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One of the very first basics of the double slit experiment is that you have a single photon source that is directed at a reference screen. There, in contrast to the physics of e.g. a pistol bullet, the photon particles are distributed completely randomly. Is this correct?

Then I have difficulties to understand this. Because if I can develop a single photon source and direct it to a target and there all photons hit somewhere, I would assume that my device has a problem and not that there is a physical component here due to which all photons distribute themselves randomly on the target.

In addition, I do not understand, in which area random distribution takes place? Somehow a target area must be defined from the photon source, otherwise the photons would be distributed on the whole screen and beyond.

So is the photon source actually a "light beam" like a laser beam, which in the double-slit experiment completely covers the slits and within this beam, however, only single photons are emitted?

user276771
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  • see https://www.sps.ch/artikel/progresses/wave-particle-duality-of-light-for-the-classroom-13/ referred in my answer here https://physics.stackexchange.com/questions/388026/is-the-double-slit-experiment-performed-measuring-single-photons/388050#388050 – anna v Feb 12 '23 at 05:22
  • Every laser has a tiny aperture ... and the diffraction is huge .... the beam is not a beam until a lense is added if front! – PhysicsDave Feb 12 '23 at 16:18

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You take any source of light and create a beam of desired width using solid non-transparent obstacles. Then you put dark half-transparent absorbers in the path of the beam, e.g. darkened glass slabs. When there is enough of these obstacles in the path, the light will get so weak that there is a good argument for there being less than one quantum of energy $hf$ between the double slit and screen at any time. With a weak-enough light, the screen where the beam falls will evolve quasi-random spots where the screen interaction with the light resulted in chemical change (or electric current detection in CCD detector). The whole pattern is not completely random, because it still manifests interference stripes.

Ján Lalinský
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  • Re, "any source of light," If you want to see a strong interference pattern, then it had better be a coherent source of light. If the photons don't have all the same wavelength, then different wavelengths will create patterns with different spacing between the light and dark bands. If the photons don't all appear to emanate from the same single point, then the patterns won't all be in the same place. Either way, the interference pattern will become less distinct. – Solomon Slow Feb 12 '23 at 04:39
  • Indeed, to see the interference stripes the light has to come from single source, ideally of single wavelength, e.g. laser. – Ján Lalinský Feb 12 '23 at 04:42
  • You can take any source of light and make it coherent by passing it through a spatial filter (blocks all photons except those that appear to come from one particular point,) and passing it through a narrow pass-band color filter (blocks all photons except those in a narrow band of wavelengths.) This is extremely inefficient because you're throwing away most of the photons, but if your goal is to do the single-photon experiment, then efficiency of the light source is not going to be a significant concern. – Solomon Slow Feb 12 '23 at 04:42