The original experiment did not rely on the emission of individual photons. It just observed an interference pattern that classical particles would not exhibit.

The original experiment did not rely on the emission of individual photons. It just observed an interference pattern that classical particles would not exhibit. Later experiments that relied on individual photons could detect them individually [0] Here's an article on single photon sources [1] [0](https://en.wikipedia.org/wiki/Double-slit_experiment#/media/File:Wave-particle_duality.gif https://en.wikipedia.org/wiki/Double-slit_experiment#/media/...) [1](https://en.wikipedia.org/wiki/Single-photon_source https://en.wikipedia.org/wiki/Single-photon_source)