WEBVTT Kind: captions Language: en 00:00:02.850 --> 00:00:04.970 Have you ever seen one of these? 00:00:04.970 --> 00:00:06.049 I sure have. 00:00:06.049 --> 00:00:07.756 There’s one right here! 00:00:08.069 --> 00:00:11.860 I have a very burned-in memory of these things which has survived 00:00:11.860 --> 00:00:15.591 all this time to make a video for you. 00:00:15.591 --> 00:00:18.801 These curiosities have been staples of novelty stores 00:00:18.801 --> 00:00:23.334 and science museum gift shops for many, many years. 00:00:23.334 --> 00:00:25.980 One common name for them is the light mill. 00:00:25.980 --> 00:00:29.869 See, it’s turning because there’s a light shining on it. 00:00:29.869 --> 00:00:33.004 If I make the light brighter, well, 00:00:33.004 --> 00:00:34.504 it will speed up. 00:00:34.504 --> 00:00:35.752 And if the light goes out, 00:00:35.752 --> 00:00:38.280 it will slow down and eventually stop. 00:00:39.297 --> 00:00:42.389 There’s some sort of physics goin' on in there, I just know it! 00:00:42.858 --> 00:00:45.302 While light mill may be a common name, 00:00:45.302 --> 00:00:47.037 a better one is radiometer. 00:00:47.037 --> 00:00:50.002 An even more better one is Crookes Radiometer, 00:00:50.002 --> 00:00:52.676 named for its inventor Sir Willam Crookes. 00:00:52.676 --> 00:00:54.949 Fun fact, Richard Nixon had one! 00:00:54.949 --> 00:00:56.426 But he denied it. 00:00:56.426 --> 00:00:58.585 Anyway, invented in 1873 00:00:58.585 --> 00:01:02.897 the vanes of this device will, when it’s exposed to light, rotate. 00:01:02.897 --> 00:01:07.291 Now if you’re wondering why it’s called a radiometer and not a "lightiometer," 00:01:07.291 --> 00:01:10.355 well that’s good! It’s important to the point I want to make here. 00:01:10.355 --> 00:01:16.509 As a hint, keep in mind that visible light is only a small fraction of the electromagnetic spectrum. 00:01:16.509 --> 00:01:19.144 The construction of these things is pretty simple. 00:01:19.144 --> 00:01:23.428 The main structure is a glass envelope not unlike an ordinary lightbulb. 00:01:23.428 --> 00:01:26.827 You have a large volume containing a glass support stem, 00:01:26.827 --> 00:01:29.352 but rather than supporting an incandescent filament 00:01:29.352 --> 00:01:31.659 that stem supports a pin. 00:01:31.659 --> 00:01:34.240 Atop the pin is a little glass hat doohickey, 00:01:34.240 --> 00:01:37.680 and that hat thingamajig is holding onto four vanes. 00:01:37.680 --> 00:01:42.033 The sharp pin and smooth glass create a near-frictionless bearing 00:01:42.033 --> 00:01:44.873 allowing the vanes to rotate with little effort. 00:01:44.873 --> 00:01:50.367 Of course the signature feature of the vanes is that one side is black and the other side is white. 00:01:50.367 --> 00:01:55.501 They’re arranged so that the black sides always face in the counter-clockwise direction of rotation 00:01:55.501 --> 00:01:56.927 (as viewed from above) 00:01:56.927 --> 00:01:59.832 and the white sides face the clockwise direction. 00:01:59.832 --> 00:02:02.339 But don’t let the fact that this is black-and-white fool you. 00:02:02.339 --> 00:02:05.791 The way this actually works is anything but black and white. 00:02:05.791 --> 00:02:07.547 An interesting fact about these 00:02:07.547 --> 00:02:12.063 is that finding out how they work was and still is tricky business. 00:02:12.063 --> 00:02:15.137 Even when you buy one from a reputable science shop, 00:02:15.137 --> 00:02:19.672 it will often come with an explanation like the one printed on the bottom right here. 00:02:19.672 --> 00:02:23.819 “This fascinating radiometer spins from the heat of the sun. 00:02:23.819 --> 00:02:27.283 The vanes in the radiometer are dark and light in color. 00:02:27.283 --> 00:02:29.286 The dark vanes absorb the rays, 00:02:29.286 --> 00:02:31.710 the light vanes reflect the rays.” 00:02:32.361 --> 00:02:34.509 I agree that this is fascinating 00:02:34.509 --> 00:02:38.610 and that the dark side absorbs and the white side reflects... 00:02:38.610 --> 00:02:40.759 but that’s misleading. 00:02:40.759 --> 00:02:44.756 I guess technically it didn’t explain anything about why it moves, 00:02:44.756 --> 00:02:48.255 but it’s leading you to the phenomenon of light pressure. 00:02:48.255 --> 00:02:52.285 That’s the pressure of light not, like, "light pressure." 00:02:52.285 --> 00:02:55.153 In fact, it’s what our friend Billy Crookes thought was happening 00:02:55.153 --> 00:02:57.715 when he first dreamt up this spinny thing. 00:02:57.715 --> 00:03:00.414 You may have heard that light has weight. 00:03:00.414 --> 00:03:03.864 That’s the weight of light, not lightweight. 00:03:03.864 --> 00:03:07.397 But it is very, very lightweight this light weight. 