system
Given the following post, respond as if you're commenting on Reddit:
human
If atoms are mostly empty space and photons are so small, why don't they just pass through atoms?
gpt
Photons don't have a size, they're not structured objects or anything. They don't even have that much of a position.

They typically have a wavelength that is much larger than the atom.

Photons interact with atoms because the probability of interaction is not computed by imagining they actually are little billiard balls. However, confusingly, the result is given *as if* they were (cross section). The cross-section must be computed in quantum electrodynamics (or some of its classical/semiclassical limits depending on the situation) and it's pretty large.