(soft instrumental music) - This is one of my favorite
experiments of all time because if you think
about it deeply enough, it becomes really counter-intuitive and you have to dig even deeper than that to solve the mystery at the core of it. You may have seen this
experiment on YouTube already, but I can always guarantee that you haven't seen an
explanation of how it works. I certainly wasn't able to find one. It's an experiment that
shows how sugar in solution can twist light. Specifically, it can change the direction of polarized light. You probably know what
polarized light is already but just to recap. So light is an oscillation in the electric and magnetic fields that permeate the universe. So, you know, if light
is traveling towards you that represents an oscillation
in the electric field and perpendicular to that, an oscillation in the magnetic field. For simplicity sake, it's a good idea to just
ignore the magnetic field and just focus on the electric field just cause it makes diagrams cleaner, but just know that the magnetic field is oscillating as well and it's perpendicular
to the electric field. So you've got light traveling towards you, here's the oscillation
in the electric field and it's going up and down but it could just as easily
be going side to side. In fact, it could be oscillating
any of those directions. But because it's a
quantum mechanical system, actually it can be a superposition of all those different directions. And that's what unpolarized light is. It's light that is in a super
position of the electric field oscillating in all different directions. If you pass unpolarized light
through a polarizing filter like this one, then it restricts all those oscillations down into just one direction. So the light reaching the
camera from my face is polarized and I can change the direction
of that polarized light by, you know, turning
the filter like this. This computer monitor has
a polarizing filter in it, like most computer monitors actually. So all the light coming from this monitor is already polarized. If I put another
polarizing filter in front and change the angle,
you can see the effect. So if I put the polarizing
filter at 90 degrees to the polarization of the
light coming from the monitor then it all gets blocked. But if I line them up, then the polarizing filter
lets all the light through. Here's the crazy part though, if I put this cylinder full of sugar water between the monitor and the filter, now look some of the light gets through and that's because the sugar is twisting the polarized light. It's changing the direction of it so that some of it can now
get through the filter. The more sugar you put in the way, the more the light turns. So I've chosen a concentration of sugar that turns the light about 90 degrees. It's approximate because different wavelengths
respond differently which is why the color changes
as I rotate the filter. This whole thing was mind
blowing to past Steve. I couldn't figure out how
this jumble of molecules that are all oriented
in different directions could possibly have this effect, could possibly lead to a net turning in the clockwise direction
and always clockwise, never counter-clockwise, that doesn't make sense. Like surely if there's a
molecule in the solution that slightly turns the light clockwise then there's gonna be another molecule oriented in a different way that will turn it slightly
counter-clockwise. The net result should be no twist in the direction of the
polarized light at all. Like it felt like a
glitch in the universe. That's not the solution by the way, to get to the answer we need to look again at the superposition of
different polarized states. So imagine you've got some lights and it's in a superposition of two states, one is polarized horizontally, the other is polarized vertically. So you've got an electric field oscillating up and down in one state, and you've got the electric field oscillating side to
side in the other state. What does the superposition look like? We can actually just think about it like summing up those two waves. So look at this point
in space, for example, what is the sum of the displacement of the electric field at this point? Well, it's displaced a
little bit vertically and a little bit horizontally. The superposition of those two states is just this point here, right? It's a little bit vertical and
it's a little bit horizontal. So it's out here. If you look at the superposition of the two states at this point, well the electric field is not
displaced at all from zero, from the access at this point. So the superposition is zero as well. Then at this point, you're a
little bit out to the right and you're a little bit down. And so the super position at that point is diagonally
down and to the right. And if we do that for all
the points along the axis, then we get this result here. In other words, the
superposition of these two states can just be thought of as a third state which is at 45 degrees to those two. Let's do that same exercise again but this time shift, the
vertically polarized state forward through a quarter of a wavelength. Let's see what happens now, looking at this point, the superposition must be
just out to the left there because the vertical component is zero. At this point, the superposition must be
just vertically downwards because the horizontal component is zero. At this point, the superposition
must be to the right because the vertical component is zero. At this point, the superposition
must be vertically upwards because the horizontal component is zero. Let's join those points together and this is the result,
this helix, this spiral. So this is clockwise
circularly polarized light and you can have counter-clockwise circularly polarized light as well. So you can think of
circularly polarized light as the superposition of two
