eli5: why does glass absorb infrared and ultraviolet light, but not visible light?
In: Physics
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Materials are made of atoms.
Each different type of atom, compound or molecule interacts with light differently.
This is mainly because of how their electrons are arranged around the atoms and molecules.
Some compounds absorb some frequencies of light and not others.
The atoms in glass used for windows don’t interact with visible light because their electrons aren’t arranged in a way to do so. But they do absorb UV. That’s why they’re good for windows!
Other glasses are good at absorbing X-rays and so are good to use in X-ray machines.
Things to google: electronic band structure & optical absorption
It has to do with how light interacts with matter.
To absorb light, you need to have things work just right. You may have heard that light is quantized, what this means is that it only gets absorbed in specific chunks, one photon at a time. And all the energy of that photon has to go somewhere.
It turns out there are a few different places for that energy to go, and since each color of light has different energy, those different absorption mechanisms affect the colors differently.
Ultraviolet has the highest energy, it’s absorbed into the electrons in a material, kicking them up in energy or ejecting them from the atoms entirely. Infrared light is absorbed into the vibrations of the atoms and molecules in a material. For glass, visible light isn’t high enough energy to be absorbed by the electrons and too high to be absorbed as a vibration. Remember, it’s all or nothing – you can’t absorb half a photon. It gets a bit more complicated since you also have to absorb the momentum of the photon, and not matching the quantized momentum kick will lead to the photon not getting absorbed either.
Different materials have different thresholds for these absorption methods, and a huge difference is whether things are metals or not. Metals have completely different architectures for their electrons, but the basic concepts of “need to absorb a whole photon” still apply.
Not sure of the exact answer, but I did want to point out one thing. If there was another material that absorbed visible light, but not infrared and/or ultraviolet light, we’d just see it as a normal solid material. We see glass as different from all other solids because we can see through it. If our vision was based on the infrared spectrum, what we think of as glass now wouldn’t be glass to us anymore
Due to how electromagnetic waves (light) interact with atoms, many/most materials can block/absorb some wavelength ranges and other wavelengths pass through them. For example wifi signals can pass through walls to some thickness.
Glass just happens to pass through the range which human eyes can see. If humans had evolved to see in a different wavelength range, may be glass would be an opaque material like ceramic and iron or copper would be a see through material. The tags infrared, UV and visible are just based on human vision and have nothing special about them with respect to electromagnetic waves.
“Glass” is a big concept, and different types of glass absorb or reflect different wavelengths of light differently.
Ultraviolet for instance is neither reflected nor absorbed by most types of glass used in the home or vehicles. We have the technology to make UV-blocking glass, it just has a slight cost premium. You, the consumer, can buy it! I highly recommend it, because the damage that UV light does to the interior of your house and car is real, but especially the damage it does to your physical body. You don’t want to get skin cancer or premature aging.
Infrared has less horrible effects than skin cancer but its effects are more relevant to your day-to-day in that it affects your home heating/cooling bill, or in the case of homes in the US, whether your home is livable at all when the grid goes down. Wealthy homebuyers and architects love using glass, but the panes they typically use are cheap (so they can use a lot of them) and without a powerful HVAC system will turn your house into an oven during the day and an icebox at night. Different panes like triple-pane glass, different formulations of glass, different impurities, or just using shutters can all completely change this. Green engineers redesigning the suburban home consider glass one of their top priorities, as much as saltwater batteries and solar panels.
Visible light can in fact be blocked by glass, and even blocked one-way, as in one-way mirrors. “Colored” glass by definition blocks all visible light except one color, and that’s medieval technology. I’ve even seen privacy glass that can be electronically turned off (completely transparent) or on (opaque), and solar glass that not only absorbs but photosynthesizes impinging light.
Some of the other top responses do a somewhat decent job of approaching the answers to this problem but honestly, the real answer is somewhere between “we sort of know” and “you really need a PhD in physics or material science”. For example, take most of the top responses as ask this:
What’s the difference between frosted glass (found in shower doors or interior separation), silvered glass (also called a mirror), a lens, tinted glass (your car windows), polarized glass (your sunglasses), and low-emissivity glass (your double pane windows) and you’ll very very quickly run into some terrifying quantum phenomena and run into issues involving coherence, polarization vectors, complex index of refraction, permittivity tensors, evanescent fields and plasmon-polariton interactions. It really is a very simple question with a horrendously complicated answer that is barely captured by even one textbook.
So why do some varieties of glass absorb IR and UV? Because over the past millennium, we tried adding random shit to hot sand and cooling it in different ways until we found things that did that and it is in the nature of those types of glass to do that.
I wouldn’t think infrared is absorbed. Have you been in a greenhouse? It’s hot AF. That heat got in there thanks to the radiating of heat via IR.
My understanding could be way off. Please correct me if I’m wrong.
p.s. glass reflects visible light too; that’s why you can see (a partial) reflection in glass! Whether a photon is reflected or not is governed by QM probabilities
This isn’t a full answer, more a comment. But if glass didn’t allow visible light through we wouldn’t use it as a window – we’d find some other material. And since we don’t see in infrared or ultraviolet then we don’t care if our windows absorb those frequencies. Glass is a Goldilocks material for this reason.
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