Welcome back everybody. Alright, so we're still talking about light, and it's interaction with matter. And how we're going to use those our understanding of the nature of light to decipher the properties of stars. Okay, last time we introduced this weirdness of quantum mechanics that light can behave as both a wave. Which is something like dropping a stone in water, and watching the waves ripple outward. Or a particle, which is something like a bullet, and, and you know, if you think about it, those are so totally, such totally different kinds of phenomena you'd be okay if you were to say like, that seems crazy, how could it possibly be that way? Well it is that way, nature's just that way and we have to sort of figure out how to you know, reconcile them and after a hundred years of quantum mechanics. Or quantum physics. We haven't really been able to reconcile them conceptually. We do have a powerful mathematics, which allows us to build things like computers. but, we really don't have an interpretation that allows us to understand or represent what these wave particles are and that's just sort of, you know, what perhaps quantum mechanics just telling us that at the very, level of the very small, nature gets very weird. So what I'd to do now is talk, go to the next level of our understanding of light and matter. Very important for astronomy but also it illustrates another of the parts of the weirdnesses of quantum mechanics. So one thing physicists recognized early on was that if you were to take a vial and fill it up with gas, say just hydrogen gas, and then run an electric current through that gas to heat it up what you found is it would glow. But it wouldn't glow with a continuous spectrum like blackbody spectrum. You wouldn't see a rainbow of colors if you were to take that light from the glowing tube and pass it through a prism All you would see were a few particular wavelengths of light being emitted by the gas. And more than that, if you were to change the, the vial of, the container of gas, and use a different gas. You would find once again, if you heated it up. You would find that their were only a particular, a few wavelengths being represented, but it would be different wavelengths than the other, element you used. So if you put in hydrogen, you'd see one range, one particular set of, wavelengths being emitted. And if you were to use sodium, you'd look, you'd see another particular set of wavelengths being represented. In fact, every element had it's own fingerprint of light. Had it's own fingerprint of particular wavelengths of light being emited. And this presented astronomers and physicists with an enormous puzzle. What model could they come up with of atoms that could, would, that would allow, that would only produce when you heated it up only certain wavelengths of light to be emitted? And physicists puzzled over this for years. And in order to solve the problem, they had to do something very dramatic. So let's imagine that we have. We're going to come up with a model for an atom. Let's imagine an atom being, let's take the simplest atom. Hydrogen, which is a proton, a positively charged proton, and around it we're going to have an electron, spinning around it. And the thing to understand is that, if the electron wants to move closer in or further away from the proton from the nucleus. It has to either gain or lose energy. If we want the electron to move further out, we've got to give it some energy to pull it farther away from the, the, neu, from the proton, and if it's further out and we want it to drop in it's got to give up energy somehow. Now that fact that only particular wavelengths of light were being shown in the spectra told us something about how the ele, electron was absorbing or emitting photons, emitting light, such that it could jump from one orbit around the electron to the other. Now, what physisicts would have expected is that the electron could be anywhere. It could be orbiting here, it could be orbiting, you know, there. It could be orbiting even further. There's no, restriction on the radius of the orbit, the size of the orbit. But in order to understand why only specific wave lengths were being emitted, the physicists had to imagine that the electrons could only be in very specific orbits. They could only be you know, one unit distance away from the proton, or two units, or three units, but never 1.5, or 1.6, or 1.2. So there were like, rail road tracks burned around the proton. That was the only place the electron could be. And what that allowed them to do then is say okay, if I want an electron to go from very close to the nucleus to very far from the nucleus then it only had, it could only make specific jumps. It could only jump from very, one very specific orbit to the next specific orbit and it would do that by absorbing a photon. And that leads us to the very important idea of absorption lines. So if I have a collection of atoms, that are, have, that most of the atoms, the electrons are in the lowest energy level, and I pass a flood of light of all different wavelengths through that box of atoms, the only photons that are going to be absorbed are those that correspond to energy jumps. That energy, the jumps between the the orbits. And so what that means is that what I expect to see if I looked at the spectrum of light, I would expect to see only certain wavelengths of light being taken out of the, the incoming light. I would see the atoms removing only those eh, wavelengths of light that corresponded to jumps. And we call that an absorption spectre. So if I shine black body radiation into a box of atoms on the other side, look what comes out on the other side. I'd expect to see still a black body spectrum but with certain chunks taken out at specific wavelengths. Some certain features absorbed. Eh, certain photons absorbed. And so, this is exactly in fact what we see in stars and the Sun is that we see a rainbow of light, we see a black body spectrum, continuous spectrum. But there are certain wavelengths removed and those wavelengths correspond to these photons being absorbed by the atoms in the in the light. sorry, by the atoms in the gas in the star. Likewise we can look at the other side, the flipside of this process which is where if we have a box of atoms where many of the atoms have, for whatever reasons, been kicked into higher orbits, and so they're orbiting very far from the, their nucleus. They can spontaneously drop back down and get closer to the nucleus. But in doing so, they have to give up energy. But because there's only certain orbits you can jump from. You can only go from a high orbit to one of the specified lower orbits via quantum mechanics. What that means is, there's only certain kinds of light that can be emitted by that atom. There's certain wavelengths of light that are going to be emitted by that atom. And that's what we call an emission spectra. So if we had a box of hot gas rarefied hot gas, so it wasn't black body, so the atoms weren't able to collide and jiggle together a lot the way they can in a black body. But what I would expect to have happen then is, the only light I am going to see coming out of that is light reflecting the specific quantum jumps that are possible for that kind of gas. And in that way, the light emitted or absorbed by a particular element is actually a fingerprint of that element. So, Hydrogen will only have certain wavelengths of light that it can emit or absorb. Helium will have only certain wavelengths of light that it can emit or absorb, all the way down every different kind of element has a different fingerprint in terms of its light and. This is enormously powerful for astronomy, because what it means is, is by analyzing the light coming from stars, coming from clouds of gas I can extract from it, by looking at it's spectrum, by looking at either the emission lines or the absorption lines, I can tell what that object was made out of. Even if it's millions of light years away, I can tell whether there's helium or carbon, or nitrogen, in that object. And I can exactly tell how much helium, nitrogen, and carbon are in there. So that is why quantum mechanics which is you know, specifies the fact that there are these discrete kinds of orbits, there's only certain orbits around a, an, a, around a nucleus. And that these quantum jumps. You may have heard the term, quantum jump. The quantum jumps are the jumps that the electron makes when it's jumping from a higher level to a lower level. Or a lower level to a higher level. These very specific quantum jumps, become so important for astronomy. They are the key to us understanding the fundamental con-, conditions. Inside astronomical objects. [BLANK_AUDIO]
