1
00:00:02,850 --> 00:00:04,970
Have you ever seen one of these?

2
00:00:04,970 --> 00:00:06,049
I sure have.

3
00:00:06,049 --> 00:00:07,756
There’s one right here!

4
00:00:08,069 --> 00:00:11,860
I have a very burned-in memory of these things which has survived

5
00:00:11,860 --> 00:00:15,591
all this time to make a video for you.

6
00:00:15,591 --> 00:00:18,801
These curiosities have been staples of novelty stores

7
00:00:18,801 --> 00:00:23,334
and science museum gift shops for many, many years.

8
00:00:23,334 --> 00:00:25,980
One common name for them is the light mill.

9
00:00:25,980 --> 00:00:29,869
See, it’s turning because there’s a light shining on it.

10
00:00:29,869 --> 00:00:33,004
If I make the light brighter, well,

11
00:00:33,004 --> 00:00:34,504
it will speed up.

12
00:00:34,504 --> 00:00:35,752
And if the light goes out,

13
00:00:35,752 --> 00:00:38,280
it will slow down and eventually stop.

14
00:00:39,297 --> 00:00:42,389
There’s some sort of physics goin' on in there, I just know it!

15
00:00:42,858 --> 00:00:45,302
While light mill may be a common name,

16
00:00:45,302 --> 00:00:47,037
a better one is radiometer.

17
00:00:47,037 --> 00:00:50,002
An even more better one is Crookes Radiometer,

18
00:00:50,002 --> 00:00:52,676
named for its inventor Sir Willam Crookes.

19
00:00:52,676 --> 00:00:54,949
Fun fact, Richard Nixon had one!

20
00:00:54,949 --> 00:00:56,426
But he denied it.

21
00:00:56,426 --> 00:00:58,585
Anyway, invented in 1873

22
00:00:58,585 --> 00:01:02,897
the vanes of this device will, when it’s exposed to light, rotate.

23
00:01:02,897 --> 00:01:07,291
Now if you’re wondering why it’s called
a radiometer and not a "lightiometer,"

24
00:01:07,291 --> 00:01:10,355
well that’s good! 
It’s important to the point I want to make here.

25
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.

26
00:01:16,509 --> 00:01:19,144
The construction of these things is pretty simple.

27
00:01:19,144 --> 00:01:23,428
The main structure is a glass envelope not unlike an ordinary lightbulb.

28
00:01:23,428 --> 00:01:26,827
You have a large volume containing a glass support stem,

29
00:01:26,827 --> 00:01:29,352
but rather than supporting an incandescent filament

30
00:01:29,352 --> 00:01:31,659
that stem supports a pin.

31
00:01:31,659 --> 00:01:34,240
Atop the pin is a little glass hat doohickey,

32
00:01:34,240 --> 00:01:37,680
and that hat thingamajig is holding onto four vanes.

33
00:01:37,680 --> 00:01:42,033
The sharp pin and smooth glass create a near-frictionless bearing

34
00:01:42,033 --> 00:01:44,873
allowing the vanes to rotate with little effort.

35
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.

36
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

37
00:01:55,501 --> 00:01:56,927
(as viewed from above)

38
00:01:56,927 --> 00:01:59,832
and the white sides face the clockwise direction.

39
00:01:59,832 --> 00:02:02,339
But don’t let the fact that this is black-and-white fool you.

40
00:02:02,339 --> 00:02:05,791
The way this actually works is anything but black and white.

41
00:02:05,791 --> 00:02:07,547
An interesting fact about these

42
00:02:07,547 --> 00:02:12,063
is that finding out how they work was and still is tricky business.

43
00:02:12,063 --> 00:02:15,137
Even when you buy one from a reputable science shop,

44
00:02:15,137 --> 00:02:19,672
it will often come with an explanation like the one printed on the bottom right here.

45
00:02:19,672 --> 00:02:23,819
“This fascinating radiometer spins from the heat of the sun.

46
00:02:23,819 --> 00:02:27,283
The vanes in the radiometer are dark and light in color.

47
00:02:27,283 --> 00:02:29,286
The dark vanes absorb the rays,

48
00:02:29,286 --> 00:02:31,710
the light vanes reflect the rays.”

49
00:02:32,361 --> 00:02:34,509
I agree that this is fascinating

50
00:02:34,509 --> 00:02:38,610
and that the dark side absorbs and the white side reflects...

51
00:02:38,610 --> 00:02:40,759
but that’s misleading.

52
00:02:40,759 --> 00:02:44,756
I guess technically it didn’t explain anything about why it moves,

53
00:02:44,756 --> 00:02:48,255
but it’s leading you to the phenomenon of light pressure.

54
00:02:48,255 --> 00:02:52,285
That’s the pressure of light not, like, 
"light pressure."

55
00:02:52,285 --> 00:02:55,153
In fact, it’s what our friend Billy Crookes thought was happening

56
00:02:55,153 --> 00:02:57,715
when he first dreamt up this spinny thing.

57
00:02:57,715 --> 00:03:00,414
You may have heard that light has weight.

