Have you ever seen one of these?
I sure have.
There’s one right here!
I have a very burned-in memory of these things which has survived
all this time to make a video for you.
These curiosities have been staples of novelty stores
and science museum gift shops for many, many years.
One common name for them is the light mill.
See, it’s turning because there’s a light shining on it.
If I make the light brighter, well,
it will speed up.
And if the light goes out,
it will slow down and eventually stop.
There’s some sort of physics goin' on in there, I just know it!
While light mill may be a common name,
a better one is radiometer.
An even more better one is Crookes Radiometer,
named for its inventor Sir Willam Crookes.
Fun fact, Richard Nixon had one!
But he denied it.
Anyway, invented in 1873
the vanes of this device will, when it’s exposed to light, rotate.
Now if you’re wondering why it’s called a radiometer and not a "lightiometer,"
well that’s good! It’s important to the point I want to make here.
As a hint, keep in mind that visible light is only a small fraction of the electromagnetic spectrum.
The construction of these things is pretty simple.
The main structure is a glass envelope not unlike an ordinary lightbulb.
You have a large volume containing a glass support stem,
but rather than supporting an incandescent filament
that stem supports a pin.
Atop the pin is a little glass hat doohickey,
and that hat thingamajig is holding onto four vanes.
The sharp pin and smooth glass create a near-frictionless bearing
allowing the vanes to rotate with little effort.
Of course the signature feature of the vanes is that one side is black and the other side is white.
They’re arranged so that the black sides always face in the counter-clockwise direction of rotation
(as viewed from above)
and the white sides face the clockwise direction.
But don’t let the fact that this is black-and-white fool you.
The way this actually works is anything but black and white.
An interesting fact about these
is that finding out how they work was and still is tricky business.
Even when you buy one from a reputable science shop,
it will often come with an explanation like the one printed on the bottom right here.
“This fascinating radiometer spins from the heat of the sun.
The vanes in the radiometer are dark and light in color.
The dark vanes absorb the rays,
the light vanes reflect the rays.”
I agree that this is fascinating
and that the dark side absorbs and the white side reflects...
but that’s misleading.
I guess technically it didn’t explain anything about why it moves,
but it’s leading you to the phenomenon of light pressure.
That’s the pressure of light not, like, "light pressure."
In fact, it’s what our friend Billy Crookes thought was happening
when he first dreamt up this spinny thing.
You may have heard that light has weight.
That’s the weight of light, not lightweight.
But it is very, very lightweight this light weight.
And that’s why this explanation doesn’t work.
It also doesn’t help that it’s rotating backwards than this explanation would predict
but let’s ignore that for now and deal with the pressures involved.
The radiation pressure of sunlight on the Earth
is equivalent to about one milligram of weight per square meter,
or about 10 micronewtons of force.
That’s not nothing, and in fact when we send probes and stuff into space
we need to account for this or the sun will literally push stuff off course.
In fact with solar sails we could potentially use the sun or maybe an Earth-bound laser
to propel spacecraft on their journeys.
That is, after all, how the ancient Bajorans got to Cardassia.
Trouble is, this is insignificant at this scale.
These vanes are, maybe, 10 millimeters square.
So in direct sunlight, about .0016 micronewtons of force
is exerted by the sun on each vane.
Or about .13 micrograms.
If that sounds like very little that’s because it is.
And yet, we don’t need anything nearly as powerful as sunlight to get this to move.
Even the tiny LED from my phone’s camera will make it turn a little bit.
The light pressure exerted here is so hilariously meaningless that you can ignore it entirely.
Trust me.
It’s fine.
Even if the miniscule forces light pressures exert on this scale were enough to spin it,
the other thing is that light pressure is actually greater
when it reflects off something than what it’s absorbed.
The explanation I first heard about these
was that the black side absorbed the photons
and thus their tiny bit of momentum was transferred to it, pushing it away,
and since they bounced off the white side their momentum wasn’t transferred,
thus you get a pushing force on the black side but not on the white.
That’s just malarkey.
Doesn’t work that way.
Solar radiation pressure with perfect reflection is twice that of perfect absorption,
and while it’s hard to say whether this even matters when both surfaces in question are painted with ordinary paint,
were this the phenomenon at play this is rotating in the wrong direction.
