eli5 why are films photos negative?
In: 0
Photographic film is limited by the chemicals we can find that are light sensitive and can react in a way to form an image. One of the earlier and most popular class of chemicals used are silver halides.
It turns out that silver halides, on exposure to light, can turn into metallic silver and elemental halogen (often starting with silver bromide, so elemental bromine). At the microscopic level, embedded in the film gelatin, these tiny silver crystals appear black. So it’s the fact that the chemical exposed to light turns black and the chemical that isn’t exposed to light doesn’t, plus the latter can be washed away, leaving the metallic silver behind, that results in an image that’s the opposite of the light exposure.
Colors are more complicated, but some work by having other chemicals or dyes of the appropriate color attached to the silver halide, influencing what color light it’s sensitive to or what color it appears as. But to the extent it’s based on silver halide technology, it will still be that the part exposed to the most light will be darkest.
As u/paulmarchant points out, color slide film works differently. There were, in fact, multiple chemical systems for color photography, some of which are still available.
Latest Answers