How can a single photon or electron create a small visible dot on a photosensitive plate? 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. 
 A: 
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. 
A: 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) :)