00:03:07.397 --> 00:03:10.360 And that’s why this explanation doesn’t work. 00:03:10.360 --> 00:03:14.518 It also doesn’t help that it’s rotating backwards than this explanation would predict 00:03:14.518 --> 00:03:17.711 but let’s ignore that for now and deal with the pressures involved. 00:03:17.711 --> 00:03:20.487 The radiation pressure of sunlight on the Earth 00:03:20.487 --> 00:03:24.407 is equivalent to about one milligram of weight per square meter, 00:03:24.407 --> 00:03:26.920 or about 10 micronewtons of force. 00:03:26.920 --> 00:03:31.150 That’s not nothing, and in fact when we send probes and stuff into space 00:03:31.150 --> 00:03:35.671 we need to account for this or the sun will literally push stuff off course. 00:03:35.730 --> 00:03:40.830 In fact with solar sails we could potentially use the sun or maybe an Earth-bound laser 00:03:40.830 --> 00:03:43.034 to propel spacecraft on their journeys. 00:03:43.158 --> 00:03:46.204 That is, after all, how the ancient Bajorans got to Cardassia. 00:03:46.829 --> 00:03:50.839 Trouble is, this is insignificant at this scale. 00:03:50.839 --> 00:03:54.790 These vanes are, maybe, 10 millimeters square. 00:03:54.790 --> 00:04:00.216 So in direct sunlight, about .0016 micronewtons of force 00:04:00.216 --> 00:04:02.730 is exerted by the sun on each vane. 00:04:02.730 --> 00:04:06.385 Or about .13 micrograms. 00:04:06.385 --> 00:04:09.202 If that sounds like very little that’s because it is. 00:04:09.202 --> 00:04:13.783 And yet, we don’t need anything nearly as powerful as sunlight to get this to move. 00:04:13.783 --> 00:04:18.766 Even the tiny LED from my phone’s camera will make it turn a little bit. 00:04:18.766 --> 00:04:22.989 The light pressure exerted here is so hilariously meaningless that you can ignore it entirely. 00:04:22.989 --> 00:04:23.885 Trust me. 00:04:23.885 --> 00:04:24.908 It’s fine. 00:04:24.908 --> 00:04:30.719 Even if the miniscule forces light pressures exert on this scale were enough to spin it, 00:04:30.719 --> 00:04:34.368 the other thing is that light pressure is actually greater 00:04:34.368 --> 00:04:37.982 when it reflects off something than what it’s absorbed. 00:04:37.982 --> 00:04:40.516 The explanation I first heard about these 00:04:40.516 --> 00:04:43.235 was that the black side absorbed the photons 00:04:43.235 --> 00:04:47.784 and thus their tiny bit of momentum was transferred to it, pushing it away, 00:04:47.784 --> 00:04:51.662 and since they bounced off the white side their momentum wasn’t transferred, 00:04:51.662 --> 00:04:55.017 thus you get a pushing force on the black side but not on the white. 00:04:56.007 --> 00:04:57.361 That’s just malarkey. 00:04:57.361 --> 00:04:58.468 Doesn’t work that way. 00:04:58.468 --> 00:05:04.137 Solar radiation pressure with perfect reflection is twice that of perfect absorption, 00:05:04.137 --> 00:05:10.516 and while it’s hard to say whether this even matters when both surfaces in question are painted with ordinary paint, 00:05:10.516 --> 00:05:14.459 were this the phenomenon at play this is rotating in the wrong direction. 00:05:14.459 --> 00:05:18.148 There should actually be more force on the white side than the black, 00:05:18.148 --> 00:05:20.418 so the black side should be leading. 00:05:20.801 --> 00:05:22.240 But it’s not. 00:05:22.240 --> 00:05:25.718 And the final nail in the coffin for this particular theory? 00:05:25.718 --> 00:05:30.497 Well, what’s in that glass other than the vanes and pin and hat thingamabob 00:05:30.497 --> 00:05:32.237 is one of these. 00:05:32.237 --> 00:05:33.935 A partial vacuum. 00:05:33.935 --> 00:05:35.860 If this were a complete vacuum 00:05:35.860 --> 00:05:38.831 (or at least as complete as we know how to make one) 00:05:38.831 --> 00:05:40.307 this wouldn’t work. 00:05:40.307 --> 00:05:45.062 If it operated on the principle of radiation pressure it should work just fine - 00:05:45.062 --> 00:05:49.076 after all, radiation pressure is a thing in the vacuum of space. 00:05:49.076 --> 00:05:52.781 But this radiometer actually needs some atmosphere to function, 00:05:52.781 --> 00:05:56.479 but it needs to be a near-vacuum or there would be too much air resistance 00:05:56.479 --> 00:05:58.868 for the vanes to spin freely. 00:05:58.868 --> 00:06:03.665 Ultimately, how this works isn’t important to the point I wanted to make with this video 00:06:03.665 --> 00:06:06.902 but, uh, we’ve come this far so I might as well finish the explanation. 00:06:07.553 --> 00:06:12.144 Longtime viewers of the channel will now groan in unison as I inform you 00:06:12.144 --> 00:06:15.497 that this is in fact a heat engine. 00:06:16.148 --> 00:06:18.683 Just can’t get away from them, can we? 00:06:18.683 --> 00:06:21.415 The difference in color is important for the operation 00:06:21.415 --> 00:06:23.895 but not because of photon momentum. 