eli5: why does glass absorb infrared and ultraviolet light, but not visible light?
In: Physics
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.
Light is part of a spectrum, meaning rainbow. The colours of the rainbow are just the parts our eye can see, but they stretch beyond red (into infrared) and violet (into ultraviolet). Redder waves are bigger. Purpler waves a smaller.
All of these colours, including the ones our eye can’t see, will behave differently depending on their size. The way that the atoms are arranged in glass mean that the big ones get trapped in the spaces between the atoms. The really small ones get sucked up by the atoms themselves. But light is in the happy middle ground and gets to sneak between the atoms while still being big enough to avoid getting sucked up by them. Visible light therefore can go through it as if there is nothing there.
This happens all over the place. X-rays for example are so small they can even go through the atoms themselves, meaning they are good for seeing through things like fabric and skin – useful for an airport security scanner. Microways are big enough that a metal grid in your microwave oven window is enough to stop them from escaping.
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.
Depends on the glass composition. For instance solar green glass, which is commonly used in automotive applications has a different makeup compared to glass used on a coffee table, or glass used on a skyscraper. Solar green does a nice job blocking some UV and some infrared, and allows for most visible light because that’s what’s required by law and practically what is needed for cars.
All of this can be changed in 3 main ways: change the recipe for the glass, add a piece of laminate between two pieces of glass or add a coating to the glass. That can impact color, light transmission, heat absorption you name it.
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
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
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!
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