eli5: why does glass absorb infrared and ultraviolet light, but not visible light?
In: Physics
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.
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
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FYI: as my bleached out hardwood flooring and carpeting can attest to, glass does not block UV unless it is treated to do so.
A particular matter being transparent for a certain kind of light means that it is physically unable to tap into the energy provided at that the particular frequencies of the radition (“absorption”). Absorption happens when electrons in the matter resonate with the frequency provided. They can only resonate and hence absorb radiation when they are bound with the “right” strenght, like someone sitting on a swing: If you push in opposite directions too fast, the poor person will not move. If you push every other year, they will swing, but not gain energy either. But if you push just at the right times, they will swinger higher and higher… resonance!
Glass is transparent, because its electrons are either too stiffly bound (absorbing UV) or too losely bound (absorbing infrared), but non of them are able to absorb visible light.
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.
Wavelengths. Wifi is light. Bluetooth is light, radio, everything wireless is light. And to those wavelengths everything is clear like glass. For x rays your flesh is but your bones aren’t.
For different colored glass, the material allows only certain colors through.
Just the composition of it and how it absorbs or reflects certain wavelengths of light. Which is to do with chemistry/quantum physics and how materials interact with wavelengths of light.
A lot of decent explanations of how electrons absorbs photons, but most comments don’t answer OPs original question and many include misinformation.
Glass does not absorb (much) UV or infrared light by default. However, it is common more recently (last 10-15 years) for glass to be manufactured with a “low-e” coating (low emittance). This coating is made of a compound that specifically reflects (like a mirror) infrared and much UV light. Since UV and infrared are invisible to our human eyes, these coatings don’t look like mirrors, they look clear; but if you have infrared vision, they would look shiney.
How does a compound only reflect certain types of light? Many other commenters have attempted to explain and some are correct.
There is actually a lot of empty space in between atoms. Think of the size of an atom as a football field and the nucleus as a football in the center of the field with the electrons as M&Ms orbiting around the field and the stands. That’s a LOT of empty space between individual atoms; plenty of room for photons to slip through!
But how do most materials absorb most light and end up being opaque? Well, photons are absorbed by the electron “orbitals”, NOT by the electron (per se) or by the nucleus. The orbital is just the area within which the electron orbits, and they have weird shapes; it’s very complicated, so lets just imagine that some electrons orbit in the home team goal zone, some orbit in the away team goal zone, and some orbit in the spectator stands.
If a photon passes through the orbital of an electron, the orbital may absorb that photon; the energy of that photon is then transfered into the electron in that orbital. But here’s the thing: not all orbital absorb photons. Some orbitals will let photons pass right through them, others will only absorb photons of certain wavelengths (like UV or infrared).
Why don’t all electron orbitals absorb all photons? That has to do with complex quantum resonance and interactions between fermions (which I don’t fully understand). It also has to do with how many orbitals the atom has, how close they are to being full of electrons, what types of electron bonds they have with surrounding atoms, and many other factors that I can’t fully comment on.
Suffice to say that “low-e” coatings in glass windows are made of a material that researchers discovered that has atoms with electron orbitals that like to absorb (and then reflect) UV and infrared, but which ignore visible light. How fortunate for our energy savings that they discovered this, so that we can see beautiful things through our windows but not over-heat our buildings in the process!
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
Basically light wavelengths have different “distance”/frequency, which we observe as different colours (measured in nm), UV IR etc are not detectable by eye. The wavelengths (“distance”) needs to be identical distance in bonds between atoms or electrons to interact and pass energy (which is why light can heat up materials), if distance is not perfect it just passed through like in case of visible light and glass. Some one check I’m pretty sure it’s correct
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