00:06:23.895 --> 00:06:28.299 Instead it’s simply that the black side heats up faster than the white side 00:06:28.299 --> 00:06:30.562 when exposed to incoming energy. 00:06:30.562 --> 00:06:33.453 Both are being heated thanks to blackbody absorption, 00:06:33.453 --> 00:06:36.235 but the black side gets hotter faster. 00:06:36.235 --> 00:06:39.086 This heats up the air molecules that are next to it. 00:06:39.086 --> 00:06:41.388 And that’s why it moves. 00:06:41.388 --> 00:06:45.242 If you want to see more details and experimentation related to this, 00:06:45.242 --> 00:06:47.525 check out this video from Applied Science. 00:06:47.525 --> 00:06:50.655 It’s really worth a watch (as is the entire channel!). 00:06:50.655 --> 00:06:54.913 But a very quick summary is that two phenomena are involved here. 00:06:54.913 --> 00:07:00.002 The first is simply that the hotter black side causes gas molecules that bounce into it 00:07:00.002 --> 00:07:04.097 to bounce away more forcefully than they do against the cooler white side. 00:07:04.097 --> 00:07:06.888 And the other phenomenon is thermal transpiration, 00:07:06.888 --> 00:07:12.843 which is basically a fancy thing to call forces that occur on gases at different temperatures. 00:07:12.843 --> 00:07:17.623 The temperature difference between the two sides creates a slight difference in this force, 00:07:17.623 --> 00:07:20.003 which in the end helps the vanes rotate. 00:07:20.759 --> 00:07:22.920 Now that we know how these things work, 00:07:22.920 --> 00:07:25.655 let’s talk about why they’re so interesting to me. 00:07:25.655 --> 00:07:29.588 I said that I had a memory burned into my brain regarding these things. 00:07:29.588 --> 00:07:32.237 Long ago, when I was maybe… eight? 00:07:32.237 --> 00:07:36.159 I don’t remember exactly, but my parents were thinking about moving. 00:07:36.159 --> 00:07:39.221 We toured a model home of a new housing development, 00:07:39.221 --> 00:07:42.116 I think it was called Sudden Valley if I remember right, 00:07:42.116 --> 00:07:47.193 anyway the homebuilder had set up a demonstration using two of these radiometers 00:07:47.193 --> 00:07:51.355 to tout the benefits of their home’s amazing new windows. 00:07:51.355 --> 00:07:53.540 See, this isn’t just a "lightiometer" 00:07:53.540 --> 00:07:58.281 because any radiation that will cause the vanes to heat up through blackbody absorption 00:07:58.281 --> 00:08:00.029 will make this move. 00:08:00.029 --> 00:08:03.897 Infrared is one such source of non-visible radiation. 00:08:03.897 --> 00:08:08.986 You may have caught that the label said heat from the sun, and not just light, makes it run. 00:08:08.986 --> 00:08:12.668 Indeed, intense sources of heat will make this thing move. 00:08:12.668 --> 00:08:14.951 But, radiant sources of heat - 00:08:14.951 --> 00:08:17.014 in other words, intense infrared - 00:08:17.014 --> 00:08:19.005 will make it fly. 00:08:19.005 --> 00:08:21.537 Take a look - here’s an ordinary space heater. 00:08:21.537 --> 00:08:24.684 This one isn’t meant to be a source of radiant heat, 00:08:24.684 --> 00:08:28.081 instead it heats the air through blowing it over hot wires. 00:08:28.081 --> 00:08:33.131 But these wires don’t get hot enough to produce much medium- and short-wave infrared radiation, 00:08:33.131 --> 00:08:36.793 so while the radiometer does move, it’s not a whole lot. 00:08:36.793 --> 00:08:40.131 Now take a look at what it does in front of this toaster oven. 00:08:40.131 --> 00:08:43.454 The heating elements here are in fact quartz heat lamps, 00:08:43.454 --> 00:08:46.527 and these eventually start to incandesce intensely. 00:08:46.527 --> 00:08:50.049 They’re so hot they’re comfortably out of the red zone, and into the orange. 00:08:50.049 --> 00:08:51.523 Almost yellow. 00:08:51.523 --> 00:08:56.776 But that means they’re also throwing out tons of shortwave infrared, and indeed - 00:08:56.776 --> 00:08:59.256 the radiometer agrees. 00:08:59.256 --> 00:09:02.759 In something like a toaster oven and indeed an ordinary toaster 00:09:02.759 --> 00:09:05.792 we want to have plenty of infrared radiation. 00:09:05.792 --> 00:09:09.645 Heating something through hot air alone produces a very different effect 00:09:09.645 --> 00:09:12.149 compared to heating it with radiant energy. 00:09:12.149 --> 00:09:17.508 Lots of IR will be better at browning the surface of something without drying it out, 00:09:17.508 --> 00:09:20.983 as the energy becomes absorbed at the surface of whatever’s being heated 00:09:20.983 --> 00:09:25.344 and so in a sense that surface becomes its own source of heat. 00:09:25.344 --> 00:09:30.577 For a slice of bread, that means the surface of the slice gets very hot very quickly, 00:09:30.577 --> 00:09:36.069 but the inside warms slowly as the heat from the surface gradually seeps through. 