Why Sugar Always Twists Light To The Right - Optical Rotation

Summary

This video delves into a fascinating experiment demonstrating how sugar in a solution can twist polarized light. The narrator explains the quantum mechanics behind polarized light, the concept of circular and linear polarization, and how sugar's molecular structure affects light's behavior. The key insight is that sugar molecules have a handedness, causing the light to rotate as it passes through the solution. The video also emphasizes the counterintuitive nature of the experiment and provides a clear, visual breakdown of the science involved. The narrator concludes by recommending the Blinkist app for concise non-fiction book summaries.

Takeaways

• 🔬 Sugar solutions can twist polarized light, a fascinating phenomenon that challenges intuition.
• 💡 Light is an oscillation in the electric and magnetic fields, and unpolarized light oscillates in all directions.
• 🔍 Polarized light occurs when unpolarized light passes through a polarizing filter, restricting oscillation to one direction.
• 🌀 A sugar solution twists the direction of polarized light due to its molecular structure, and the amount of twist depends on the sugar concentration.
• ⚛️ Linearly polarized light can be understood as a superposition of two circularly polarized states—one clockwise and one counterclockwise.
• 🍬 The handedness (chirality) of sugar molecules causes circularly polarized light to have different interactions, leading to a twist in linearly polarized light.
• 🔄 Clockwise and counterclockwise circularly polarized light components experience different refractive indices as they pass through the sugar molecules, causing phase shifts and resulting in a twist of the light.
• 🧬 The twist is consistent because sugar molecules lack mirror symmetry, meaning their handedness is preserved no matter how they are oriented in the solution.
• 📚 The video highlights the complexity of light polarization and how molecular structure can affect its behavior, offering a deeper understanding than commonly available explanations.
• 🎧 The speaker also promotes Blinkist, an app that summarizes books into 15-minute audio or text summaries, with recommendations for several non-fiction titles.

Q & A

What is the main focus of the experiment described in the script?

The experiment focuses on how sugar in solution can twist light, specifically changing the direction of polarized light.

What is polarized light?

Polarized light is light whose electric field oscillates in a single direction, as opposed to unpolarized light which oscillates in multiple directions.

How does a polarizing filter work?

A polarizing filter works by restricting the oscillations of light to a single direction, allowing only light polarized in that direction to pass through.

What happens when polarized light passes through a sugar solution?

When polarized light passes through a sugar solution, the sugar molecules cause the light to twist, changing its direction of polarization.

Why does the color change as the filter is rotated in the experiment?

The color changes as the filter is rotated because different wavelengths of light respond differently to the sugar solution, causing a change in the degree of polarization.

What is the significance of the sugar solution turning the light about 90 degrees?

Turning the light about 90 degrees indicates that the sugar solution has a significant effect on the polarization of the light, demonstrating the experiment's counter-intuitive nature.

Why does the script mention the need to consider the superposition of different polarized states?

The script mentions the superposition of different polarized states to explain how light can be in a combination of polarization directions, which is crucial for understanding how sugar can twist light.

What is the role of the sugar molecule's handedness in the experiment?

The handedness of sugar molecules is fundamental because it means they lack mirror symmetry, causing circularly polarized light to interact differently with the molecules, which results in a net rotation of the light's polarization.

How does the script use the analogy of pasta to explain the twisting of light?

The script uses the analogy of pasta to illustrate how light might interact differently with a molecule based on its orientation, although it clarifies that sugar molecules do not actually look like pasta.

What is the conclusion about the orientation of sugar molecules and the twisting of light?

The conclusion is that the orientation of sugar molecules, which have a handedness, causes a net rotation in the polarization of light because their mirror-image counterparts cannot be created by simply flipping the molecules.

Why does the script mention the term 'dextrose' in relation to sugar?

The script mentions 'dextrose' to highlight that glucose, or dextrose, turns polarized light to the right, which is why it is referred to as 'dextro', indicating a clockwise direction.

Outlines

00:00

🔬 The Counterintuitive Experiment

This paragraph introduces an experiment that shows how sugar in a solution can twist polarized light. The explanation begins by recapping how light is an oscillation in electric and magnetic fields, and how polarized light is restricted to oscillate in one direction. A demonstration is provided using polarizing filters and sugar water, revealing that sugar can twist the direction of polarized light. This effect is most notable when more sugar is added, and different wavelengths of light react differently, leading to a color change. The presenter expresses fascination at this phenomenon and the mystery behind how molecules can consistently twist light in the same direction, prompting deeper exploration into the physics of polarized light.

05:01

💡 Superposition and Polarized Light

The focus here shifts to understanding polarized light in terms of superposition. It explains how light can exist in a superposition of states, particularly horizontal and vertical polarization. By summing up these oscillations at different points, a third state is created, which is polarized at 45 degrees. The paragraph then introduces circularly polarized light, showing how it forms by shifting one component by a quarter wavelength. Clockwise and counterclockwise circularly polarized light is depicted, and the superposition of these forms is central to the understanding of light's behavior in the experiment.

