Why are air, water, and glass transparent?

Many non-conductive materials can be transparent. Conductive materials like metals absorb light because their free electrons interact with photons. Non-conductive materials don’t absorb photons in the same way. So, most transparent materials tend to be non-conductive.

That includes numerous polymers like Plexiglas:

And countless crystals can be transparent. Even table salt is transparent if you grow it in large crystals. I work with some really weird crystals for infrared optics, stuff I’d never consider to be transparent but are. (Sodium chloride is, in fact, one of the options for optics. It just doesn’t handle humidity too well.)

(Table salt)

Solid carbon can be transparent, too.

A key factor is preparation. Table salt generally doesn’t look transparent because it is a pile of shattered crystals filled with voids, defects, and contaminants.

Likewise, this pile of glass used to be transparent. Now light has to pass through a maze of surfaces.

Grow table salt as a large crystal and then polish it and it’ll be transparent:

Aluminum oxynitride is generally made by sintering many small particles of the ceramic, resulting in opaque and translucent materials like this:

But with a sintering aide to close the pores in the ceramic, you can get panels like this and launch a thousand bad news articles about “transparent aluminum.” (AlON is not “transparent aluminum.”)

Likewise, a pile of alumina (aluminum oxide) dust is not transparent:

And when you sinter it into useful parts, alumina’s often opaque:

But when you grow the alumina as large crystals and polish it, you can get some very hard, transparent crystals like the panels on the nose of this Sniper targeting pod. (Apple also tried and failed to make alumina screens for its iPhones.)

And when nature grows a large alumina crystal, it often botches the job and lets it get contaminated with metals. For some reason, people pay a lot for these dirty, transparent crystals (called sapphire):

And ruby…

Then there are liquids that are transparent, like (almost) pure alcohol:

Liquid nitrogen:

And even hydrocarbons like gasoline (quite a stew of hydrocarbons) are transparent:

The first step to finding a transparent material is usually to identify a non-conductive material. Often no one bothers making them transparent, but they can. Even something like wood

https://youtu.be/d8cRJjfznEM