linearly polarized states that are perpendicular to each other, where one of them is offset
by a quarter of a wavelength. You know, if you search online for things like circularly polarized light and superposition, you'll find diagrams like this one. What's interesting is, there's a diagram that you won't find, or at least I wasn't able to find and it's really important
for this explanation. And it's a diagram of the
opposite of what we just did because it turns out
that the superposition of two circularly polarized states, one clockwise, one anti-clockwise gives you linearly polarized light. So that this is maybe the first time this diagram is appearing
on the internet ever, something that I have been able to find almost certainly in video form. Look, you've got two States here, one is polarized in the
counter-clockwise direction, one is polarized in the
clockwise direction. Let's look at some points
and see how they add up. So look, at this point the two
waves are in the same place, so the superposition must
be there at that point. Here, you've only got horizontal
components in each wave and they're in opposite directions so they cancel out. Here, the two waves meet again, so the superposition
must be at that point. Here, the two horizontal
components cancel out, so it must be here. So let's draw all those points in and see what the result is. It's linearly, polarized light. So that's really important. You can think of linearly polarized light as the superposition of
two polarized states, one circularly polarized
in the clockwise direction, the other circularly polarized in the counter-clockwise direction. So what happens when light like this passes through a solution of sugar like the light coming from my monitor? Well, I want you to imagine that the solution of sugar
is like this bag of pasta. It's actually nothing
like this bag of pasta, but I want you to imagine
that it is just for a minute. So you've got all these
molecules, all jumbled up, all in different orientations. Let's have a look at what happens when the light passes through a single one of those molecules and it happens to be oriented
vertically like this. So the light is linearly polarized, which we can think of as the superposition of two circularly polarized states going in opposite directions. And hopefully you can
see that those two states will have a very different experience of traveling through
this molecule of pasta. The clockwise circularly polarized state is in step with the pasta, it's nestled into the
grooves of the pasta. Whereas the anti-clockwise state is in opposition to those turns. It's constantly bumping
into those flaps of pasta. For the purposes of illustration, I've chosen a wavelength of light that matches the spacing
of the pasta's spirals. In reality, it's not gonna
be like that most of the time but hopefully you can see
regardless of the wavelength that those two different directions of circularly polarized light will have a different experience of passing through the pasta. Why does that matter? Well, you probably already know that light travels more slowly in glass. And so the peaks and troughs of the wave have to bunch up to compensate. In other words, the
wavelength that goes down. And that's true when light
passes through anything actually not just glass. It's also true when light
passes through a molecule like our spirally pasta molecule here. Let's suppose hypothetically that the clockwise circularly polarized component of our light interacts more strongly with the pasta because it spends more time
traveling through those flaps. Well, in that case we
would expect that component to be slowed down down more. To relate that back to the language that we used to talk about light, you would say that the index
of refraction for this pasta is higher for clockwise
circularly polarized light than anti-clockwise
circularly polarized light. The two components of light see
a different refractive index when they interact with this pasta. So if the counter-clockwise
component of the light is traveling more
quickly through the pasta than the clockwise component of the light, it will have traveled further by the time it exits the
pasta at the top there. In other words, the counter-clockwise
component would have shifted up relative to the
counter-clockwise component. What does that do to the
superposition of these two states? Hopefully you can see from this animation that if you shift one of
the components or states of polarized light forward, it causes the superposition
in yellow to rotate. So if we have a solution of sugar where all the molecules are all pointing in the same direction, they're all aligned vertically like this, all throughout the solution, then we would expect the
light to twist as it comes up. And to reiterate why, that's because this
linearly polarized light is the superposition of two circularly polarized light states and they have a different
experience as they travel through. One goes more slowly than the other, so by the time they leave, the phase between them has shifted and that twists the superposition. But a solution of sugar is not like that, it's a jumble of molecules
in all different directions. And this is where we have to correct my miss assumption at the start, because my thinking was like, look if you've got a, you know, pasta, or a molecule oriented this way in space and that's twisting light like this and all you need to do
is find another molecule that's oriented the other way and that will twist it
back the way it came. So the net result of all
these jumbled up molecules should be no rotation at all. But that's a mistake because look, what happens if I turn
this pasta upside down? It's genuinely unremarkable,
like not very much happens. Look at the direction of the spiral, so as you move up through
the pasta from bottom to top the edge moves from left to right. And if we switch it upside down, maybe we expect that to reverse so that it goes from right to left. But look, as turn it