58
00:03:00,414 --> 00:03:03,864
That’s the weight of light, not lightweight.

59
00:03:03,864 --> 00:03:07,397
But it is very, very lightweight this light weight.

60
00:03:07,397 --> 00:03:10,360
And that’s why this explanation doesn’t work.

61
00:03:10,360 --> 00:03:14,518
It also doesn’t help that it’s rotating backwards than this explanation would predict

62
00:03:14,518 --> 00:03:17,711
but let’s ignore that for now and deal with the pressures involved.

63
00:03:17,711 --> 00:03:20,487
The radiation pressure of sunlight on the Earth

64
00:03:20,487 --> 00:03:24,407
is equivalent to about one milligram of weight per square meter,

65
00:03:24,407 --> 00:03:26,920
or about 10 micronewtons of force.

66
00:03:26,920 --> 00:03:31,150
That’s not nothing, and in fact when we send probes and stuff into space

67
00:03:31,150 --> 00:03:35,671
we need to account for this or the sun will literally push stuff off course.

68
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

69
00:03:40,830 --> 00:03:43,034
to propel spacecraft on their journeys.

70
00:03:43,158 --> 00:03:46,204
That is, after all, how the ancient Bajorans got to Cardassia.

71
00:03:46,829 --> 00:03:50,839
Trouble is, this is insignificant at this scale.

72
00:03:50,839 --> 00:03:54,790
These vanes are, maybe, 10 millimeters square.

73
00:03:54,790 --> 00:04:00,216
So in direct sunlight, about .0016 micronewtons of force

74
00:04:00,216 --> 00:04:02,730
is exerted by the sun on each vane.

75
00:04:02,730 --> 00:04:06,385
Or about .13 micrograms.

76
00:04:06,385 --> 00:04:09,202
If that sounds like very little that’s because it is.

77
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.

78
00:04:13,783 --> 00:04:18,766
Even the tiny LED from my phone’s camera will make it turn a little bit.

79
00:04:18,766 --> 00:04:22,989
The light pressure exerted here is so hilariously meaningless that you can ignore it entirely.

80
00:04:22,989 --> 00:04:23,885
Trust me.

81
00:04:23,885 --> 00:04:24,908
It’s fine.

82
00:04:24,908 --> 00:04:30,719
Even if the miniscule forces light pressures
exert on this scale were enough to spin it,

83
00:04:30,719 --> 00:04:34,368
the other thing is that light pressure is actually greater

84
00:04:34,368 --> 00:04:37,982
when it reflects off something than what it’s absorbed.

85
00:04:37,982 --> 00:04:40,516
The explanation I first heard about these

86
00:04:40,516 --> 00:04:43,235
was that the black side absorbed the photons

87
00:04:43,235 --> 00:04:47,784
and thus their tiny bit of momentum was transferred to it, pushing it away,

88
00:04:47,784 --> 00:04:51,662
and since they bounced off the white side their momentum wasn’t transferred,

89
00:04:51,662 --> 00:04:55,017
thus you get a pushing force on the black side but not on the white.

90
00:04:56,007 --> 00:04:57,361
That’s just malarkey.

91
00:04:57,361 --> 00:04:58,468
Doesn’t work that way.

92
00:04:58,468 --> 00:05:04,137
Solar radiation pressure with perfect reflection is twice that of perfect absorption,

93
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,

94
00:05:10,516 --> 00:05:14,459
were this the phenomenon at play this is rotating in the wrong direction.

95
00:05:14,459 --> 00:05:18,148
There should actually be more force on the white side than the black,

96
00:05:18,148 --> 00:05:20,418
so the black side should be leading.

97
00:05:20,801 --> 00:05:22,240
But it’s not.

98
00:05:22,240 --> 00:05:25,718
And the final nail in the coffin for this particular theory?

99
00:05:25,718 --> 00:05:30,497
Well, what’s in that glass other than the vanes and pin and hat thingamabob

100
00:05:30,497 --> 00:05:32,237
is one of these.

101
00:05:32,237 --> 00:05:33,935
A partial vacuum.

102
00:05:33,935 --> 00:05:35,860
If this were a complete vacuum

103
00:05:35,860 --> 00:05:38,831
(or at least as complete as we know how to make one)

104
00:05:38,831 --> 00:05:40,307
this wouldn’t work.

105
00:05:40,307 --> 00:05:45,062
If it operated on the principle of radiation pressure it should work just fine -

106
00:05:45,062 --> 00:05:49,076
after all, radiation pressure is a thing in the vacuum of space.

107
00:05:49,076 --> 00:05:52,781
But this radiometer actually needs some atmosphere to function,

108
00:05:52,781 --> 00:05:56,479
but it needs to be a near-vacuum or there would be too much air resistance

109
00:05:56,479 --> 00:05:58,868
for the vanes to spin freely.

110
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

111
00:06:03,665 --> 00:06:06,902
but, uh, we’ve come this far so I might as well finish the explanation.