00:09:36.069 --> 00:09:40.663 You’ve felt this before when near a campfire or under a heat lamp. 00:09:40.663 --> 00:09:45.460 Infrared radiation can travel through the air or even the vacuum of space, 00:09:45.460 --> 00:09:50.073 and it’s only when it becomes absorbed by something that the heat energy becomes apparent. 00:09:50.073 --> 00:09:53.358 That warm tingly sensation from a radiant source of heat 00:09:53.358 --> 00:09:59.020 is how you feel your skin being heated not by warm air, but by radiation. 00:09:59.020 --> 00:10:04.060 That’s one of the reasons quartz heat lamps are sometimes used in places like bus shelters 00:10:04.060 --> 00:10:06.152 or open train platforms. 00:10:06.152 --> 00:10:09.250 They can warm you up even when it’s very cold outside 00:10:09.250 --> 00:10:13.192 because they transmit that heat through the air to your skin. 00:10:13.192 --> 00:10:16.674 Their goal isn’t to heat the air, but you. 00:10:16.674 --> 00:10:22.470 An important note here is that the glass envelope itself is absorbing some of this infrared radiation. 00:10:22.470 --> 00:10:25.402 Because the heating element is visibly incandescing 00:10:25.402 --> 00:10:29.658 we can conclude that there’s a broad spectrum of infrared being emitted here, 00:10:29.658 --> 00:10:36.512 and ordinary silica glass absorbs IR well in the region of 2.8 to 4.5 micrometers. 00:10:36.512 --> 00:10:42.642 Ultimately that makes the glass hot which warms up the air molecules inside the envelope. 00:10:42.642 --> 00:10:46.967 An interesting side-effect of this is that when taken away from the radiation, 00:10:46.967 --> 00:10:53.578 the radiometer actually begins to spin backwards as it releases its excess energy to the surrounding air. 00:10:53.578 --> 00:10:55.824 This isn’t really important to the point I’m making 00:10:55.824 --> 00:11:01.494 but it does further demonstrate how the rotational action has nothing to do with light pressure, 00:11:01.494 --> 00:11:03.832 but is a thermal phenomenon. 00:11:03.832 --> 00:11:06.411 Speaking of the point I’m trying to make… 00:11:06.411 --> 00:11:07.917 what is that? 00:11:07.917 --> 00:11:11.366 Ah, right, uh, going back to that model home demo. 00:11:11.366 --> 00:11:15.568 To demonstrate the amazingness of their amazing new windows of amazement, 00:11:15.568 --> 00:11:19.399 the model home had a little demo consisting of two floodlights, 00:11:19.399 --> 00:11:23.186 two radiometers, and in between the radiometers and floodlights 00:11:23.186 --> 00:11:25.418 were two different window panes. 00:11:25.418 --> 00:11:27.973 One was your ordinary garden variety window, 00:11:27.973 --> 00:11:30.565 and the other was this amazing new whatever it was. 00:11:30.565 --> 00:11:34.534 The radiometer in front of the ordinary window was spinning quite quickly, 00:11:34.534 --> 00:11:40.285 but the radiometer on the other side of the amazing window wasn’t spinning nearly as fast. 00:11:40.285 --> 00:11:43.497 This served to create an immediately intuitive sense 00:11:43.497 --> 00:11:48.016 of how the better glass prevented heat energy from passing through it. 00:11:48.016 --> 00:11:51.565 Now, the scientifically-inclined and skeptical among you 00:11:51.565 --> 00:11:55.136 will no doubt realize that this could easily have been rigged. 00:11:55.136 --> 00:11:58.921 After all, who’s to say that the two radiometers performed equally? 00:11:59.468 --> 00:12:03.194 Sadly, at the age of eight I wasn’t yet thinking about these things, 00:12:03.194 --> 00:12:08.189 otherwise I would have switched those two radiometers to make sure the result was the same. 00:12:08.189 --> 00:12:11.319 Soon as I get that time machine perfected I’ll go back and do it! 00:12:11.319 --> 00:12:13.937 But as it happens, this home is fairly new 00:12:13.937 --> 00:12:16.627 and it has even newer windows than that model home did. 00:12:16.627 --> 00:12:20.324 At least, if my understanding of linear time is correct. 00:12:20.324 --> 00:12:23.208 While I don’t have a second window to compare to, 00:12:23.208 --> 00:12:25.990 I can just… open one. 00:12:25.990 --> 00:12:30.560 Here's a great demonstration of how much infrared light this window is blocking. 00:12:30.560 --> 00:12:34.438 Were it not for the window, the radiometer would be in direct sunlight. 00:12:34.438 --> 00:12:38.255 And you can see that it's actually not turning all that quickly. 00:12:38.255 --> 00:12:41.794 If I open the window, though, it immediately speeds up. 00:12:41.794 --> 00:12:44.771 Here we see using the same radiometer 00:12:44.771 --> 00:12:49.186 that the window drastically reduces the total amount of energy passing through it 00:12:49.186 --> 00:12:53.368 while only marginally decreasing the visible light that makes it through. 00:12:53.368 --> 00:12:54.605 Neat! 00:12:54.605 --> 00:12:58.525 How this is accomplished doesn’t really matter for the purposes of this video, 00:12:58.525 --> 00:13:00.204 but let’s talk about it anyway! 