 So, now that we understand both fluorescence and phosphorescence and the differences between them, it can be a little tempting to think that everything you see that glows is either fluorescing or phosphorescing, but that's actually not true. There's a lot of different types of luminescence, a lot of different types of giving off light, and we can distinguish between a few other flavors of photo-luminescent effects. So, first of all, just as a reminder, we have fluorescence and phosphorescence. Fluorescence, if I pull up a picture over here of some examples of fluorescent objects, either a fluorescent light bulb or some fluorescent markers or the fluorescent pieces that are embedded in the printing on dollar bills, both in the U.S. and in other countries. Those are all examples of fluorescence. We know they fluoresce because if we turn off the source of energy that's exciting the molecules, then the glowing stops immediately. And that's because that's a very rapid process that's not spin forbidden as opposed to phosphorescence. And if we, I'll show you some examples of phosphorescent objects, so glow-in-the-dark stars on a child's bedroom wall or plastic glow-in-the-dark toys that you have to charge up in the light glow for a few minutes after the light is turned off. Those are phosphorescent. The emission is spin forbidden for a phosphorescent process. But in both of those examples, the source of the excitation is the same. In both cases we shine a high energy photon on the molecule, it gives us back a lower energy photon either much later in the case of phosphorescence or immediately within nanoseconds or so in the case of fluorescence. So for both of those cases we've excited the molecule with light and it gives back light of a lower energy, lower frequency, longer wavelength. There's other types of luminescence where the excitation doesn't happen with a photon. For example, if I pull up a few more examples here, you're all familiar with glow sticks and because of the color of the glow sticks you might immediately think that's fluorescence or phosphorescence going on. If you think about it, you realize that can't be the case because, first of all, they glow for some period of time so it can't be fluorescence but you don't have to charge them up with light. You don't have to put them under a light source to make them glow. In fact, if you've ever used a glow stick, you know that you have to break them. There's a small glass tube inside of this tube that you break, mixed two chemical reagents together. When those two reagents mix with one another, that chemical reaction is the source of energy. That's a completely different type of photo luminescence that we call a chemiluminescence and the source of the energy that causes those photons to be emitted is in fact a chemical reaction. Other than that, the excitation happens from a different source. This chemical reaction provides enough energy to get the molecule into some electronically excited state. After that, the emission may take place in much the same way so it may be much the same types of light that are given off. It's just the source of the excitation that's different. Another example, if I switch to another example, I have here a picture of some fireflies. That's supposed to be a firefly glowing on some pieces of grass or a glowing jellyfish. These are examples of living creatures that give off light that they produce themselves. Those are examples of bioluminescence. The only difference between a bioluminescent living organism and a chemiluminescent glow stick is that in this case the chemical reactions are being produced inside the body of some living organism. We would call that a biochemical reaction. There's still chemistry going on that excites molecules into an electronically excited state. Because that chemical reaction is taking place inside a living organism, we call that a biochemical reaction instead of a chemical reaction. We can distinguish bioluminescence glowing of some living creature from chemiluminescence where we've engineered the chemical reaction ourselves in a laboratory somewhere. As one slightly more exotic example of a type of luminescence, I'll show you a picture of an additional type of luminescence called triboluminescence. The best known example of which might be this wintergreen lifesaver effect. If you've ever done this yourself or heard about it, if you take a wintergreen lifesaver, specifically wintergreen, it doesn't work with other flavors, and you either chew it in the dark yourself, it will give off little sparks, not electrical sparks, but it will give off photons, it will create light. Here's an example of crushing a wintergreen lifesaver in a wrench. That's enough to cause the wintergreen lifesaver to emit light. It will luminesce, and the source of the energy in this case is friction or mechanical work. So we've put enough energy into the molecules of this wintergreen lifesaver by crushing it, by compressing it, by causing the friction in the molecules to rub against one another, and that is enough energy to excite some of the molecules into an electronically excited state when they fall back down and they give off visible photons. So that's an example of triboluminescence. Tribal in this case comes from the word, and that root is based in the word for friction, so this is luminescence due to friction or due to mechanical work. So the point is not all light that is emitted by chemical molecules is the same type of luminescence depending on where the source of the energy that gave rise to that emission comes from. We categorize those different types of photoluminescence differently.