The old fashioned black & white photography
In the old days (last century) photographers and hobbyists printed their own photos in their own dark room on photographic paper.
It still exists and still can be done!!
That paper contains a layer of very fine cristals of Silver bromide.
At the moment that light radiation (energy) reaches such a cristal, it is - in a way - affected:
AgBr(s) Ag(s) + Br2(g)
Ag(s) is black, and Br2
(escapes as a gas)
On the paper, immediatly after the lightning, you still can see nothing of this process, but that affection occurred on many places of the paper.
Then, you must submerge this paper in a developer bath (1).
Every affected cristal (has already a littlebit of neutral silver) wil start to react with the substances in that bath: hydroxyde and dihydroxy benzene.
2AgBr(s) + 2KOH(aq) + H2O + OH-
2Ag(black) + more substances
The more light fell on the cristals, the more silver will appear and the more black will be the result.
At the end there is still a lot of unused silverbromide and that - of course - may not stay there.
That would cause problems at the moment the film of the photopaper enters a light room.
The remained silverbromide must be taken away (fixed) with a solution of ammoniumtio sulfate.
AgBr(s) + 2(NH4)2S2O3(aq)
NH4Br(aq) + (NH4)3(AgS4O6)(aq)
At the end it is only a question of drying paper and film.