112
00:06:07,553 --> 00:06:12,144
Longtime viewers of the channel will now groan in unison as I inform you

113
00:06:12,144 --> 00:06:15,497
that this is in fact a heat engine.

114
00:06:16,148 --> 00:06:18,683
Just can’t get away from them, can we?

115
00:06:18,683 --> 00:06:21,415
The difference in color is important for the operation

116
00:06:21,415 --> 00:06:23,895
but not because of photon momentum.

117
00:06:23,895 --> 00:06:28,299
Instead it’s simply that the black side heats up faster than the white side

118
00:06:28,299 --> 00:06:30,562
when exposed to incoming energy.

119
00:06:30,562 --> 00:06:33,453
Both are being heated thanks to blackbody absorption,

120
00:06:33,453 --> 00:06:36,235
but the black side gets hotter faster.

121
00:06:36,235 --> 00:06:39,086
This heats up the air molecules that are next to it.

122
00:06:39,086 --> 00:06:41,388
And that’s why it moves.

123
00:06:41,388 --> 00:06:45,242
If you want to see more details and experimentation related to this,

124
00:06:45,242 --> 00:06:47,525
check out this video from Applied Science.

125
00:06:47,525 --> 00:06:50,655
It’s really worth a watch (as is the entire channel!).

126
00:06:50,655 --> 00:06:54,913
But a very quick summary is that two phenomena are involved here.

127
00:06:54,913 --> 00:07:00,002
The first is simply that the hotter black side causes gas molecules that bounce into it

128
00:07:00,002 --> 00:07:04,097
to bounce away more forcefully than they do against the cooler white side.

129
00:07:04,097 --> 00:07:06,888
And the other phenomenon is thermal transpiration,

130
00:07:06,888 --> 00:07:12,843
which is basically a fancy thing to call forces that occur on gases at different temperatures.

131
00:07:12,843 --> 00:07:17,623
The temperature difference between the two sides
creates a slight difference in this force,

132
00:07:17,623 --> 00:07:20,003
which in the end helps the vanes rotate.

133
00:07:20,759 --> 00:07:22,920
Now that we know how these things work,

134
00:07:22,920 --> 00:07:25,655
let’s talk about why they’re so interesting to me.

135
00:07:25,655 --> 00:07:29,588
I said that I had a memory burned into my brain regarding these things.

136
00:07:29,588 --> 00:07:32,237
Long ago, when I was maybe… eight?

137
00:07:32,237 --> 00:07:36,159
I don’t remember exactly, but my parents were thinking about moving.

138
00:07:36,159 --> 00:07:39,221
We toured a model home of a new housing development,

139
00:07:39,221 --> 00:07:42,116
I think it was called Sudden Valley if I remember right,

140
00:07:42,116 --> 00:07:47,193
anyway the homebuilder had set up a demonstration using two of these radiometers

141
00:07:47,193 --> 00:07:51,355
to tout the benefits of their home’s amazing new windows.

142
00:07:51,355 --> 00:07:53,540
See, this isn’t just a "lightiometer"

143
00:07:53,540 --> 00:07:58,281
because any radiation that will cause the vanes to heat up through blackbody absorption

144
00:07:58,281 --> 00:08:00,029
will make this move.

145
00:08:00,029 --> 00:08:03,897
Infrared is one such source of non-visible radiation.

146
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.

147
00:08:08,986 --> 00:08:12,668
Indeed, intense sources of heat will make this thing move.

148
00:08:12,668 --> 00:08:14,951
But, radiant sources of heat -

149
00:08:14,951 --> 00:08:17,014
in other words, intense infrared -

150
00:08:17,014 --> 00:08:19,005
will make it fly.

151
00:08:19,005 --> 00:08:21,537
Take a look - here’s an ordinary space heater.

152
00:08:21,537 --> 00:08:24,684
This one isn’t meant to be a source of radiant heat,

153
00:08:24,684 --> 00:08:28,081
instead it heats the air through blowing it over hot wires.

154
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,

155
00:08:33,131 --> 00:08:36,793
so while the radiometer does move, it’s not a whole lot.

156
00:08:36,793 --> 00:08:40,131
Now take a look at what it does in front of this toaster oven.

157
00:08:40,131 --> 00:08:43,454
The heating elements here are in fact quartz heat lamps,

158
00:08:43,454 --> 00:08:46,527
and these eventually start to incandesce intensely.

159
00:08:46,527 --> 00:08:50,049
They’re so hot they’re comfortably out of the red zone, and into the orange.

160
00:08:50,049 --> 00:08:51,523
Almost yellow.

161
00:08:51,523 --> 00:08:56,776
But that means they’re also throwing out
tons of shortwave infrared, and indeed -

162
00:08:56,776 --> 00:08:59,256
the radiometer agrees.

163
00:08:59,256 --> 00:09:02,759
In something like a toaster oven and indeed an ordinary toaster

164
00:09:02,759 --> 00:09:05,792
we want to have plenty of infrared radiation.

165
00:09:05,792 --> 00:09:09,645
Heating something through hot air alone produces a very different effect

166
00:09:09,645 --> 00:09:12,149
compared to heating it with radiant energy.