There should actually be more force on the white side than the black,
so the black side should be leading.
But it’s not.
And the final nail in the coffin for this particular theory?
Well, what’s in that glass other than the vanes and pin and hat thingamabob
is one of these.
A partial vacuum.
If this were a complete vacuum
(or at least as complete as we know how to make one)
this wouldn’t work.
If it operated on the principle of radiation pressure it should work just fine -
after all, radiation pressure is a thing in the vacuum of space.
But this radiometer actually needs some atmosphere to function,
but it needs to be a near-vacuum or there would be too much air resistance
for the vanes to spin freely.
Ultimately, how this works isn’t important to the point I wanted to make with this video
but, uh, we’ve come this far so I might as well finish the explanation.
Longtime viewers of the channel will now groan in unison as I inform you
that this is in fact a heat engine.
Just can’t get away from them, can we?
The difference in color is important for the operation
but not because of photon momentum.
Instead it’s simply that the black side heats up faster than the white side
when exposed to incoming energy.
Both are being heated thanks to blackbody absorption,
but the black side gets hotter faster.
This heats up the air molecules that are next to it.
And that’s why it moves.
If you want to see more details and experimentation related to this,
check out this video from Applied Science.
It’s really worth a watch (as is the entire channel!).
But a very quick summary is that two phenomena are involved here.
The first is simply that the hotter black side causes gas molecules that bounce into it
to bounce away more forcefully than they do against the cooler white side.
And the other phenomenon is thermal transpiration,
which is basically a fancy thing to call forces that occur on gases at different temperatures.
The temperature difference between the two sides creates a slight difference in this force,
which in the end helps the vanes rotate.
Now that we know how these things work,
let’s talk about why they’re so interesting to me.
I said that I had a memory burned into my brain regarding these things.
Long ago, when I was maybe… eight?
I don’t remember exactly, but my parents were thinking about moving.
We toured a model home of a new housing development,
I think it was called Sudden Valley if I remember right,
anyway the homebuilder had set up a demonstration using two of these radiometers
to tout the benefits of their home’s amazing new windows.
See, this isn’t just a "lightiometer"
because any radiation that will cause the vanes to heat up through blackbody absorption
will make this move.
Infrared is one such source of non-visible radiation.
You may have caught that the label said heat from the sun, and not just light, makes it run.
Indeed, intense sources of heat will make this thing move.
But, radiant sources of heat -
in other words, intense infrared -
will make it fly.
Take a look - here’s an ordinary space heater.
This one isn’t meant to be a source of radiant heat,
instead it heats the air through blowing it over hot wires.
But these wires don’t get hot enough to produce much medium- and short-wave infrared radiation,
so while the radiometer does move, it’s not a whole lot.
Now take a look at what it does in front of this toaster oven.
The heating elements here are in fact quartz heat lamps,
and these eventually start to incandesce intensely.
They’re so hot they’re comfortably out of the red zone, and into the orange.
Almost yellow.
But that means they’re also throwing out tons of shortwave infrared, and indeed -
the radiometer agrees.
In something like a toaster oven and indeed an ordinary toaster
we want to have plenty of infrared radiation.
Heating something through hot air alone produces a very different effect
compared to heating it with radiant energy.
Lots of IR will be better at browning the surface of something without drying it out,
as the energy becomes absorbed at the surface of whatever’s being heated
and so in a sense that surface becomes its own source of heat.
For a slice of bread, that means the surface of the slice gets very hot very quickly,
but the inside warms slowly as the heat from the surface gradually seeps through.
You’ve felt this before when near a campfire or under a heat lamp.
Infrared radiation can travel through the air or even the vacuum of space,
and it’s only when it becomes absorbed by something that the heat energy becomes apparent.
That warm tingly sensation from a radiant source of heat
is how you feel your skin being heated not by warm air, but by radiation.
That’s one of the reasons quartz heat lamps are sometimes used in places like bus shelters
or open train platforms.
They can warm you up even when it’s very cold outside
because they transmit that heat through the air to your skin.
Their goal isn’t to heat the air, but you.
An important note here is that the glass envelope itself is absorbing some of this infrared radiation.
