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The photon or electron is just one subatomic particle, but if it hits the film and creates a dot visible to the human eye (btw, modern technology can do this), then the dot must be a collection of millions of atoms or molecules on the screen that have been transformed via chemical reactions triggered by that single photon or electron.

How exactly can it happen?

(Presumably the photon hits just one atom in the photosensitive plate (as in a double slit experiment), thus changing only that one single atom, a happening that is still microscopic and invisible to the human eye.)

Edit: If I rephrase the question and say "a few photons", they are still completely microscopic and my question will be the same.

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    $\begingroup$ Not sure how it's done today, but google for photomultiplier. $\endgroup$
    – pfnuesel
    Commented Jan 23, 2015 at 15:12
  • $\begingroup$ I assume it's similar to how a geiger-muller tube can produce an audible sound from just one high-energy particle produced by radioactive decay. $\endgroup$
    – Time4Tea
    Commented Jan 23, 2015 at 15:22
  • $\begingroup$ THIS article says we cannot see a single photonhttp://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html $\endgroup$
    – Paul
    Commented Jan 23, 2015 at 15:46
  • $\begingroup$ I believe in this double-slit experiment (hitachi.com/rd/portal/research/em/doubleslit.html) there is an amplifying apparatus (more like a photomultiplier) somewhere between the detector and the screen. $\endgroup$
    – Metrica
    Commented Jan 23, 2015 at 16:06

2 Answers 2

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but if it hits the film and creates a dot visible

I guess that You think of a classical film made from silver halide crystals in gelatin? Your assumption is quite good, only the "atom" is not the right thing. Research on the most sensitive films showed that about 4 absorbed photons are needed to transform one silver halide crystal into the "latent" picture form. This form is a electron trapped in some crystal imperfection, called a "trap" (trace amounts of sulfide are important for this) This "latent" sensiticed crystal then is preferably reduced to silver when the film is immersed into the developer, a catalytic effect. Crystals without this trapped electrons will not be reduced, at least not within the usual temperature and time used for that development. What is important here, this "development" (reaction with a reducing chemical) is a amplifying process, reducing millions of silver atoms (the whole crystal) induced by one of that trapped electrons.

PS The geiger counter tube and the photomultiplier mentioned in the comments above are good examples for similar action, because both contain "built in" amplifiers, but physical, whereas the photographic film was chemically amplified.

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  • $\begingroup$ Illuminating answer indeed! Thank you so much! So on the photographic film, what the photon has done is still microscopic. Could you please say a few words about the amplifying process, i.e., how one of the trapped electrons reduces millions of silver atoms? $\endgroup$
    – Metrica
    Commented Jan 23, 2015 at 16:05
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    $\begingroup$ "", i.e., how one of the trapped electrons reduces millions of silver atoms?"" The reduction is dome by the developer! The trapped electron is a catalyst for this reaction initially. After some silver is developed in the crystal, this already reduced silver acts as a catalyt for the further reaction. The reduction is all or nothing for one of those crystals. Chemically this is called autocatalysis. $\endgroup$
    – Georg
    Commented Jan 23, 2015 at 16:14
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    $\begingroup$ @Metrica when an answer is helpful to you, it is the custom in our site to upvote the answer of the person that helped you. So, Georg deserves an upvote, I guess. $\endgroup$
    – Sofia
    Commented Jan 23, 2015 at 16:16
  • $\begingroup$ @Sofia Yes of course. Still not familiar with this site, I didn't know it yet. $\endgroup$
    – Metrica
    Commented Jan 23, 2015 at 16:26
  • $\begingroup$ @Metrica : very well, you will learn, step by step. $\endgroup$
    – Sofia
    Commented Jan 23, 2015 at 16:28
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ALL reactions are a matter of energy, even when at the surface they may appear as something chemical, physical, .. etc. The visible results of the chemical reaction involves not only that one particle but also the kinetic energy and the transfer of that energy from the collision.

An everyday example would be to let a rock fall on the sand. The kinetic energy from the fall will dissipate (once the rock hits the ground) in the work of displacing the sand it hits. Your particle follows the exact same principle only in your case it will dissipate it's kinetic energy by chemically "displacing" the surrounding atoms.

Hope that made things a bit clearer (and if I misunderstood, apologies as this is my first answer/post) :)

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  • $\begingroup$ ""The reaction that you perceive as "chemical" in nature is simply the effects of a high ENERGY collision."" This is simply wrong. Chemistry is about one too some eV. $\endgroup$
    – Georg
    Commented Jan 23, 2015 at 17:01
  • $\begingroup$ "Chemistry is about one too some eV".. apologies, I can see now that my answer implied something different than what you said. I'd edited my answer to be more precise/valid. $\endgroup$
    – micmanos
    Commented Jan 23, 2015 at 17:17

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