system
Given the following post, respond as if you're commenting on Reddit:
human
What causes glass to be transparent? If atoms are mostly empty space, then why can't photons pass through that space and make every substance transparent?
gpt
> If atoms are mostly empty space, then why can't photons pass through that space 

Solids are mostly empty space. But the maximum dimension of the empty space (the biggest area of empty space you could find without any atomic nucleus in it) is very small compared to a photon. 

The spacing between atoms in a "typical" solid, speaking very roughly, is around 0.1 to 1 nm. 

The "size" of a photon is roughly the same as its wavelength, so around 400 - 700 nm.

As an analogy, chicken wire (In case you're from somewhere where this isn't a common term, it's the kind of wire net fencing used to enclose chickens) is mostly empty space, but you still can't throw a basketball through a wall of it, because the basketball is much bigger than the largest openings in the chicken wire.