167
00:09:12,149 --> 00:09:17,508
Lots of IR will be better at browning the
surface of something without drying it out,

168
00:09:17,508 --> 00:09:20,983
as the energy becomes absorbed at the surface
of whatever’s being heated

169
00:09:20,983 --> 00:09:25,344
and so in a sense that surface becomes its own source of heat.

170
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,

171
00:09:30,577 --> 00:09:36,069
but the inside warms slowly as the heat from the
surface gradually seeps through.

172
00:09:36,069 --> 00:09:40,663
You’ve felt this before when near a campfire
or under a heat lamp.

173
00:09:40,663 --> 00:09:45,460
Infrared radiation can travel through the air or even the vacuum
of space,

174
00:09:45,460 --> 00:09:50,073
and it’s only when it becomes absorbed by something that the heat energy becomes apparent.

175
00:09:50,073 --> 00:09:53,358
That warm tingly sensation from a radiant source of heat

176
00:09:53,358 --> 00:09:59,020
is how you feel your skin being heated not by warm air, but by radiation.

177
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

178
00:10:04,060 --> 00:10:06,152
or open train platforms.

179
00:10:06,152 --> 00:10:09,250
They can warm you up even when it’s very cold outside

180
00:10:09,250 --> 00:10:13,192
because they transmit that heat through the air to your skin.

181
00:10:13,192 --> 00:10:16,674
Their goal isn’t to heat the air, but you.

182
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.

183
00:10:22,470 --> 00:10:25,402
Because the heating element is visibly incandescing

184
00:10:25,402 --> 00:10:29,658
we can conclude that there’s a broad spectrum of infrared being emitted here,

185
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.

186
00:10:36,512 --> 00:10:42,642
Ultimately that makes the glass hot which
warms up the air molecules inside the envelope.

187
00:10:42,642 --> 00:10:46,967
An interesting side-effect of this is that
when taken away from the radiation,

188
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.

189
00:10:53,578 --> 00:10:55,824
This isn’t really important to the point I’m making

190
00:10:55,824 --> 00:11:01,494
but it does further demonstrate how the rotational action has nothing to do with light pressure,

191
00:11:01,494 --> 00:11:03,832
but is a thermal phenomenon.

192
00:11:03,832 --> 00:11:06,411
Speaking of the point I’m trying to make…

193
00:11:06,411 --> 00:11:07,917
what is that?

194
00:11:07,917 --> 00:11:11,366
Ah, right, uh, going back to that model home demo.

195
00:11:11,366 --> 00:11:15,568
To demonstrate the amazingness of their amazing
new windows of amazement,

196
00:11:15,568 --> 00:11:19,399
the model home had a little demo consisting of two floodlights,

197
00:11:19,399 --> 00:11:23,186
two radiometers, and in between the radiometers and floodlights

198
00:11:23,186 --> 00:11:25,418
were two different window panes.

199
00:11:25,418 --> 00:11:27,973
One was your ordinary garden variety window,

200
00:11:27,973 --> 00:11:30,565
and the other was this amazing new whatever it was.

201
00:11:30,565 --> 00:11:34,534
The radiometer in front of the ordinary window
was spinning quite quickly,

202
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.

203
00:11:40,285 --> 00:11:43,497
This served to create an immediately intuitive
sense

204
00:11:43,497 --> 00:11:48,016
of how the better glass prevented heat energy from passing through it.

205
00:11:48,016 --> 00:11:51,565
Now, the scientifically-inclined and skeptical  among you

206
00:11:51,565 --> 00:11:55,136
will no doubt realize that this could easily have been rigged.

207
00:11:55,136 --> 00:11:58,921
After all, who’s to say that the two radiometers performed equally?

208
00:11:59,468 --> 00:12:03,194
Sadly, at the age of eight I wasn’t yet thinking about these things,

209
00:12:03,194 --> 00:12:08,189
otherwise I would have switched those two radiometers to make sure the result was the same.

210
00:12:08,189 --> 00:12:11,319
Soon as I get that time machine perfected I’ll go back and do it!

211
00:12:11,319 --> 00:12:13,937
But as it happens, this home is fairly new

212
00:12:13,937 --> 00:12:16,627
and it has even newer windows than that model home did.

213
00:12:16,627 --> 00:12:20,324
At least, if my understanding of linear time is correct.

214
00:12:20,324 --> 00:12:23,208
While I don’t have a second window to compare to,

215
00:12:23,208 --> 00:12:25,990
I can just… open one.

216
00:12:25,990 --> 00:12:30,560
Here's a great demonstration of how much infrared light this window is blocking.

217
00:12:30,560 --> 00:12:34,438
Were it not for the window, the radiometer
would be in direct sunlight.

218
00:12:34,438 --> 00:12:38,255
And you can see that it's actually not turning all that quickly.

219
00:12:38,255 --> 00:12:41,794
If I open the window, though, it immediately speeds up.