00:13:00.204 --> 00:13:06.395 This window, like many out there, features low-emissivity glass (often called low-E). 00:13:06.395 --> 00:13:09.897 At the time it was made, a coating was deposited on the glass 00:13:09.897 --> 00:13:13.566 which will reject much of the near-infrared light that hits it. 00:13:13.566 --> 00:13:16.502 Since glass will absorb certain IR wavelengths, 00:13:16.502 --> 00:13:20.478 we want those wavelengths to be reflected away before it even hits the glass, 00:13:20.478 --> 00:13:23.299 otherwise the glass would be heated by this energy 00:13:23.299 --> 00:13:27.037 and end up allowing much of that into the space anyway. 00:13:27.037 --> 00:13:29.958 While visible light does plenty of heating by itself, 00:13:29.958 --> 00:13:34.751 a slight majority of the sun’s energy that reaches the Earth’s surface is infrared. 00:13:34.751 --> 00:13:39.338 Windows that can reflect just this portion of the sun’s energy away 00:13:39.338 --> 00:13:42.331 reduce the total amount of energy that passes through them 00:13:42.331 --> 00:13:45.535 and thus lower the amount of heating caused by the sun 00:13:45.535 --> 00:13:48.824 all without appearing darker to our eyes. 00:13:48.860 --> 00:13:51.249 And these windows are great at this! 00:13:51.249 --> 00:13:53.831 Not only does the radiometer demonstrate this nicely, 00:13:53.831 --> 00:13:58.723 but in fact you can tell just by feeling the difference between the sunlight on your skin 00:13:58.723 --> 00:14:01.929 and the sunlight that makes it through the glass on your skin. 00:14:01.929 --> 00:14:05.370 There’s a significant, noticeable difference. 00:14:05.370 --> 00:14:09.339 Voiceover me is here to tell you about another demo we can do! 00:14:09.339 --> 00:14:13.509 This halogen floodlight produces quite a lot of directed infrared. 00:14:13.509 --> 00:14:17.396 Don’t think about that too much or it might spoil a later demo but, uh 00:14:17.396 --> 00:14:21.731 anyway if I put it outside and shine it at the radiometer through the window, 00:14:21.731 --> 00:14:24.080 you can see it doesn’t move all that much. 00:14:24.080 --> 00:14:26.227 If, however, I bring it back inside 00:14:26.227 --> 00:14:29.615 and point it directly at the radiometer from about the same distance, 00:14:29.615 --> 00:14:32.033 it spins very rapidly. 00:14:32.033 --> 00:14:35.725 That window’s doing a lot to keep the infrared from coming through. 00:14:35.725 --> 00:14:39.285 And to be clear, ordinary glass doesn’t do hardly anything at all 00:14:39.285 --> 00:14:40.848 as seen here. 00:14:40.848 --> 00:14:43.532 An interesting side-effect of this low-E coating 00:14:43.532 --> 00:14:48.294 is that the internal reflections in the glass become color-shifted as they repeat. 00:14:48.294 --> 00:14:50.234 And the way they shift is surprising. 00:14:50.234 --> 00:14:52.071 I can’t really make heads or tails of it. 00:14:52.071 --> 00:14:53.782 It’s not consistent at all, 00:14:53.782 --> 00:14:58.461 and there’s a noticeable difference between the cool white diodes and the warm white diodes here 00:14:58.461 --> 00:15:03.658 (this is one of my portable RGBW studio lights with both white color temps lit). 00:15:03.658 --> 00:15:07.510 However, the very far repeated reflections all look pretty blue, 00:15:07.510 --> 00:15:10.299 which makes sense as that’s farthest from infrared. 00:15:10.299 --> 00:15:16.006 Wavelengths even remotely close to IR probably get rejected by the eighth reflection or so. 00:15:16.006 --> 00:15:19.842 And actually, one thing I was hoping I could demonstrate but wasn’t able to 00:15:19.842 --> 00:15:22.416 was that depending on the angle of the sun, 00:15:22.416 --> 00:15:29.062 sometimes you’ll see an intense pink or magenta cast in light that’s reflected off these windows. 00:15:29.062 --> 00:15:32.510 I was playing around with the floodlight but could only barely see it. 00:15:32.510 --> 00:15:37.500 I was hoping to show it to you because it demonstrates a significant drawback of low-E glass: 00:15:37.500 --> 00:15:41.506 it can actually cause damage to various things that end up in the sun 00:15:41.506 --> 00:15:44.629 and also get hit with a reflection off a window. 00:15:44.629 --> 00:15:48.127 Those objects end up getting more IR than direct sunlight, 00:15:48.127 --> 00:15:52.221 and with modern building designs with tons of windows this can become problematic, 00:15:52.221 --> 00:15:54.048 especially in urban settings. 00:15:54.048 --> 00:15:59.415 And especially especially if architects get creative with curved building surfaces. 00:15:59.415 --> 00:16:03.339 Now I deleted a section here in editing, so here’s a sloppy segue. 00:16:03.339 --> 00:16:05.418 What about in the winter time? 00:16:05.418 --> 00:16:11.172 This low-E technology can also help keep heat inside the home from leaving in the winter. 