system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If glass is tightly packed atoms (solid), how can it be completely transparent?
gpt
Density of atoms doesn't necessarily relate to the opacity of a material.

Basically, any atom has a cloud of electrons, which has specific energy levels. When light hits these electrons, they can cause these electrons to change energy levels - this means they can absorb specific frequencies. This is called atomic absorption.

When you have bulk material, like in glass, you no longer have specific, discrete energy levels. Instead, energy levels combine to form _bands_, and the energy difference is called a _band gap_. Just like atomic absorption, only photons with enough energy to cross the band gap can be absorbed.

So the answer is that glass has a band gap in the UV range, so visible light passes through fairly easily.