system
Given the following post, respond as if you're commenting on Reddit:
human
What makes glass transparent?

At an atomic level, what allows glass to be like 1cm thick and still be transparent, but for example a cloth of only a few mm’s thick to be opaque?
(Hope this makes sense, English is my second language)
gpt
Two things make glass transparent. 


Firstly, it's molecular/atomic structure simply does not have an available state to absorb the energy of visible light. 


Light is made of photons. Depending on the frequency (colour within the visible spectrum but extending beyond it on either side), each photon carries a different amount of energy. Low frequency, like radio waves, have very little. High frequency, like x-rays, have a lot. In the visible spectrum, blue has about twice the energy of red per photon. That's not to say blue light is always more powerful, you can simply have more or less photons to get the same amount of power. 


Things like exciting electrons or causing molecules to rotate or vibrate take very specific amounts of energy. It's kind of like the concept of resonant frequency that you likely have heard of. The wrong frequency does nothing while the right one excites it. Similar idea here. 


So each material has certain energies that can excite it. And certain frequencies of light consist of particles of a given energy. Put the two together, and only certain frequencies of light can excite a given material. And exciting it can cause reflection or absorption of the light, depending on whether it gives it back or not. If the light can't interact with the material at all, it passes through pretty much unaffected. Ie, transparent. 


Glass just happens to have the right molecular/atomic structure that visible light doesn't interact with it. It's actually opaque to infrared and ultraviolet, just past the visible spectrum. Everyday glass is far from the only thing that's transparent to visible light, there's actually a lot of types of glass that are different materials, lot's of plastics, ice, water, air, diamond, quartz, sugar, etc. Other materials strongly block visible light. All metals for example strongly block most kinds of light, right from from radio waves up to the UV spectrum, as the free electrons that make them electronical conductora can absorb near any amount of energy. Even a few nanometers of metals can block light. 


Secondly, and actually the important one for your cloth comparison, it's fairly well organized on a more macroscopic scale. Small imperfections and defects, random jagged edges, random small particles, etc, cause random scattering of light. This highly distorts a clear image from making it through, so you'll never see through it. As well it cause more of it to be absorbed or reflected quicker, though not necessarily as fast as an truly opaque material. You can see light through thin cloth for example, you just can't see a clear image. 


A large quartz crystal (very similar to glass) is clear, smashed into sand (which is melted and then solidified to make glass) it is not really transparent. Water is clear, turbulent waves aren't. Ice is clear, snow is not. A hard candy is transparent, powered sugar is not. Polyethylene terephthalate plastic is clear (your plastic pop/water bottle), Polyethylene terephthalate plastic spun into very fine fibres and then all tangled up into a your polyester shirt is not. You'll notice the trend here is very unorganized transparent things just end up white.