00:16:11.172 --> 00:16:13.357 Everything radiates heat energy 00:16:13.357 --> 00:16:15.739 (unless it’s at absolute zero, of course), 00:16:15.739 --> 00:16:18.456 and if that energy goes through a window… 00:16:18.456 --> 00:16:20.016 well now it’s outside. 00:16:20.016 --> 00:16:24.036 If the window can reflect long-wave infrared back inward, though, 00:16:24.036 --> 00:16:28.013 it will be able to prevent much of that radiant energy from being lost through it. 00:16:28.013 --> 00:16:31.417 Instead it will end up on another inside surface where it will re-radiate 00:16:31.417 --> 00:16:36.138 and maybe hit the window again who knows but the important part is it didn’t leave. 00:16:36.138 --> 00:16:37.587 Voiceover me again. 00:16:37.587 --> 00:16:40.551 I was gonna do some sort of whiteboard drawing to show this 00:16:40.551 --> 00:16:45.256 but as it turns out this can be demonstrated with the radiometer and the floodlight and the window. 00:16:45.256 --> 00:16:48.003 Here I’m pointing the floodlight out the window 00:16:48.003 --> 00:16:52.736 at an angle so that the reflected light will land on the radiometer inside the room. 00:16:52.736 --> 00:16:56.303 Though most of the visible was unimpeded as it left the room, 00:16:56.303 --> 00:17:01.170 the IR was reflected right back and the vanes are spinning quite quickly to prove it. 00:17:01.170 --> 00:17:02.356 Pretty neat. 00:17:02.356 --> 00:17:06.879 I had assumed that the inside coating might be focused more on long-wave infrared 00:17:06.879 --> 00:17:12.306 but given how fast the radiometer is spinning here I’m going to assume that assumption was wrong. 00:17:12.306 --> 00:17:18.916 However, the benefit of rejecting infrared from the outside becomes a drawback in the winter. 00:17:18.916 --> 00:17:20.882 When you need to heat your home, 00:17:20.882 --> 00:17:26.306 windows which are blocking a majority of the free solar radiation coming at them 00:17:26.306 --> 00:17:28.002 are kind of a bummer. 00:17:28.002 --> 00:17:32.124 And indeed, in the winter these windows are kind of a bummer. 00:17:32.124 --> 00:17:36.544 It would be great if we could develop some sort of selectable Low-E window. 00:17:36.544 --> 00:17:42.433 Maybe the glass could be reversible so it reflects short and medium-wave IR only on one side. 00:17:42.433 --> 00:17:47.632 Turning it inward would thus allow for more IR transmission in the winter. 00:17:47.632 --> 00:17:49.035 But maybe that wouldn’t work. 00:17:49.035 --> 00:17:53.057 Truthfully I don’t know how much the side that the coating is on actually matters, 00:17:53.057 --> 00:17:57.629 but one of you probably does and I hope you’ll let us know down there. 00:17:57.629 --> 00:18:01.020 There is, though, a solution to this problem that we know of today 00:18:01.020 --> 00:18:03.948 which is incorporated in the passive house design. 00:18:03.948 --> 00:18:06.853 Because the angle of the sun changes through the year 00:18:06.853 --> 00:18:09.574 with higher angles in the summer and lower ones in the winter 00:18:09.574 --> 00:18:12.874 (especially where it gets cold enough to need a lot of seasonal heat) 00:18:12.874 --> 00:18:14.870 we actually have a workaround. 00:18:14.870 --> 00:18:19.030 A structure like this can be fitted with solar-control windows up here, 00:18:19.030 --> 00:18:21.675 and standard windows below this awning. 00:18:21.675 --> 00:18:25.982 In the summer months, the awning prevents direct sunlight from hitting these standard windows, 00:18:25.982 --> 00:18:28.122 so they don’t need to reject IR. 00:18:28.122 --> 00:18:33.164 It will only hit the transoms, providing plenty of natural light but with minimal solar heating. 00:18:33.164 --> 00:18:39.052 But in the winter, the sun’s low angle now allows sunlight through the low windows as well, 00:18:39.052 --> 00:18:41.294 providing some free heating. 00:18:41.294 --> 00:18:45.442 Still, though, I think developing a window that can reject infrared in the summer 00:18:45.442 --> 00:18:48.951 but allow it through in the winter is a worthwhile cause, 00:18:48.951 --> 00:18:54.057 but the mechanism of how the glass rejects infrared is really beside the point. 00:18:54.057 --> 00:18:57.331 We were talking about radiometers, at one point. 00:18:57.331 --> 00:18:58.221 Uh... 00:18:58.221 --> 00:19:01.621 What I find fascinating about the radiometer is that, 00:19:01.621 --> 00:19:05.861 while it certainly isn’t a precise tool giving any useful units, 00:19:05.861 --> 00:19:10.549 it can be very useful in demonstrating the concept of radiant energy. 00:19:10.549 --> 00:19:13.901 For example, here’s another fantastic use for one. 00:19:14.234 --> 00:19:20.328 By now I hope we’re all pretty aware of why incandescent lightbulbs are all but gone, 00:19:20.328 --> 00:19:25.054 but a pair of radiometers would have made a great in-store display. 