eli5: why does glass absorb infrared and ultraviolet light, but not visible light?
In: Physics
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!
Latest Answers