220
00:12:41,794 --> 00:12:44,771
Here we see using the same radiometer

221
00:12:44,771 --> 00:12:49,186
that the window drastically reduces the total amount of energy passing through it

222
00:12:49,186 --> 00:12:53,368
while only marginally decreasing the visible light that makes it through.

223
00:12:53,368 --> 00:12:54,605
Neat!

224
00:12:54,605 --> 00:12:58,525
How this is accomplished doesn’t really matter for
the purposes of this video,

225
00:12:58,525 --> 00:13:00,204
but let’s talk about it anyway!

226
00:13:00,204 --> 00:13:06,395
This window, like many out there, features
low-emissivity glass (often called low-E).

227
00:13:06,395 --> 00:13:09,897
At the time it was made, a coating was deposited
on the glass

228
00:13:09,897 --> 00:13:13,566
which will reject much of the near-infrared light that hits it.

229
00:13:13,566 --> 00:13:16,502
Since glass will absorb certain IR wavelengths,

230
00:13:16,502 --> 00:13:20,478
we want those wavelengths to be reflected away before it even hits the glass,

231
00:13:20,478 --> 00:13:23,299
otherwise the glass would be heated by this energy

232
00:13:23,299 --> 00:13:27,037
and end up allowing much of that into the space anyway.

233
00:13:27,037 --> 00:13:29,958
While visible light does plenty of heating by itself,

234
00:13:29,958 --> 00:13:34,751
a slight majority of the sun’s energy that reaches the Earth’s surface is infrared.

235
00:13:34,751 --> 00:13:39,338
Windows that can reflect just this portion
of the sun’s energy away

236
00:13:39,338 --> 00:13:42,331
reduce the total amount of energy that passes through them

237
00:13:42,331 --> 00:13:45,535
and thus lower the amount of heating caused by the sun

238
00:13:45,535 --> 00:13:48,824
all without appearing darker to our eyes.

239
00:13:48,860 --> 00:13:51,249
And these windows are great at this!

240
00:13:51,249 --> 00:13:53,831
Not only does the radiometer demonstrate this nicely,

241
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

242
00:13:58,723 --> 00:14:01,929
and the sunlight that makes it through the glass on your skin.

243
00:14:01,929 --> 00:14:05,370
There’s a significant, noticeable difference.

244
00:14:05,370 --> 00:14:09,339
Voiceover me is here to tell you about another demo we can do!

245
00:14:09,339 --> 00:14:13,509
This halogen floodlight produces quite a lot of directed infrared.

246
00:14:13,509 --> 00:14:17,396
Don’t think about that too much or it might spoil a later demo but, uh

247
00:14:17,396 --> 00:14:21,731
anyway if I put it outside and shine it at the radiometer through
the window,

248
00:14:21,731 --> 00:14:24,080
you can see it doesn’t move all that much.

249
00:14:24,080 --> 00:14:26,227
If, however, I bring it back inside

250
00:14:26,227 --> 00:14:29,615
and point it directly at the radiometer from about the same distance,

251
00:14:29,615 --> 00:14:32,033
it spins very rapidly.

252
00:14:32,033 --> 00:14:35,725
That window’s doing a lot to keep the infrared
from coming through.

253
00:14:35,725 --> 00:14:39,285
And to be clear, ordinary glass doesn’t do hardly anything at all

254
00:14:39,285 --> 00:14:40,848
as seen here.

255
00:14:40,848 --> 00:14:43,532
An interesting side-effect of this low-E coating

256
00:14:43,532 --> 00:14:48,294
is that the internal reflections in the glass become color-shifted as they repeat.

257
00:14:48,294 --> 00:14:50,234
And the way they shift is surprising.

258
00:14:50,234 --> 00:14:52,071
I can’t really make heads or tails of it.

259
00:14:52,071 --> 00:14:53,782
It’s not consistent at all,

260
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

261
00:14:58,461 --> 00:15:03,658
(this is one of my portable RGBW studio lights with both white color temps lit).

262
00:15:03,658 --> 00:15:07,510
However, the very far repeated reflections
all look pretty blue,

263
00:15:07,510 --> 00:15:10,299
which makes sense as that’s farthest from infrared.

264
00:15:10,299 --> 00:15:16,006
Wavelengths even remotely close to IR probably
get rejected by the eighth reflection or so.

265
00:15:16,006 --> 00:15:19,842
And actually, one thing I was hoping I could
demonstrate but wasn’t able to

266
00:15:19,842 --> 00:15:22,416
was that depending on the angle of the sun,

267
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.

268
00:15:29,062 --> 00:15:32,510
I was playing around with the floodlight but
could only barely see it.

269
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:

270
00:15:37,500 --> 00:15:41,506
it can actually cause damage to various things
that end up in the sun

271
00:15:41,506 --> 00:15:44,629
and also get hit with a reflection off a window.

272
00:15:44,629 --> 00:15:48,127
Those objects end up getting more IR than
direct sunlight,

273
00:15:48,127 --> 00:15:52,221
and with modern building designs with tons of windows this can become problematic,

274
00:15:52,221 --> 00:15:54,048
especially in urban settings.

