How does photographic film work?

The discovery and invention of photographic film starts with one of those wonderful events in science: a complete accident. In 1727, a German scientist named Johan Heinrich Schulze was mixing a batch of silver nitrate when he discovered something amazing. Exposing the silver nitrate to light made it turn purple! Schulze found that by cutting out letters and affixing them to his flask, he could make letter shadows on the solution, areas that wouldn’t change. Only the parts of the solution exposed to light would change colour in this way. Other scientists took Schultze’s discovery and tried to make practical photographic film by dipping paper into the solution. The problem, though, was that the image wasn’t “fixed”. That meant that once the film was taken out into the daylight, it all turned dark, effectively blacking out the image. Almost a hundred years later, scientists learned that a chemical with a mouthful of a name, sodium hyposulfite, could remove the unexposed silver in a “ ...  more