system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did Theodore Case and Lee De Forest use a light bulb to transfer sound onto film?
gpt
If you look at sound as an electrical signal you will have a voltage that changes over time.   

Instead of using it to power an electromagnet to move a speaker membrane, you can use it to power a lamp, its brightness will depend on the voltage. If you then have a light-sensitive photocell that produces a current that depends on the amount of light that hits it you have to know created a way to transfer sound with light.

Photographic film is a medium that recodes the amount of light that hits it so by shining the light that is controlled by the sound on it when you move the film you can recode that pattern. When the film is developed you can shine a light through the film and the amount that hits the other side will have the shape of the initial sound wave.

There are lots of technical detail of exactly how those steps are done, and how you expose and read it from a small area of the film. There might be some non-linear effects you need to handle with amplifiers and/or filters but what is described above is the general idea of how you store sound on photographic film.