275
00:15:54,048 --> 00:15:59,415
And especially especially if architects get
creative with curved building surfaces.

276
00:15:59,415 --> 00:16:03,339
Now I deleted a section here in editing, so
here’s a sloppy segue.

277
00:16:03,339 --> 00:16:05,418
What about in the winter time?

278
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.

279
00:16:11,172 --> 00:16:13,357
Everything radiates heat energy

280
00:16:13,357 --> 00:16:15,739
(unless it’s at absolute zero, of course),

281
00:16:15,739 --> 00:16:18,456
and if that energy goes through a window…

282
00:16:18,456 --> 00:16:20,016
well now it’s outside.

283
00:16:20,016 --> 00:16:24,036
If the window can reflect long-wave infrared back inward, though,

284
00:16:24,036 --> 00:16:28,013
it will be able to prevent much of that radiant energy from being lost through it.

285
00:16:28,013 --> 00:16:31,417
Instead it will end up on another inside surface
where it will re-radiate

286
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.

287
00:16:36,138 --> 00:16:37,587
Voiceover me again.

288
00:16:37,587 --> 00:16:40,551
I was gonna do some sort of whiteboard drawing
to show this

289
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.

290
00:16:45,256 --> 00:16:48,003
Here I’m pointing the floodlight out the window

291
00:16:48,003 --> 00:16:52,736
at an angle so that the reflected light will land on the radiometer inside the room.

292
00:16:52,736 --> 00:16:56,303
Though most of the visible was unimpeded as
it left the room,

293
00:16:56,303 --> 00:17:01,170
the IR was reflected right back and the vanes are spinning quite quickly to prove it.

294
00:17:01,170 --> 00:17:02,356
Pretty neat.

295
00:17:02,356 --> 00:17:06,879
I had assumed that the inside coating might
be focused more on long-wave infrared

296
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.

297
00:17:12,306 --> 00:17:18,916
However, the benefit of rejecting infrared
from the outside becomes a drawback in the winter.

298
00:17:18,916 --> 00:17:20,882
When you need to heat your home,

299
00:17:20,882 --> 00:17:26,306
windows which are blocking a majority of the free solar radiation coming at them

300
00:17:26,306 --> 00:17:28,002
are kind of a bummer.

301
00:17:28,002 --> 00:17:32,124
And indeed, in the winter these windows
are kind of a bummer.

302
00:17:32,124 --> 00:17:36,544
It would be great if we could develop some
sort of selectable Low-E window.

303
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.

304
00:17:42,433 --> 00:17:47,632
Turning it inward would thus allow for more
IR transmission in the winter.

305
00:17:47,632 --> 00:17:49,035
But maybe that wouldn’t work.

306
00:17:49,035 --> 00:17:53,057
Truthfully I don’t know how much the side
that the coating is on actually matters,

307
00:17:53,057 --> 00:17:57,629
but one of you probably does and I hope you’ll
let us know down there.

308
00:17:57,629 --> 00:18:01,020
There is, though, a solution to this problem that we know of today

309
00:18:01,020 --> 00:18:03,948
which is incorporated in the passive house design.

310
00:18:03,948 --> 00:18:06,853
Because the angle of the sun changes through
the year

311
00:18:06,853 --> 00:18:09,574
with higher angles in the summer and lower ones in the winter

312
00:18:09,574 --> 00:18:12,874
(especially where it gets cold enough to need a lot of seasonal heat)

313
00:18:12,874 --> 00:18:14,870
we actually have a workaround.

314
00:18:14,870 --> 00:18:19,030
A structure like this can be fitted with solar-control
windows up here,

315
00:18:19,030 --> 00:18:21,675
and standard windows below this awning.

316
00:18:21,675 --> 00:18:25,982
In the summer months, the awning prevents
direct sunlight from hitting these standard windows,

317
00:18:25,982 --> 00:18:28,122
so they don’t need to reject IR.

318
00:18:28,122 --> 00:18:33,164
It will only hit the transoms, providing plenty
of natural light but with minimal solar heating.

319
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,

320
00:18:39,052 --> 00:18:41,294
providing some free heating.

321
00:18:41,294 --> 00:18:45,442
Still, though, I think developing a window
that can reject infrared in the summer

322
00:18:45,442 --> 00:18:48,951
but allow it through in the winter is a worthwhile cause,

323
00:18:48,951 --> 00:18:54,057
but the mechanism of how the glass rejects infrared is really beside the point.

324
00:18:54,057 --> 00:18:57,331
We were talking about radiometers, at one point.

325
00:18:57,331 --> 00:18:58,221
Uh...

326
00:18:58,221 --> 00:19:01,621
What I find fascinating about the radiometer is that,

327
00:19:01,621 --> 00:19:05,861
while it certainly isn’t a precise tool giving any useful units,

328
00:19:05,861 --> 00:19:10,549
it can be very useful in demonstrating the concept of radiant energy.

329
00:19:10,549 --> 00:19:13,901
For example, here’s another fantastic use for one.

