I Can See Clearly Now, Sorta

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I saw this via @JenLucPiquant, and I have to say, the video only explains half the answer. It’s true that if a photon doesn’t have enough energy to promote an electron to a higher energy level, it won’t be absorbed — there is no allowable state for the electron, so the photon passes through unscathed. That’s all good. It explains why a material that is transparent in the visible part of the spectrum becomes opaque somewhere in the UV.

But what this doesn’t explain is why the same transparent material also becomes opaque in the IR. Here are a few transmission curves, and we see for BK7 glass, it is indeed transparent in the visible and the transmission falls sharply when we get to 300 nm. But over at the other end of the graph, we also see that the transmission falls when we get out near 2 – 3 microns, i.e. photons that have a bit more than 10% of the energy to bridge the gap being discussed in the video. It’s not just that, either. There are more substances on that data page, and other materials still, all with the same general behavior; the details are different, which is nice, because you can find a material that is transparent a little further into the UV, or out into the IR, according to your needs. That way you aren’t limited to strictly being in the visible part of the spectrum for your work.

Why is this the case? A visible photon has a few electron-Volts of energy, and that’s not enough to promote an electron, but the material becomes opaque at photon energies of less than one eV! The key here is that photons and molecules are choosier than was implied in the video. Not only do you have to have enough energy to complete the excitation, you can’t have too much, either. A photon cannot give up only part of its energy in an absorption — it’s all or nothing. For simple systems, like an atom, the photon energy has to be exactly the right value. (This being quantum mechanics, though, “exact” means “exact to within the limits of the Heisenberg Uncertainty Principle,” but it’s still close enough for government work). For composite systems, the energy levels can thicken into bands, but the same principle holds: too little, or too much energy, landing you in a region where the electron’s energy isn’t permitted, and there will be no interaction. Transparent materials are those that lack those energy levels. The light can’t interact, so it passes through.

2 thoughts on “I Can See Clearly Now, Sorta

  1. OK, I’m thinking about this. You’re saying the situation where material is opaque to lower energy photons but transparent at higher is the opposite case from the video, i.e., higher energy photons have too much energy to be absorbed, resulting in transparency, lower energy ones are “just right.” I assume, then, that lower still would again be transmitted, correct?

    Also, what explains a green glass filter? White light falls on it, it’s transparent only to that specific energy, all others (in the visible spectrum) have an energy level that can be absorbed? I’ll speculate that there are various types of atoms and that there’s a “match” in there somewhere for everything except the “green” photons. Is this close?

  2. Yes — your wi-fi and cordless/cellular phone signals make it through walls, for example.

    Color is from the energy gap being not spanning the entire visible part of the spectrum. Clear transparent crystals can be “doped” with impurities to give them color. This changes the absorption profile of the material.

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