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.

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.

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.

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.