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My question: In this version of the experiment, doesn't the single photon source's direction define which slit the particle passes through.

Also, with reference to the apparatus described here, where there is an additional single slit, does it mean to say that diffraction happens even for single photon's at the (first) single slit.

I understood what the experiment says after the particles pass through the slit, but I'm not able to understand how the single particle/photon source set up works.

Hope someone can help me understand how that works.

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Since no one has responded yet, I'm posting an answer I found on reddit, which helped me understand this a bit better.

Every beam of light has a diameter. It does not matter if it is a beam of high or low intensity (= number of photons).

If the distance between your slits is larger than your beam width, all of the photons will hit the part between the slits and no light will go through your double slit. So ... you construct your double slit so that your laser (or whatever light-source you are using) will illuminate both slits.

The whole point of the double slit experiment is that it proves light doesn't exist as particles. Light is a wave that kind of looks like a particle, sometimes. You cannot rationalize the experiment by thinking about photons as particles because that isn't what they are.

Credit: https://www.reddit.com/r/askscience/comments/2d1118/in_the_double_slit_experiment_why_doesnt_the/cjl0bed

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In the usual double slit set-up, it is assumed that the incident field is a plane wave. An ideal plane wave, and hence the photons that it supports, have an infinite extent and always cover both slits.

It's important to note that this is true even if there is only one photon in the field.

In an experiment, we cannot make a plane wave, so we make a beam wider than the slit distance.

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