00:19:25.054 --> 00:19:31.347 Really, it would be the same exact demo as in the model home but without the window glass. 00:19:31.347 --> 00:19:36.159 And to make sure I’m not foolin’ you, well I’ll just use the same radiometer. 00:19:36.159 --> 00:19:39.843 I only have one of 'em after all so we’ll split screen it! 00:19:39.843 --> 00:19:43.208 The large amount of IR emitted from the incandescent bulb 00:19:43.208 --> 00:19:48.242 heats the vanes of the radiometer quickly and intensely, causing rapid motion. 00:19:48.242 --> 00:19:50.227 But the LED bulb next to it, 00:19:50.227 --> 00:19:54.743 producing none of the medium and shortwave IR that the incandescent light does, 00:19:54.743 --> 00:19:57.042 hardly makes it turn at all. 00:19:57.042 --> 00:19:58.972 Sure, it still turns a bit - 00:19:58.972 --> 00:20:03.456 the visible light is absorbed by the black paint and also makes it heat up. 00:20:03.456 --> 00:20:06.740 But the total amount of energy being radiated from the light bulb 00:20:06.740 --> 00:20:09.854 is a small fraction compared to the incandescent, 00:20:09.854 --> 00:20:12.151 yet the bulb is just as bright. 00:20:12.151 --> 00:20:16.472 Our eyes don’t care about most of the radiation that an incandescent bulb creates, 00:20:16.472 --> 00:20:19.699 so why waste energy making it in the first place? 00:20:19.699 --> 00:20:23.501 And this is where I think the radiometer can teach us a valuable lesson. 00:20:23.501 --> 00:20:27.005 Sure, I could grab a power meter and put each light bulb on there 00:20:27.005 --> 00:20:29.191 to give you the difference with raw numbers. 00:20:29.191 --> 00:20:32.150 But this is far more… visceral. 00:20:32.150 --> 00:20:33.086 Intuitive. 00:20:33.086 --> 00:20:34.772 Plainly evident. 00:20:34.772 --> 00:20:38.128 It doesn’t require comparing numbers to see that there’s a difference. 00:20:38.128 --> 00:20:40.685 There just… is one. 00:20:40.685 --> 00:20:43.342 Similarly, I could go window shopping 00:20:43.342 --> 00:20:47.387 (that’s shopping for windows, not window shopping) 00:20:47.387 --> 00:20:50.493 and take a look at the IR rejection specifications, 00:20:50.493 --> 00:20:52.813 R-values, and other data, 00:20:52.813 --> 00:20:56.987 but not everyone will understand why those data are important. 00:20:56.987 --> 00:21:01.582 Using something that provides you with clear evidence of infrared radiation 00:21:01.582 --> 00:21:05.969 without requiring any contextualizing or prior understanding 00:21:05.969 --> 00:21:11.432 makes the knowledge and importance of measures to abate infrared more accessible. 00:21:11.432 --> 00:21:15.759 I would argue that you don’t even need much awareness of what the radiometer does 00:21:15.759 --> 00:21:18.820 for it to be an effective demonstration tool. 00:21:18.820 --> 00:21:26.617 But admittedly I can’t unlearn that so perhaps the intuitiveness I feel this provides is more unique to me. 00:21:26.617 --> 00:21:33.125 And honestly, I’m not even sure eight year old me grasped the point of the demo at the model home. 00:21:33.180 --> 00:21:36.502 It may have just stuck in my mind because these things are cool, 00:21:36.502 --> 00:21:39.626 dare I say nifty. 00:21:39.626 --> 00:21:41.502 I feel like I got the point of it because 00:21:41.502 --> 00:21:44.880 I remember there being two radiometers and two different windows 00:21:44.880 --> 00:21:47.361 and one of them was spinning faster than the other, 00:21:47.361 --> 00:21:53.143 but it could very well be the case that the point of the demo only dawned on me years later. 00:21:53.143 --> 00:21:57.420 And unfortunately, that’s something my time machine won’t be able to tell me, 00:21:57.420 --> 00:22:00.722 at least not without asking my former self 00:22:00.722 --> 00:22:04.247 and who knows what the consequences of that might be? 00:22:04.247 --> 00:22:07.480 It could turn that kid into some sort of… 00:22:07.480 --> 00:22:08.586 influencer. 00:22:08.586 --> 00:22:09.598 Eugh! 00:22:09.598 --> 00:22:13.715 Anyway, while William Crookes was wrong about how this thing works 00:22:13.715 --> 00:22:15.955 and it’s mainly relegated to the 00:22:15.955 --> 00:22:18.749 “well that’s neat!” category of existence, 00:22:18.749 --> 00:22:22.958 I think devices like this are more useful than we perhaps realize. 00:22:22.958 --> 00:22:26.059 Sometimes it’s not the accuracy of the tool that matters 00:22:26.059 --> 00:22:29.052 but how easily it demonstrates something. 00:22:29.052 --> 00:22:33.369 If a 19th century scientific curiosity can demonstrate to an eight year old 00:22:33.369 --> 00:22:37.413 that some windows - or some light bulbs - are better than others, 00:22:37.413 --> 00:22:41.620 who knows what might be out there which can do the same for other concepts? 