tl;dr? For most of the time we’ve had photography, you couldn’t just snap a picture on your phone. You had to use specially made film. And taking the perfect photography meant you had to master some chemistry, from capturing light, to developing an image that you can see. When photons enter a film camera, these packets of light energy fly through the lens and hit a piece of plastic covered in what’s called an emulsion. And for you chemists out there, that might be kind of a misleading name. In chemistry, an emulsion is a mixture of two or more liquids that normally can’t be combined, like the oils and vinegar in a tasty vinaigrette. The stuff coating a film strip is technically closer to a suspension, which has tiny solid bits floating around in a fluid. Typically, it’s silver halide crystal floating around in gelatin, which solidifies on the film. Silver halides are just atoms of silver bound to atoms of any element known as a halogen, which includes things like bromine, chlorine, and iodine. Another name for a halide is a salt. Like, ordinary table salt is a halide made from sodium and chlorine. Halogens can react with metals to form halide crystals, in which the atoms become arranged in a very orderly grid called a lattice. And all those atoms are ionized: they either lose an electron or gain one to give them a charge. Until they’re struck by light. When a photon hits a crystal of silver bromide on a film strip, for instance, it knocks an electron off a negatively-charged bromide ion to create an atom of bromine. Then, that electron moves around the crystal lattice until it settles into a defect. And eventually it can combine with a nearby silver ion to turn it into a neutrally-charged silver atom.
As more light hits the emulsion this happens over and over again, and creates small pockets of metallic silver. There’s still plenty of silver bromide left over, though. The photons don’t change every halide. Only some of the silver atoms need to be changed in order for the film to be considered exposed. You still can’t see anything on the film at this point, though. In order to make that image appear clearly, the film needs to be developed. With another chemical reaction! Chemical developer vary depending on the kind of film you’re using. But the basic idea is that they react with the remaining silver halide crystals to create an image you can see. The important thing is that developers include reducing agents: chemicals that generally donate electrons to other compounds during a chemical reaction. In this case, the recipients of those electrons are the remaining silver ions. The silver ions in the exposed halide crystals, the ones with metallic silver in them, will have more reactions with the developer, creating more metallic silver in those grains. Now, you might assume that the metallic silver will look like shiny spoons in a fancy dining set. But in order to have that mirror-like surface, the silver atoms have to be arranged in a rigid, uniform pattern so they all reflect light the same way. When you look closely at a photography, though, you can see that the film grains are bumpy, and uneven. On a piece of film, metallic silver actually looks dark. If more photons hit a silver halide crystal, more reactions with the developer can happen, and that spot on the film strip becomes darker. Now, generally, when you take a picture, your image will have brighter areas and darker areas. There’s more light coming from the sky into your camera lens than from a shadow under a tree. And the more light that hits the film, the more metallic silver you get after it’s developed. So that part of the image will look darker. Basically, shadows will look bright, and highlights will look darker. Because the light and dark information has been reversed. We call this is a negative image. Photographers use negatives to make copies of their pictures. They shine light through the negative onto another piece of photographic paper, to make the exposure reaction happen again. But this time, the dark parts of the negative block light from reaching the copy. So when the copy is developed, the areas where the highlights were in real life show up brighter on the print. And vice versa with the shadows. This new photo is a positive image, like what you originally saw!
So if you get your hands on a film camera, give it a shot. It’s not the same as talking a selfie with your phone, but with a little chemistry knowledge, film isn’t so mysterious after all.