330
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,

331
00:19:20,328 --> 00:19:25,054
but a pair of radiometers would have made
a great in-store display.

332
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.

333
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.

334
00:19:36,159 --> 00:19:39,843
I only have one of 'em after all so we’ll split screen it!

335
00:19:39,843 --> 00:19:43,208
The large amount of IR emitted from the incandescent
bulb

336
00:19:43,208 --> 00:19:48,242
heats the vanes of the radiometer quickly and intensely, causing rapid motion.

337
00:19:48,242 --> 00:19:50,227
But the LED bulb next to it,

338
00:19:50,227 --> 00:19:54,743
producing none of the medium and shortwave IR that the incandescent light does,

339
00:19:54,743 --> 00:19:57,042
hardly makes it turn at all.

340
00:19:57,042 --> 00:19:58,972
Sure, it still turns a bit -

341
00:19:58,972 --> 00:20:03,456
the visible light is absorbed by the black paint and also makes it heat up.

342
00:20:03,456 --> 00:20:06,740
But the total amount of energy being radiated
from the light bulb

343
00:20:06,740 --> 00:20:09,854
is a small fraction compared to the incandescent,

344
00:20:09,854 --> 00:20:12,151
yet the bulb is just as bright.

345
00:20:12,151 --> 00:20:16,472
Our eyes don’t care about most of the radiation
that an incandescent bulb creates,

346
00:20:16,472 --> 00:20:19,699
so why waste energy making it in the first place?

347
00:20:19,699 --> 00:20:23,501
And this is where I think the radiometer can
teach us a valuable lesson.

348
00:20:23,501 --> 00:20:27,005
Sure, I could grab a power meter and put each
light bulb on there

349
00:20:27,005 --> 00:20:29,191
to give you the difference with raw numbers.

350
00:20:29,191 --> 00:20:32,150
But this is far more… visceral.

351
00:20:32,150 --> 00:20:33,086
Intuitive.

352
00:20:33,086 --> 00:20:34,772
Plainly evident.

353
00:20:34,772 --> 00:20:38,128
It doesn’t require comparing numbers to
see that there’s a difference.

354
00:20:38,128 --> 00:20:40,685
There just… is one.

355
00:20:40,685 --> 00:20:43,342
Similarly, I could go window shopping

356
00:20:43,342 --> 00:20:47,387
(that’s shopping for windows, not window shopping)

357
00:20:47,387 --> 00:20:50,493
and take a look at the IR rejection specifications,

358
00:20:50,493 --> 00:20:52,813
R-values, and other data,

359
00:20:52,813 --> 00:20:56,987
but not everyone will understand why those data are important.

360
00:20:56,987 --> 00:21:01,582
Using something that provides you with clear
evidence of infrared radiation

361
00:21:01,582 --> 00:21:05,969
without requiring any contextualizing or prior understanding

362
00:21:05,969 --> 00:21:11,432
makes the knowledge and importance of measures to abate infrared more accessible.

363
00:21:11,432 --> 00:21:15,759
I would argue that you don’t even need much awareness
of what the radiometer does

364
00:21:15,759 --> 00:21:18,820
for it to be an effective demonstration tool.

365
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.

366
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.

367
00:21:33,180 --> 00:21:36,502
It may have just stuck in my mind because
these things are cool,

368
00:21:36,502 --> 00:21:39,626
dare I say nifty.

369
00:21:39,626 --> 00:21:41,502
I feel like I got the point of it because

370
00:21:41,502 --> 00:21:44,880
I remember there being two radiometers and two different windows

371
00:21:44,880 --> 00:21:47,361
and one of them was spinning faster than the other,

372
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.

373
00:21:53,143 --> 00:21:57,420
And unfortunately, that’s something my time
machine won’t be able to tell me,

374
00:21:57,420 --> 00:22:00,722
at least not without asking my former self

375
00:22:00,722 --> 00:22:04,247
and who knows what the consequences of that might be?

376
00:22:04,247 --> 00:22:07,480
It could turn that kid into some sort of…

377
00:22:07,480 --> 00:22:08,586
influencer.

378
00:22:08,586 --> 00:22:09,598
Eugh!

379
00:22:09,598 --> 00:22:13,715
Anyway, while William Crookes was wrong about
how this thing works

380
00:22:13,715 --> 00:22:15,955
and it’s mainly relegated to the

381
00:22:15,955 --> 00:22:18,749
“well that’s neat!” category of existence,

382
00:22:18,749 --> 00:22:22,958
I think devices like this are more useful than we perhaps realize.

383
00:22:22,958 --> 00:22:26,059
Sometimes it’s not the accuracy of the tool
that matters

384
00:22:26,059 --> 00:22:29,052
but how easily it demonstrates something.

385
00:22:29,052 --> 00:22:33,369
If a 19th century scientific curiosity can
demonstrate to an eight year old

386
00:22:33,369 --> 00:22:37,413
that some windows - or some light bulbs - are better than others,

387
00:22:37,413 --> 00:22:41,620
who knows what might be out there which can do the same for other concepts?