Because the heating element is visibly incandescing
we can conclude that there’s a broad spectrum of infrared being emitted here,
and ordinary silica glass absorbs IR well in the region of 2.8 to 4.5 micrometers.
Ultimately that makes the glass hot which warms up the air molecules inside the envelope.
An interesting side-effect of this is that when taken away from the radiation,
the radiometer actually begins to spin backwards as it releases its excess energy to the surrounding air.
This isn’t really important to the point I’m making
but it does further demonstrate how the rotational action has nothing to do with light pressure,
but is a thermal phenomenon.
Speaking of the point I’m trying to make…
what is that?
Ah, right, uh, going back to that model home demo.
To demonstrate the amazingness of their amazing new windows of amazement,
the model home had a little demo consisting of two floodlights,
two radiometers, and in between the radiometers and floodlights
were two different window panes.
One was your ordinary garden variety window,
and the other was this amazing new whatever it was.
The radiometer in front of the ordinary window was spinning quite quickly,
but the radiometer on the other side of the amazing window wasn’t spinning nearly as fast.
This served to create an immediately intuitive sense
of how the better glass prevented heat energy from passing through it.
Now, the scientifically-inclined and skeptical among you
will no doubt realize that this could easily have been rigged.
After all, who’s to say that the two radiometers performed equally?
Sadly, at the age of eight I wasn’t yet thinking about these things,
otherwise I would have switched those two radiometers to make sure the result was the same.
Soon as I get that time machine perfected I’ll go back and do it!
But as it happens, this home is fairly new
and it has even newer windows than that model home did.
At least, if my understanding of linear time is correct.
While I don’t have a second window to compare to,
I can just… open one.
Here's a great demonstration of how much infrared light this window is blocking.
Were it not for the window, the radiometer would be in direct sunlight.
And you can see that it's actually not turning all that quickly.
If I open the window, though, it immediately speeds up.
Here we see using the same radiometer
that the window drastically reduces the total amount of energy passing through it
while only marginally decreasing the visible light that makes it through.
Neat!
How this is accomplished doesn’t really matter for the purposes of this video,
but let’s talk about it anyway!
This window, like many out there, features low-emissivity glass (often called low-E).
At the time it was made, a coating was deposited on the glass
which will reject much of the near-infrared light that hits it.
Since glass will absorb certain IR wavelengths,
we want those wavelengths to be reflected away before it even hits the glass,
otherwise the glass would be heated by this energy
and end up allowing much of that into the space anyway.
While visible light does plenty of heating by itself,
a slight majority of the sun’s energy that reaches the Earth’s surface is infrared.
Windows that can reflect just this portion of the sun’s energy away
reduce the total amount of energy that passes through them
and thus lower the amount of heating caused by the sun
all without appearing darker to our eyes.
And these windows are great at this!
Not only does the radiometer demonstrate this nicely,
but in fact you can tell just by feeling the difference between the sunlight on your skin
and the sunlight that makes it through the glass on your skin.
There’s a significant, noticeable difference.
Voiceover me is here to tell you about another demo we can do!
This halogen floodlight produces quite a lot of directed infrared.
Don’t think about that too much or it might spoil a later demo but, uh
anyway if I put it outside and shine it at the radiometer through the window,
you can see it doesn’t move all that much.
If, however, I bring it back inside
and point it directly at the radiometer from about the same distance,
it spins very rapidly.
That window’s doing a lot to keep the infrared from coming through.
And to be clear, ordinary glass doesn’t do hardly anything at all
as seen here.
An interesting side-effect of this low-E coating
is that the internal reflections in the glass become color-shifted as they repeat.
And the way they shift is surprising.
I can’t really make heads or tails of it.
It’s not consistent at all,
and there’s a noticeable difference between the cool white diodes and the warm white diodes here
(this is one of my portable RGBW studio lights with both white color temps lit).
However, the very far repeated reflections all look pretty blue,
which makes sense as that’s farthest from infrared.
Wavelengths even remotely close to IR probably get rejected by the eighth reflection or so.
And actually, one thing I was hoping I could demonstrate but wasn’t able to
was that depending on the angle of the sun,
sometimes you’ll see an intense pink or magenta cast in light that’s reflected off these windows.