10:02

🌈 The Twist of Light Explained

This paragraph digs deeper into how light interacts with sugar molecules, which have handedness. The explanation uses a metaphor involving pasta to illustrate how circularly polarized light components interact differently with molecules depending on their orientation. The clockwise component, more aligned with the molecule’s structure, is slowed down more than the counterclockwise component. This difference in speed alters the superposition of the two components, causing the overall polarized light to twist. The twist results from the difference in refractive index experienced by the two components due to the handedness of sugar molecules.

15:03

🌀 Handedness and Symmetry in Molecules

The final paragraph explains how the handedness of sugar molecules contributes to the light's twisting effect. It contrasts molecules that have mirror symmetry, where the light's experience can be reversed, with sugar molecules that lack such symmetry. These asymmetrical molecules can't be simply flipped to undo the light's experience, leading to a consistent twist in one direction. The paragraph concludes with a brief mention of sugar's alternative name, dextrose, which reflects its tendency to twist light to the right. The video ends with a plug for Blinkist, a service that summarizes non-fiction books into short, digestible formats.

Keywords

💡Polarized Light

Polarized light refers to light waves in which the oscillations of the electric field are restricted to a single plane. In the video, polarized light is used to demonstrate how sugar solutions can twist the direction of light. The experiment shows how passing polarized light through sugar water alters the orientation of the light, illustrating the interaction between light and molecules with 'handedness.'

💡Superposition

Superposition is a quantum mechanical concept where a system exists in multiple states at once. In the video, unpolarized light is described as being in a superposition of oscillating electric fields in various directions. The superposition of circularly and linearly polarized light is key to understanding how light interacts with sugar molecules, twisting its polarization.

💡Circularly Polarized Light

Circularly polarized light is light where the electric field rotates in a circular motion as the light propagates. This can be either clockwise (right-handed) or counter-clockwise (left-handed). The video explains how linearly polarized light can be viewed as a superposition of two circularly polarized light waves, which helps explain the interaction of light with sugar molecules.

💡Handedness

Handedness, or chirality, refers to objects or molecules that do not have mirror symmetry, such as left and right hands. In the video, sugar molecules are described as having handedness, meaning their mirror images are distinct from each other. This handedness is crucial in explaining why light passing through a sugar solution always twists in the same direction (clockwise).

💡Refractive Index

The refractive index measures how much light slows down as it passes through a material. The video discusses how the refractive index differs for clockwise and counter-clockwise circularly polarized light when passing through sugar molecules. This difference causes one component of the light to slow down more, leading to a phase shift and a twist in the polarization of the light.

💡Phase Shift

A phase shift occurs when one wave moves ahead or lags behind another, altering their relative positions. The video explains how circularly polarized light experiences a phase shift when one component moves faster through sugar than the other. This phase shift causes the superposition of the two waves to twist, changing the polarization of the light.

💡Sugar Solution

A sugar solution is a mixture of sugar and water. In the video, the sugar solution is used to demonstrate how certain molecules can twist polarized light. The sugar molecules, which have handedness, cause polarized light to rotate as it passes through the solution. The degree of twist increases with the concentration of sugar.

💡Linear Polarization

Linear polarization refers to light waves where the electric field oscillates in a single plane. In the video, the presenter uses a polarizing filter to produce linearly polarized light from the unpolarized light coming from a computer monitor. This linearly polarized light is key to understanding how it interacts with sugar molecules in the experiment.

💡Molecule Orientation

Molecule orientation refers to the spatial arrangement of molecules. The video explores how sugar molecules, which are randomly oriented in solution, all share a handedness that causes light to twist in a consistent direction. This explanation refutes the initial assumption that random orientations would cancel out the effect.

💡Optical Activity

Optical activity is the ability of a substance to rotate the plane of polarized light. The video focuses on the optical activity of sugar solutions, explaining how the chirality of sugar molecules causes the rotation of polarized light. This phenomenon is used to explain why sugar is often referred to as 'dextrose,' meaning it twists light to the right.

Highlights

The experiment shows how sugar in solution can twist polarized light.

Polarized light is light that has restricted oscillation to one direction, controlled by a polarizing filter.

Sugar solution twists the polarized light, and the degree of twist increases with the sugar concentration.

The superposition of two linearly polarized light states creates a new state polarized at 45 degrees.

When light passes through sugar, clockwise and counter-clockwise circularly polarized light components experience different speeds.

The difference in speed between the two circularly polarized components causes the linear polarization to rotate.

Despite sugar molecules being randomly oriented, they still consistently twist light due to their handedness, not mirror symmetry.

Circularly polarized light is the superposition of two linearly polarized light states shifted by a quarter wavelength.

The experiment visually shows the effect using a cylinder filled with sugar water between a polarizing filter and a light source.

As the light passes through sugar, the clockwise circularly polarized light interacts more strongly than the counter-clockwise component.

The handedness of sugar molecules explains why light is consistently twisted in one direction, either clockwise or counter-clockwise.

Glucose is also called dextrose, referring to its ability to twist light to the right.

The relationship between circular polarization and sugar molecules highlights the fundamental concept of handedness in molecular structures.

Molecules that do not have handedness, such as those with mirror symmetry, do not cause any net rotation of polarized light.

The key finding: linearly polarized light is the superposition of clockwise and counter-clockwise circularly polarized states.

Transcripts

(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)