We may ask, is the source of light really producing single photons or is it producing single packages of photons? Obviously it is the latter. This would indicate that light produced by a chaotic source such as the sun or incandescent bulb, is really semi-coherent. And of course it must be for us to see it. If it were truely noncoherent all the waves would cancel each other out statistically and we would not be able to see it at all. So the answer to the situation is that the source of light with its filters are producing packages of photons which then are broken down into smaller packages at the first slit by diffraction and then after some of the smaller packages pass through the second two slits they interfer on the photographic plate producing the diffraction pattern.