2 00:00:10,783 --> 00:00:13,357 So far, most of the things we've talked about 3 00:00:13,357 --> 00:00:16,900 in this class have been pretty good black bodies. 4 00:00:16,900 --> 00:00:21,730 So, we've sort of been slinging, sort of been slinging epsilon values around. 5 00:00:21,730 --> 00:00:23,800 but just calling them one, in my mind, at 6 00:00:23,800 --> 00:00:27,210 least, because most things are pretty good blackbodies. 7 00:00:27,210 --> 00:00:30,750 Most condensed matter is pretty good at absorbing 8 00:00:30,750 --> 00:00:33,740 and emitting all the different frequencies in infrared light. 9 00:00:33,740 --> 00:00:36,980 But gases are different because they're so simple 11 00:00:36,980 --> 00:00:39,730 They only have very simple modes of vibration. 12 00:00:39,730 --> 00:00:43,240 If you take carbon dioxide and you 13 00:00:43,240 --> 00:00:46,690 freeze it into dry ice, into condensed form, 14 00:00:47,700 --> 00:00:50,030 it's got all these other things around it. 15 00:00:50,030 --> 00:00:51,740 It's like a blop of jello. 16 00:00:51,740 --> 00:00:55,170 It has all different kinds of vibrational frequencies. 17 00:00:55,170 --> 00:00:57,860 And can absorb everything, in a condensed state. 18 00:00:57,860 --> 00:01:01,180 But when the CO2 is floating off by itself, and not touching 19 00:01:01,180 --> 00:01:05,550 anything else very much. It can only vibrate in very specific ways. 20 00:01:05,550 --> 00:01:11,130 Gases are very choosy about the kinds of light they can absorb and emit. 22 00:01:12,330 --> 00:01:15,430 So the way we've done this, 23 00:01:15,430 --> 00:01:20,860 For black bodies, epsilon is a value close to one, 24 00:01:20,860 --> 00:01:24,790 and that works for the ground and for the pane of glass in our simple model. 25 00:01:24,790 --> 00:01:27,020 But for gases, in a real atmosphere, 26 00:01:27,020 --> 00:01:30,930 epsilon value is not at all one, it's much smaller than that, 27 00:01:30,930 --> 00:01:34,800 because gases are so picky. 28 00:01:34,800 --> 00:01:39,220 So, just as an aside, before we go on, I want to explain that 29 00:01:39,220 --> 00:01:43,490 most of the colors that we observe, most of the light that we see in nature 30 00:01:43,490 --> 00:01:52,070 around us is got its color not because of absorption and emission by vibration 31 00:01:52,070 --> 00:01:52,930 of the atoms. 32 00:01:52,930 --> 00:01:58,690 But actually because the electrons exist at different energy levels. 33 00:01:58,690 --> 00:02:02,510 And they can hop up to higher energy levels and pop back down again. 34 00:02:02,510 --> 00:02:06,680 And when they do that they, they absorb or give off energy. 35 00:02:06,680 --> 00:02:11,130 So, most things that are good dyes, like the red stuff 36 00:02:11,130 --> 00:02:15,850 in this chalk, has energy levels that are fairly close together. 37 00:02:15,850 --> 00:02:17,370 And so when an electron 38 00:02:17,370 --> 00:02:23,140 falls down that much it gives off light that has that much energy. 39 00:02:23,140 --> 00:02:26,980 Which turns out to be in the visible range and so we can see it. 40 00:02:26,980 --> 00:02:30,430 But gases are so simple that the energy 41 00:02:30,430 --> 00:02:32,670 levels of their electrons are so far apart. 42 00:02:32,670 --> 00:02:38,810 That what they mostly do is, is make light in the ultraviolet that we can't see. 43 00:02:38,810 --> 00:02:43,570 And this is why gases are transparent to the visible light that we can see. 44 00:02:43,570 --> 00:02:48,100 Some exceptions to that are chlorine gas, is actually green. 45 00:02:48,100 --> 00:02:52,450 Chlorine is a big atom with lots of electrons 46 00:02:52,450 --> 00:02:54,670 on the outside and some of them are bound sort of loosely. 47 00:02:54,670 --> 00:02:58,380 And so you can actually see it in visible light, 48 00:02:58,380 --> 00:03:02,680 and then NO2 is kind of a brown color. 49 00:03:02,680 --> 00:03:06,150 This is something that you find in urban smog. 50 00:03:06,150 --> 00:03:07,530 When they have ozone alert days. 51 00:03:07,530 --> 00:03:08,630 If you can see 52 00:03:08,630 --> 00:03:11,960 far off into the distance, you see this sort of brown layer. 53 00:03:11,960 --> 00:03:14,380 And that's what this is. 54 00:03:14,380 --> 00:03:19,060 That's mostly what creates the colors that we see. 55 00:03:19,060 --> 00:03:22,060 But what we're interested in, for the energy in the atmosphere, 56 00:03:22,060 --> 00:03:28,150 is back to molecular and atomic vibrations. 57 00:03:29,590 --> 00:03:33,751 For a gas to absorb light or to emit light, because it's always a two wa street, 58 00:03:33,751 --> 00:03:37,000 if it can absorb, it can also emit, 59 00:03:37,000 --> 00:03:38,710 two things must be true. 60 00:03:38,710 --> 00:03:41,370 One is that the frequency of the light has 61 00:03:41,370 --> 00:03:44,930 to be pretty close to the frequency of the vibration, 62 00:03:44,930 --> 00:03:48,730 has to be kind of a match, you have to be well tuned. 