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I have read explanations and watched videos about the double slit experiment and the interference pattern and I believe I understand these.

However, I believe (correct me if I'm wrong) it is said, that a single slit experiment shows that the light/photon acts as a particle. But from what I've seen, the laser light passing through a single slit still spreads out, much like a water-wave or sound passing through a narrow gap doesn't only arrive in the "visible" area of the slit, but breaks at the corners and this is also follows the light-as-a-wave math (for example see a video by DrPhysicsA on YouTube at around the 20min mark, where he shows how the single slit light distribution is explained by the wave model).

So what is an easy experiment (even maybe a doit-at-home experiment, like a laser pointer and a dual slit with aluminum foil) to show that light also acts as a particle.

Not sure if that matters, but I've seen videos/images, where they show that on photographic film you can see the photon-particles as dots and only in doing the experiment multiple times they spread out in a probabilistic way that eventually forms the waves/interference pattern. I'm also aware that in quantum theory, like with electron spin, a 90° spin only pans out probabilistically over multiple experiments.

But I guess my question is: What is the most simple way (possibly DIY) to demonstrate that light has a particle quality (because I think a single slit doesn't do that, but I may be wrong or may have looked at the wrong youtube videos)?

EDIT: I looked at the comments and I kind of see, that apparently with above description is sending people on all kinds of tangents, so let me rephrase the question: What is the easiest to understand, most convincing and possible (easiest to DIY) experiment, that shows that light is not only a wave (for this there are ample experiments and indication), but also a particle (I guess there are digital photon counter apparatuses, but there must have been such experiments before computers).

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    $\begingroup$ Here; and here, and here; and here., but repeat for weak light: photons. $\endgroup$ Aug 2, 2023 at 17:35
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    $\begingroup$ Want this for one slit, right? $\endgroup$ Aug 2, 2023 at 18:05
  • $\begingroup$ Water waves and sound waves travel in a medium. Photons do not need a medium. Photons are always measured as individual particles. Photons diffract around edges or scatter off edges. A slit has two edges and this creates the overall spreading. EVERY light phenomenon can be explained with individual photon particles but only some can be explained with waves. Besides, what is a light wave if not billions of individual and coherent photons? $\endgroup$ Aug 2, 2023 at 20:01
  • $\begingroup$ Many experiments can show/interprete particle like behaviour of photons .... but they can never exclusively prove that light is a particle. If you have a very small wave in the EM field and it is able to transfer momentum to whatever you use to detect it with .... then is it a wave or a particle? Does it matter if a particle can have wave properties .... that's ok as well. $\endgroup$ Aug 2, 2023 at 22:55
  • $\begingroup$ @PhysicsDave I didn't mean exclusively. See my edit of the question. $\endgroup$ Aug 3, 2023 at 16:31

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One possibility to show the particle behaviour of light is to look at the photoelectric effect.

Basic Setup: You shine light on a metal plate and measure the number of electrons coming out of the plate. A certain amount of energy (specific to the material you use) is needed to 'kick' one electron out of the material. Interestingly,(in historic terms), the number of electrons (in reality one measures a current) does not increase with the intensity of the light you shine on the plate, but rather depends on the frequency of the light. This behaviour cannot be explained by a classical electromagnetic wave picture, but rather suggests that light comes in discrete packets (-> particles, named photons) which need to have the needed energy to kick the electron out. There is plenty of information about the photoelectric effect, but here is the paper of Einstein's explanation for which he got the Nobel Prize: http://astro1.panet.utoledo.edu/~ljc/PE_eng.pdf

Here would be a DIY implementation of the experiment: https://www.aapt.org/programs/contests/upload/beehler.pdf

I hope it helps and answers your question!

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  • $\begingroup$ Oops, I had initially dismissed this answer because it got downvoted by someone and on first glimpse didn't seem relevant. But after writing my own answer, I looked again and found that this one actually answers my initial question and even provides a DIY (yay!). I'll thus mark this as correct. Kudos for the new contributor! $\endgroup$ Aug 4, 2023 at 8:17
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You can use water waves and 2 slits to demonstrate interference too. While the underlying constituents of water are discrete (water molecules), this isn't evidence that they are operating as waves at the molecular level.

Similarly, I can have a classical light wave and exhibit classical diffraction. Your question relates to doing a measurement to show the particle nature. To be able to do this you need to be able to have a measurement device (eg. photomultiplier tube (PMT)) that actually measures a single particle of light.

So, if you use have 2 slits and shine light on it, and you place a PMT behind each of the slits. Then if you reduce the intensity such that you have only one "click" at only once PMT at a time, say only once a day (a very low intensity indeed). Then you remove the PMTs and and place your photographic film behind the slits, over time you'll build up the interference pattern. Did the particle go through one slit, given that you initially measured that it only went through one slit at a time? Answer: the particle went through both slits.

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  • $\begingroup$ Water waves and sound waves travel in a medium. Photons do not need a medium. Photons are always measured as individual particles. Photons diffract around edges or scatter off edges. A slit has two edges and this creates the overall spreading. EVERY light phenomenon can be explained with individual photon particles but only some can be explained with waves. Besides, what is a light wave if not billions of individual and coherent photons? $\endgroup$ Aug 2, 2023 at 19:41
  • $\begingroup$ I'm not sure what the point of your statement is. Every light phenomena can also be explained by a wave, but only some need to be explained with particles. If you use an electromagnetic wave at carrier frequency of 1 MHz, it'll be very difficult to measure a photon, but all its physics is described classically as a wave. $\endgroup$
    – JQK
    Aug 2, 2023 at 21:10
  • $\begingroup$ The fact that a medium is needed or not needed is not a necessary aspect of this question. $\endgroup$
    – JQK
    Aug 2, 2023 at 21:14
  • $\begingroup$ The EM field sees individual slits when the PMTs are close and both slits when the PMTs/screen are far. The EM field sees it all ..... most importantly the excited electron in the excited atom is already strongly interacting with the EM field, much before real photon emission. $\endgroup$ Aug 2, 2023 at 22:52
  • $\begingroup$ @JQK I'd tend to make this the right answer bc. it at least addresses the essence of my question (upvote for that at least), but maybe see the question edit. $\endgroup$ Aug 3, 2023 at 16:32
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Well, answering my own question (Note: this was written before I noticed the answer that I finally marked as correct, see my comment there).

Turns out that apparently Einstein got the Nobel price for discovering the particle like (quantized) nature of light.

https://www.nobelprize.org/prizes/physics/1921/einstein/facts/

If metal electrodes are exposed to light, electrical sparks between them occur more readily. For this photoelectric effect to occur, the light waves must be above a certain frequency, however. According to physics theory, the light's intensity should be critical. In one of several epoch-making studies beginning in 1905, Albert Einstein explained that light consists of quanta—packets with fixed energies corresponding to certain frequencies. One such light quantum, a photon, must have a certain minimum frequency before it can liberate an electron.

Also explained in detail in an MIT lecture with the photoelectric effect (within the first 45 minutes: https://www.youtube.com/watch?v=U6fI3brP8V4). The lecture there explains the experiment with the photoelectric effect, how the results didn't match the expectations (expectations based on non-quantized energy in light beams) and points out how Einstein came up with the formula that led to the Nobel prize.

(Note: With quantum theory, most people seem to talk about the double slit experiment as if it's the awesome thing, but apparently that was well known and understood and common knowledge before the discovery of the photons and is only used in popular science because it's easy to understand and even to demonstrate in DIY terms, while there is little talk about how the photon was discovered or how it can be demonstrated, because both are apparently non-trivial).

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