00:22:41.620 --> 00:22:43.669 No really, do you know? 00:22:43.669 --> 00:22:45.571 I... I think we should be looking into this. 00:22:45.571 --> 00:22:49.598 There’s a lot of stuff out there these days that’s complicated to communicate, 00:22:49.598 --> 00:22:53.075 and whenever that happens there’s room for misinformation. 00:22:53.075 --> 00:22:56.772 A lot of people are skeptical about the merits of technological change, 00:22:56.772 --> 00:23:01.014 even when it’s for something so mundane as improving energy efficiency. 00:23:01.014 --> 00:23:05.981 Trust me, I’m as leery as you are about much of what the Valley techbros are dreaming up these days - 00:23:05.981 --> 00:23:09.450 not all progress is truly progressive, after all. 00:23:09.450 --> 00:23:13.678 But a lot of common-sense progress gets met with skepticism these days, 00:23:13.678 --> 00:23:18.439 and maybe there are ancient thingamjigs like this that can help with that. 00:23:18.439 --> 00:23:22.421 Maybe fighting misinformation is going a little too deep, 00:23:22.421 --> 00:23:26.690 but I do earnestly think that there’s value in exploring this. 00:23:26.690 --> 00:23:29.982 There are probably all sorts of old curios out there 00:23:29.982 --> 00:23:35.998 that seem pointless by modern standards but which could actually provide a meaningful educational experience. 00:23:35.998 --> 00:23:41.822 Being able to demonstrate a concept in a purely intuitive fashion can be very powerful, 00:23:41.822 --> 00:23:46.356 and while I doubt there’s a demo rig to be built for all of life’s various lessons, 00:23:46.356 --> 00:23:51.601 I’ll bet there are more of them out there just waiting to be built than you might imagine. 00:23:51.601 --> 00:23:53.373 Thanks for watching. 00:23:54.363 --> 00:23:56.943 ♫ transpirationally smooth jazz ♫ 00:23:57.907 --> 00:24:00.099 Have you ever seen one of these? 00:24:00.099 --> 00:24:01.329 I… nope. 00:24:01.329 --> 00:24:04.226 The final nail in the coffin for this theory… 00:24:04.226 --> 00:24:05.555 oh yeah, shoot! 00:24:06.806 --> 00:24:07.751 I need a prop! 00:24:07.751 --> 00:24:11.338 Oh, and I restarted the teleprompter to the very beginning! 00:24:11.338 --> 00:24:13.645 Why do I still use this one? 00:24:14.713 --> 00:24:17.402 It’s because I’m lazy, and I’m used to it. 00:24:17.751 --> 00:24:23.494 These curiosities have been staples of novelty shops and museum science gift stores. 00:24:23.494 --> 00:24:24.222 Oops. 00:24:24.587 --> 00:24:25.958 A partial vacuum. 00:24:25.958 --> 00:24:28.696 If this were a complete [clunk] vacuum... 00:24:28.696 --> 00:24:31.768 Well that’s not gonna work, is it? 00:24:31.768 --> 00:24:34.170 Move de shtuff out of de vay! 00:24:34.170 --> 00:24:38.371 It also doesn’t help that it’s rotating backwards from that explanation. 00:24:38.371 --> 00:24:40.041 Egghhh. 00:24:40.041 --> 00:24:43.331 Ya know it’s kinda hard to look at a lightbulb and then look at a teleprompter. 00:24:43.331 --> 00:24:44.599 It’s not very easy. 00:24:44.599 --> 00:24:46.666 My eyes can’t adjust that fast. 00:24:47.239 --> 00:24:50.880 I have a very burned-in memory of these things which has made 00:24:50.880 --> 00:24:51.855 ... (mouth noises of disappointment) 00:24:53.267 --> 00:24:55.422 What word should I choose other than malarkey? 00:24:55.839 --> 00:24:56.984 [thinking, stand-by] 00:24:57.323 --> 00:24:58.458 Hogwash. 00:24:58.458 --> 00:24:59.696 Nahhh! 00:24:59.696 --> 00:25:04.231 If it operated on the princ per he blerghhhhfff. 00:25:04.231 --> 00:25:08.880 The temperature gradient between the two sides creates a slight difference in… 00:25:08.880 --> 00:25:09.872 something. 00:25:09.872 --> 00:25:12.374 Where did that word go? 00:25:12.374 --> 00:25:15.368 It’s over there. In this force. 00:25:15.368 --> 00:25:16.637 [mouth popping sounds] 00:25:16.637 --> 00:25:21.334 It needs to be a near-vacuum or there would be too much air resistance for the vanes to spin freely. 00:25:21.334 --> 00:25:22.988 But it needs a bit. 00:25:22.988 --> 00:25:24.324 And that wasn’t in the script. 00:25:24.324 --> 00:25:25.728 You’ve adlibbed. 00:25:25.728 --> 00:25:27.580 You terrible person. 00:25:27.580 --> 00:25:30.260 I said ah high bleu… great. 00:25:31.433 --> 00:25:32.714 That section I deleted? 00:25:32.714 --> 00:25:35.698 It was about how my cat, Reed, doesn't feel all that hot to the touch when he sits in the sun, 00:25:35.698 --> 00:25:39.151 despite being a black cat. I then talked about how my black hair (of which there is a surplus at the moment) 00:25:39.151 --> 00:25:41.447 is almost too hot to the touch when I'm out in the sun. 00:25:41.447 --> 00:25:45.335 So it's pretty remarkable that the cat can be in the sun without being too hot to touch. 00:25:45.335 --> 00:25:46.595 It was a riveting story. 00:25:46.595 --> 00:25:47.512 Too bad it didn't make the cut.