63 00:03:48,730 --> 00:03:52,290 But the other thing, that's very important, is that the vibration of the 64 00:03:52,290 --> 00:03:58,990 molecule has to create a fluctuating electric field, an oscillating dipole. 65 00:04:00,220 --> 00:04:04,120 And most gases in the atmosphere are not really capable of doing this. 66 00:04:04,120 --> 00:04:09,080 So the main gases in our atmosphere 67 00:04:09,080 --> 00:04:12,835 consist of molecules that have two identical atoms in them each. 68 00:04:12,835 --> 00:04:17,670 O2 or N2 are mostly what the atmosphere is made out of. 69 00:04:17,670 --> 00:04:19,960 Those are totally symmetric, one oxygen is 70 00:04:19,960 --> 00:04:24,350 the same as another and if you vibrate it, 71 00:04:24,350 --> 00:04:25,580 when they're farther apart or closer together, 72 00:04:25,580 --> 00:04:28,410 it's still always symmetric, 73 00:04:28,410 --> 00:04:31,330 there isn't a plus on one side and a minus on another side. 74 00:04:31,330 --> 00:04:35,350 And that's why these major gases in the atmosphere are not greenhouse gases. 75 00:04:35,350 --> 00:04:38,260 They don't affect the climate of the earth in a greenhouse effect sort of way. 76 00:04:40,830 --> 00:04:45,580 Carbon dioxide is a symmetric molecule in its resting state. 77 00:04:45,580 --> 00:04:47,590 The carbon is in the middle and then you 78 00:04:47,590 --> 00:04:51,090 have these double bonds to the oxygens on either side. 79 00:04:51,090 --> 00:04:54,150 It doesn't really matter to us that these are double bonds instead of 80 00:04:54,150 --> 00:04:57,900 single bonds, they still act like springs and they still vibrate. 81 00:04:59,020 --> 00:05:00,830 and so at first glance, you might think this is symmetric too, 82 00:05:00,830 --> 00:05:04,240 and so it wouldn't be a greenhouse gas. 83 00:05:04,240 --> 00:05:11,310 But there are modes of vibration of this molocule that break the symmetry. 86 00:05:11,310 --> 00:05:14,660 And the one that's most important is the bend. 87 00:05:14,660 --> 00:05:17,070 You are sort of bending the thing like this, 88 00:05:17,070 --> 00:05:22,300 and when you do that the oxygens have sort of a minus charge to them, 89 00:05:22,300 --> 00:05:25,348 and that leaves sort of a positive charge on the other side, 90 00:05:25,348 --> 00:05:29,650 and then when it swings back the other way, the electric field is going to flip. 91 00:05:29,650 --> 00:05:32,610 You get this oscillating dipole here. 93 00:05:32,610 --> 00:05:36,680 This mode of vibration of the 94 00:05:36,680 --> 00:05:39,550 CO2 molecule is the one that's most important for climate. 95 00:05:41,020 --> 00:05:42,330 There are two other modes. 96 00:05:42,330 --> 00:05:45,390 There's a symmetric stretch which is kind of like this, and 97 00:05:45,390 --> 00:05:48,250 then there's a asymmetric stretch which is kind of like that. 98 00:05:48,250 --> 00:05:53,860 So the asymmetric stretch is infrared active because it's 99 00:05:53,860 --> 00:05:56,970 broken the symmetry, 100 00:05:56,970 --> 00:05:59,730 but it turns out that this mode of vibration is 101 00:05:59,730 --> 00:06:05,790 not as important to climate because there's just less light there, at this frequency. 102 00:06:05,790 --> 00:06:08,770 So this is the frequency that is the 103 00:06:08,770 --> 00:06:11,620 most important to absorbing light that's coming up from the ground. 104 00:06:13,850 --> 00:06:18,420 Other greenhouse gases, the main ones, are water, water vapor. 105 00:06:18,420 --> 00:06:23,580 Water has the oxygen has two lone pairs of electrons 106 00:06:23,580 --> 00:06:29,170 which tend to crowd the hydrogens off to one side. 107 00:06:29,170 --> 00:06:34,460 Water is an asymmetric molecule even at rest when you write it. 108 00:06:34,460 --> 00:06:39,790 It's got a dipole moment even when it's just sitting there 110 00:06:39,790 --> 00:06:43,180 There are lots of different ways you can vibrate this 111 00:06:43,180 --> 00:06:47,210 or you can just spin it around and that will create an oscillating electric field. 112 00:06:47,210 --> 00:06:50,940 Water has a very, very complicated absorption spectrum. 113 00:06:50,940 --> 00:06:54,530 It can do lots of different colors of infrared light. 114 00:06:54,530 --> 00:06:56,215 And then the other major greenhouse gas is methane 116 00:06:56,670 --> 00:07:01,970 CH4, so there's the carbon, 117 00:07:01,970 --> 00:07:04,150 and you've got four hydrogens on it. 118 00:07:04,150 --> 00:07:08,160 They are symmetrically spaced around the carbon 119 00:07:08,160 --> 00:07:11,480 in a tetrahedral shape. 120 00:07:11,480 --> 00:07:16,440 There's a triangle in the base, and a pyramid. 122 00:07:16,440 --> 00:07:18,720 And this is symmetric, too, in its resting state, 123 00:07:18,720 --> 00:07:21,930 but there's lots of ways that you can vibrate this thing, 125 00:07:22,960 --> 00:07:28,300 and have it break the symmetry, so this is also an important greenhouse gas. 126 00:07:28,300 --> 00:07:34,222 In general, any molecule that has more than two atoms is 127 00:07:34,222 --> 00:07:40,180 going to be a greenhouse gas of some sort, some strength.