I was playing around with the floodlight but could only barely see it.
I was hoping to show it to you because it demonstrates a significant drawback of low-E glass:
it can actually cause damage to various things that end up in the sun
and also get hit with a reflection off a window.
Those objects end up getting more IR than direct sunlight,
and with modern building designs with tons of windows this can become problematic,
especially in urban settings.
And especially especially if architects get creative with curved building surfaces.
Now I deleted a section here in editing, so here’s a sloppy segue.
What about in the winter time?
This low-E technology can also help keep heat inside the home from leaving in the winter.
Everything radiates heat energy
(unless it’s at absolute zero, of course),
and if that energy goes through a window…
well now it’s outside.
If the window can reflect long-wave infrared back inward, though,
it will be able to prevent much of that radiant energy from being lost through it.
Instead it will end up on another inside surface where it will re-radiate
and maybe hit the window again who knows but the important part is it didn’t leave.
Voiceover me again.
I was gonna do some sort of whiteboard drawing to show this
but as it turns out this can be demonstrated with the radiometer and the floodlight and the window.
Here I’m pointing the floodlight out the window
at an angle so that the reflected light will land on the radiometer inside the room.
Though most of the visible was unimpeded as it left the room,
the IR was reflected right back and the vanes are spinning quite quickly to prove it.
Pretty neat.
I had assumed that the inside coating might be focused more on long-wave infrared
but given how fast the radiometer is spinning here I’m going to assume that assumption was wrong.
However, the benefit of rejecting infrared from the outside becomes a drawback in the winter.
When you need to heat your home,
windows which are blocking a majority of the free solar radiation coming at them
are kind of a bummer.
And indeed, in the winter these windows are kind of a bummer.
It would be great if we could develop some sort of selectable Low-E window.
Maybe the glass could be reversible so it reflects short and medium-wave IR only on one side.
Turning it inward would thus allow for more IR transmission in the winter.
But maybe that wouldn’t work.
Truthfully I don’t know how much the side that the coating is on actually matters,
but one of you probably does and I hope you’ll let us know down there.
There is, though, a solution to this problem that we know of today
which is incorporated in the passive house design.
Because the angle of the sun changes through the year
with higher angles in the summer and lower ones in the winter
(especially where it gets cold enough to need a lot of seasonal heat)
we actually have a workaround.
A structure like this can be fitted with solar-control windows up here,
and standard windows below this awning.
In the summer months, the awning prevents direct sunlight from hitting these standard windows,
so they don’t need to reject IR.
It will only hit the transoms, providing plenty of natural light but with minimal solar heating.
But in the winter, the sun’s low angle now allows sunlight through the low windows as well,
providing some free heating.
Still, though, I think developing a window that can reject infrared in the summer
but allow it through in the winter is a worthwhile cause,
but the mechanism of how the glass rejects infrared is really beside the point.
We were talking about radiometers, at one point.
Uh...
What I find fascinating about the radiometer is that,
while it certainly isn’t a precise tool giving any useful units,
it can be very useful in demonstrating the concept of radiant energy.
For example, here’s another fantastic use for one.
By now I hope we’re all pretty aware of why incandescent lightbulbs are all but gone,
but a pair of radiometers would have made a great in-store display.
Really, it would be the same exact demo as in the model home but without the window glass.
And to make sure I’m not foolin’ you, well I’ll just use the same radiometer.
I only have one of 'em after all so we’ll split screen it!
The large amount of IR emitted from the incandescent bulb
heats the vanes of the radiometer quickly and intensely, causing rapid motion.
But the LED bulb next to it,
producing none of the medium and shortwave IR that the incandescent light does,
hardly makes it turn at all.
Sure, it still turns a bit -
the visible light is absorbed by the black paint and also makes it heat up.
But the total amount of energy being radiated from the light bulb
is a small fraction compared to the incandescent,
yet the bulb is just as bright.
Our eyes don’t care about most of the radiation that an incandescent bulb creates,
so why waste energy making it in the first place?
And this is where I think the radiometer can teach us a valuable lesson.
Sure, I could grab a power meter and put each light bulb on there
to give you the difference with raw numbers.
But this is far more… visceral.
Intuitive.
Plainly evident.
