system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If a glass is clear and water is clear, why can you see water in a glass?

I mean, it's not like there are shades of clear. Why can't we see air either?
gpt
You can see water in a glass because the two materials have different refractive indecies. RI is a measure of how light moves in a given material compared to in a vacuum. The refractive index of glass is about 1.46 and the refractive index of water is about 1.333. This difference makes light travel differently so they appear as two different objects. (RI is also why a straw appears "bent" in a glass of water).


You can't generally see air because you're surrounded by it. However, if the layer of air is thick enough you can see it. If you look up you'll see that the sky is blue. This is due to the atoms in the atmosphere scattering incoming sunlight.