[MUSIC] So far, most of the things we've talked about in this class have been pretty good black bodies. So I've sort of been slinging epsilon values around, but just calling them one in my mind at least. Because most things are pretty good, most condensed matter is pretty good at absorbing and emitting all the different frequencies of infrared light. But gases are different, because they're so simple. They only have very simple modes of vibration. If you take say, carbon dioxide and you freeze it into dry ice, into condensed form, it's got all these other things around it and so it can kind of, it's like a blop of Jello. It has all different kinds of vibrational frequencies and kind of absorbs, sort of, everything in a condensed state. But when the CO2 is floating off by itself and not touching anything else very much, it can only vibrate in very specific ways. And so, it's very gases that turns out are very choosy about the kinds of light that they can absorb and emit. So the way we've done this for black bodies, epsilon is a value close to one and that works for the ground and for the plane of glass in our simple model. But for gases in the real atmosphere, epsilon value is not at all one. It's much smaller than that, because gases are so picky. [SOUND] Just as an aside, before we go on. I want to explain that most of the colors that we observe, most of the light we see in nature around us is got its color not because of absorption and emission by vibration of the atoms, but actually because the electrons exist at different energy levels and they can hop up to higher energy levels and pop back down again. And when they do that, they absorb or give off energy. So most things that are good dyes like the red stuff in this chalk has energy levels that are fairly close together. And so when an electron falls down that much, it gives off light that has that much energy, which turns out to be in the visible range and so we can see it. But gases are so simple that the energy levels of their electrons are so far apart that what they mostly do is make light in the ultraviolet that we can't see and this is why gases are transparent to the visible light that we can see. Some exceptions to that are chlorine gas is actually green. Chlorine is a big atom with lots of electrons on the outside and some of them are bound loosely and so it has a visible, it can actually see it in visible light. And O2 is a brown color, this is something that you find in urban smog when they have ozone alert days. If you can see far off into the distance, you can see this sort of brown layer and that's what this is. And so that's mostly what does the colors that we see, but what we're interested in for the energy in the atmosphere is back to molecular and atomic vibrations. So for a gas to absorb light or to emit light, because it's always a two-way street. If it can absorb, it can also emit. Two things must be true. One is that the frequency of the light has to be pretty close to the frequency of the vibration. Has to be kind of a match, you have to tune, has to be well tuned. But the other thing that's very important is that the vibration of the molecule has to create a fluctuating electric field, an oscillating dipole and most gases in the atmosphere are not really capable of doing this. So the main gases in our atmosphere are consist of molecules that have two identical atoms in them each, O2 or N2 are mostly what the atmosphere is made out of. So those are totally symmetric. One oxygen is the same as another. And if you vibrate it when their farther apart or closer together, it's still always symmetric. There isn't a plus on one side and a minus on another side and that's why these major gases in the atmosphere are not greenhouse gases. They don't effect the climate of the Earth in the greenhouse effect sort of a way. So carbon dioxide is a symmetric molecule in its resting state. The carbon is in the middle and then we have these double bonds to the oxygens on either side. It doesn't really matter to us that these are double bonds instead of single bonds, they still sort of act like springs and they can still vibrate. And so at first glance, you might think this is symmetric too and so it wouldn't be a greenhouse gas, but there are modes of vibration of this molecule that breaks the symmetry and the one that's most important is the bend. So you're sort of bending the thing like this. And when you do that, the oxygens have sort of a minus charge to them and that leaves sort of a positive charge on the other side. And then when it swings back the other way, the electric field is gonna flip and so you've got this oscillating dipole here. And so this mode of vibration of the CO2 molecules, the one's that's most important for climate. There are two other modes, there's a symmetric stretch, which is kinda like this. And then there's asymmetric stretch, which is kinda like that. So the asymmetric stretch is Infrared active, because it's broken the symmetry as you might have figured out. But it turns out that this mode of vibration is not as important to climate, because there's just less light there at this frequency. So this is the frequency that is the most important to absorbing light that's coming up from the ground. [SOUND] Other greenhouse gases, the main ones are water vapor. Water has the oxygen has two lone pairs of electrons on it, which tend to crowd the hydrogens off to one side and so the water is an asymmetric molecule even at rest when you write it. And so it's got a dipole moment, even when it's just sitting there. And so there's lots of different ways you can vibrate this or you can just spin it around and that will create an oscillating electric field. So water has a very, very complicated absorption spectrum. It can do lots of different colors of infrared light. And then the other one, the other major greenhouse gas is methane, CH4. So there's the carbon and you got four hydrogens on it and they are symmetrically spaced around the carbon in a tetrahedral shape is sort of how I tried to draw it there. So these three triangle in base and then like a pyramid and this is symmetric too in its resting state, but there's lots of ways that you can vibrate this thing and have it break the symmetry. So this is also an important greenhouse gas. In general, any atom, any molecule that has more than two atoms is gonna be a greenhouse gas of some sort, some strength. [MUSIC]