It doesn’t require comparing numbers to see that there’s a difference.
There just… is one.
Similarly, I could go window shopping
(that’s shopping for windows, not window shopping)
and take a look at the IR rejection specifications,
R-values, and other data,
but not everyone will understand why those data are important.
Using something that provides you with clear evidence of infrared radiation
without requiring any contextualizing or prior understanding
makes the knowledge and importance of measures to abate infrared more accessible.
I would argue that you don’t even need much awareness of what the radiometer does
for it to be an effective demonstration tool.
But admittedly I can’t unlearn that so perhaps the intuitiveness I feel this provides is more unique to me.
And honestly, I’m not even sure eight year old me grasped the point of the demo at the model home.
It may have just stuck in my mind because these things are cool,
dare I say nifty.
I feel like I got the point of it because
I remember there being two radiometers and two different windows
and one of them was spinning faster than the other,
but it could very well be the case that the point of the demo only dawned on me years later.
And unfortunately, that’s something my time machine won’t be able to tell me,
at least not without asking my former self
and who knows what the consequences of that might be?
It could turn that kid into some sort of…
influencer.
Eugh!
Anyway, while William Crookes was wrong about how this thing works
and it’s mainly relegated to the
“well that’s neat!” category of existence,
I think devices like this are more useful than we perhaps realize.
Sometimes it’s not the accuracy of the tool that matters
but how easily it demonstrates something.
If a 19th century scientific curiosity can demonstrate to an eight year old
that some windows - or some light bulbs - are better than others,
who knows what might be out there which can do the same for other concepts?
No really, do you know?
I... I think we should be looking into this.
There’s a lot of stuff out there these days that’s complicated to communicate,
and whenever that happens there’s room for misinformation.
A lot of people are skeptical about the merits of technological change,
even when it’s for something so mundane as improving energy efficiency.
Trust me, I’m as leery as you are about much of what the Valley techbros are dreaming up these days -
not all progress is truly progressive, after all.
But a lot of common-sense progress gets met with skepticism these days,
and maybe there are ancient thingamjigs like this that can help with that.
Maybe fighting misinformation is going a little too deep,
but I do earnestly think that there’s value in exploring this.
There are probably all sorts of old curios out there
that seem pointless by modern standards but which could actually provide a meaningful educational experience.
Being able to demonstrate a concept in a purely intuitive fashion can be very powerful,
and while I doubt there’s a demo rig to be built for all of life’s various lessons,
I’ll bet there are more of them out there just waiting to be built than you might imagine.
Thanks for watching.
♫ transpirationally smooth jazz ♫
Have you ever seen one of these?
I… nope.
The final nail in the coffin for this theory…
oh yeah, shoot!
I need a prop!
Oh, and I restarted the teleprompter to the very beginning!
Why do I still use this one?
It’s because I’m lazy, and I’m used to it.
These curiosities have been staples of novelty shops and museum science gift stores.
Oops.
A partial vacuum.
If this were a complete [clunk] vacuum...
Well that’s not gonna work, is it?
Move de shtuff out of de vay!
It also doesn’t help that it’s rotating backwards from that explanation.
Egghhh.
Ya know it’s kinda hard to look at a lightbulb and then look at a teleprompter.
It’s not very easy.
My eyes can’t adjust that fast.
I have a very burned-in memory of these things which has made
... (mouth noises of disappointment)
What word should I choose other than malarkey?
[thinking, stand-by]
Hogwash.
Nahhh!
If it operated on the princ per he blerghhhhfff.
The temperature gradient between the two sides creates a slight difference in…
something.
Where did that word go?
It’s over there. In this force.
[mouth popping sounds]
It needs to be a near-vacuum or there would be too much air resistance for the vanes to spin freely.
But it needs a bit.
And that wasn’t in the script.
You’ve adlibbed.
You terrible person.
I said ah high bleu… great.
That section I deleted?
It was about how my cat, Reed, doesn't feel all that hot to the touch when he sits in the sun,
despite being a black cat. I then talked about how my black hair (of which there is a surplus at the moment)
is almost too hot to the touch when I'm out in the sun.
So it's pretty remarkable that the cat can be in the sun without being too hot to touch.
It was a riveting story.
Too bad it didn't make the cut.