388
00:22:41,620 --> 00:22:43,669
No really, do you know?

389
00:22:43,669 --> 00:22:45,571
I... I think we should be looking into this.

390
00:22:45,571 --> 00:22:49,598
There’s a lot of stuff out there these days
that’s complicated to communicate,

391
00:22:49,598 --> 00:22:53,075
and whenever that happens there’s room for misinformation.

392
00:22:53,075 --> 00:22:56,772
A lot of people are skeptical about the merits
of technological change,

393
00:22:56,772 --> 00:23:01,014
even when it’s for something so mundane as improving energy
efficiency.

394
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 -

395
00:23:05,981 --> 00:23:09,450
not all progress is truly progressive, after all.

396
00:23:09,450 --> 00:23:13,678
But a lot of common-sense progress gets met
with skepticism these days,

397
00:23:13,678 --> 00:23:18,439
and maybe there are ancient thingamjigs like this that can
help with that.

398
00:23:18,439 --> 00:23:22,421
Maybe fighting misinformation is going a little
too deep,

399
00:23:22,421 --> 00:23:26,690
but I do earnestly think that there’s value in exploring this.

400
00:23:26,690 --> 00:23:29,982
There are probably all sorts of old curios
out there

401
00:23:29,982 --> 00:23:35,998
that seem pointless by modern standards but which could actually provide a meaningful educational experience.

402
00:23:35,998 --> 00:23:41,822
Being able to demonstrate a concept in a purely intuitive
fashion can be very powerful,

403
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,

404
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.

405
00:23:51,601 --> 00:23:53,373
Thanks for watching.

406
00:23:54,363 --> 00:23:56,943
♫ transpirationally smooth jazz ♫

407
00:23:57,907 --> 00:24:00,099
Have you ever seen one of these?

408
00:24:00,099 --> 00:24:01,329
I… nope.

409
00:24:01,329 --> 00:24:04,226
The final nail in the coffin for this theory…

410
00:24:04,226 --> 00:24:05,555
oh yeah, shoot!

411
00:24:06,806 --> 00:24:07,751
I need a prop!

412
00:24:07,751 --> 00:24:11,338
Oh, and I restarted the teleprompter to the
very beginning!

413
00:24:11,338 --> 00:24:13,645
Why do I still use this one?

414
00:24:14,713 --> 00:24:17,402
It’s because I’m lazy, and I’m used to it.

415
00:24:17,751 --> 00:24:23,494
These curiosities have been staples of novelty
shops and museum science gift stores.

416
00:24:23,494 --> 00:24:24,222
Oops.

417
00:24:24,587 --> 00:24:25,958
A partial vacuum.

418
00:24:25,958 --> 00:24:28,696
If this were a complete [clunk] vacuum...

419
00:24:28,696 --> 00:24:31,768
Well that’s not gonna work, is it?

420
00:24:31,768 --> 00:24:34,170
Move de shtuff out of de vay!

421
00:24:34,170 --> 00:24:38,371
It also doesn’t help that it’s rotating
backwards from that explanation.

422
00:24:38,371 --> 00:24:40,041
Egghhh.

423
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.

424
00:24:43,331 --> 00:24:44,599
It’s not very easy.

425
00:24:44,599 --> 00:24:46,666
My eyes can’t adjust that fast.

426
00:24:47,239 --> 00:24:50,880
I have a very burned-in memory of these things which has made

427
00:24:50,880 --> 00:24:51,855
... (mouth noises of disappointment)

428
00:24:53,267 --> 00:24:55,422
What word should I choose other than malarkey?

429
00:24:55,839 --> 00:24:56,984
[thinking, stand-by]

430
00:24:57,323 --> 00:24:58,458
Hogwash.

431
00:24:58,458 --> 00:24:59,696
Nahhh!

432
00:24:59,696 --> 00:25:04,231
If it operated on the princ per he blerghhhhfff.

433
00:25:04,231 --> 00:25:08,880
The temperature gradient between the two sides
creates a slight difference in…

434
00:25:08,880 --> 00:25:09,872
something.

435
00:25:09,872 --> 00:25:12,374
Where did that word go?

436
00:25:12,374 --> 00:25:15,368
It’s over there. In this force.

437
00:25:15,368 --> 00:25:16,637
[mouth popping sounds]

438
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.

439
00:25:21,334 --> 00:25:22,988
But it needs a bit.

440
00:25:22,988 --> 00:25:24,324
And that wasn’t in the script.

441
00:25:24,324 --> 00:25:25,728
You’ve adlibbed.

442
00:25:25,728 --> 00:25:27,580
You terrible person.

443
00:25:27,580 --> 00:25:30,260
I said ah high bleu… great.

444
00:25:31,433 --> 00:25:32,714
That section I deleted?

445
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,

446
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)

447
00:25:39,151 --> 00:25:41,447
is almost too hot to the touch when I'm out in the sun.

448
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.

449
00:25:45,335 --> 00:25:46,595
It was a riveting story.

450
00:25:46,595 --> 00:25:47,512
Too bad it didn't make the cut.