upside down like that, it stays the same. The edge of the pasta still
goes from left to right as you move up through the pasta. In other words, it doesn't matter if you have the pasta
oriented this way or this way, the experience of light passing
through it will be the same. The clockwise and
counterclockwise components will have a different experience, and that difference in
experience will be the same regardless of which way up the pasta is, the experiences don't switch. You know this about spirals already from everyday experience. Look, here's a bolt
that has a spiral on it, and the inside of this
nut has a spiral on it, and I can turn the nut upside down, right? And it still works. The explanation isn't complete because sugar molecules
don't look like this. They look like this. So what is the fundamental attribute that these two things have in common that matters in this scenario? Well, it's handedness. They both have a handedness. In other words, they don't
have mirror symmetry. If you looked at this pasta in a mirror, the mirror image of the pasta would be different in a fundamental way, in the same way that my
hands have handedness. Like if you look at my
left hand in a mirror, you wouldn't see another
left-hand you'd see a right hand. And they're fundamentally different. Like if you could take that right hand from inside the mirror like a ghost, and try and line it up with the left hand, you wouldn't be able to there's no way to get
them to overlap perfectly. And not just because my
hands are slightly different and not just because I'm married. The same is true for these molecules here. If you took a mirror image
of this glucose molecule, the molecule would be
fundamentally different. You wouldn't be able to
rearrange that molecule, and line it up on top of this one. To really hammer the point home, let's see what happens with molecules that do
have mirror symmetry like these ones here. So look, let's orient
the molecule like that. And let's imagine light traveling up through the molecule like this. And it's again, it's a superposition of circularly polarized in that direction, and in that direction. Maybe you could persuade yourself that they would have
a different experience passing through this molecule. How would you then undo that? How would you reverse that? What all you need is the
mirror image of this molecule somewhere else in the path of the light. So let's put the mirror
image up here like that. And they really are the
mirror image that they go see again they're the mirror image there. And so it passes through one and then it passes through the other. And whatever preferential
treatment for the left-handed versus the right-handed is
undone with this mirror image. But of course, because these
molecules have mirror symmetry they're just the same molecule, right? So you'll find the mirror image of this orientation
floating around elsewhere in the solution. What about this sugar molecule then? Imagine the light passing
through this molecule. Hopefully you can persuade yourselves that clockwise circularly polarized light will have a different experience to counter-clockwise
circularly polarized light, as it passes through this molecule. To undo that effect, you would need the light to
pass through the same molecule but a mirror image of itself up here. And you can't create the in mirror image by flipping it in any direction, because the molecule has a handiness. It doesn't have mirror symmetry. You can't create the
mirror image of itself, just by reorienting it. So there you go. It's because sugar
molecules have a handedness and it's because linearly polarized light, can be thought of as the superposition of two states of
circularly polarized light, in opposite directions. By the way, you may have heard of sugar being referred to as dextrose. Dextro meaning to the right, because dextrose or glucose
turns light to the right, clockwise and counterclockwise. I've been using an app called Blinkist for a little over a year now. They're the sponsor of this video. Blinkist does something remarkable. It's an app that
condenses the key insights from non-fiction books
into 15-minute reads. They're also audio narrated, so you can listen to them in the car or while you're doing housework
or something like that. And I thought I would
share my favorite books that I've consumed in this
fashion over the last year or so. So they are "Sapiens", amazing book, "Digital Minimalism",
"Blink", appropriately, "Freakonomics", amazing book. I actually read the whole thing after reading the "Blink". "How to Talk So Kids Will Listen & Listen So Kids Will Talk", it's a parenting book. It worked really well for our kids. "How to Make Friends & Influence People". You know, it's interesting
how much that's got in common with "How to Talk So Kids Will Listen & Listen So Kids Will Talk". They're basically
instructions on how to be nice in a clever way to get what you want. It's amazing. Actually, I read the whole
books of those two as well after doing the 15-minute Blinks. They also have full audio
books now by the way, and members get them for
a vastly reduced price like 65% off on average. The first 100 people to go
to blinkist.com/stevemould will get one week absolutely
free to try it out. No strings attached,
cancel whenever you like. And if you wanna carry
on with full membership you get 25% off as well. The link is also in the description. So check out Blinkist today. I hope you enjoyed this video. If you did, don't forget to hit subscribe and I'll see you next time. (soft instrumental music)
This bends my mind in all sorts of ways! As someone who works with polarised light I’ve always struggled to understand how it actually works.
I thought the second filter was underneath the cylinder and I was so confused how light was getting through