> 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. 
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.