Light sucking flames look like magic

- In this video,

I'm going to show you how to make a black flame.

There's no trick here.

Like I haven't inverted the colors or anything like that.

The explanation for how it works is really cool.

And along the way we'll learn one weird trick

that glassblowers use to make fire invisible.

So I did say

that there was no trick photography in this footage,

but actually the fact that it's black and white

is sort of a trick.

If I put it back to full color

and then turn off the auto white balance on my camera,

you'll see that everything's very yellow.

That's because I'm illuminating the scene

with one of these old-fashioned streetlights

called a sodium streetlight.

Sodium streetlights were used for a long time

because they're incredibly efficient.

Unfortunately, sodium streetlights

make everything look weird.

Like you know there's this number,

the CRI for a light bulb,

and it tells you, how good is this light source

at making colors look the way they are naturally?

Sunlight has a CRI of 100,

and a light source that doesn't allow you

to distinguish color at all would have a CRI of zero.

And, well, sodium streetlights have a CRI of zero.

But what makes them so bad at rendering color

is also the thing that turns this flame black.

You might know this already,

but if you excite an atom in the right way,

an electron will jump up to a higher orbital,

and when it drops back down again,

it releases a photon of light.

How much energy the photon has

is just the difference between the two energy levels.

The electron is giving up some of the energy it had up here

and it's throwing it away in the form of a photon.

But because these orbitals are quantized,

their energy levels are fixed,

which means the energy of a photon

released by that transition will always be the same.

And the energy of a photon defines its color.

And for sodium, there's only really one possible transition

that exists within the visible spectrum,

which gives off photons

with about a third of a femtojoule of energy,

and they have this yellow-orange color.

You don't normally talk about

the femtojoules of a photon, by the way.

You usually talk about the wavelength,

in this case 590 nanometers.

It's actually quite unusual for an element

to only have one strong visible line like this.

For example, I've got some xenon gas in this vial,

and when I excite it

with electricity via this tiny Tesla coil,

you get this lovely color.

To see what that color is made of,

I could use a prism in the same way that Newton used a prism

to find out that sunlight

was made up of the full spectrum of colors.

Instead of a prism, though,

I'll use a diffraction grating.

You see how it split sunlight into a rainbow

just like a prism

when I put it in front of the lens of my camera.

Putting the diffraction grating in front of the xenon

shows us that although it looks white,

it's actually made up of lots of discreet lines

instead of the full spectrum.

Compare that to sodium, which has just the one line.

I keep saying that sodium has just one line,

but that's not strictly true.

There are in fact two emission lines very close together.

We just can't distinguish it with this setup.

I'll get to that later though,

because I really wanna explain this black flame thing first.

So, an atom of sodium emits a very specific color of photon

when an electron drops from the 3p orbital

to the 3s orbital.

But the opposite also happens.

If you can hit a sodium atom

with a photon that has just the right energy,

the electron will jump

from the 3s orbital to the 3p orbital,

absorbing that photon.

If the photon has too little energy,

the sodium atom won't absorb it,

but also, if it's got too much energy,

the sodium atom won't absorb it either.

So as well as having an emission spectrum,

sodium also has an absorption spectrum,

which is just the inverse of the emission spectrum.

And with that simple fact we can explain the black flame.

So there are at least two ways to excite sodium atoms

so that they give off orange light.

The sodium streetlamp uses electricity,

which when you get down to it, is actually about collisions.

Electrons streaming from the anode to the cathode

smack into those sodium atoms,

and that's what causes the jump in energy levels.

The other way to do it is with heat.

So I start off with a methanol flame.

The reason I'm choosing methanol

is because it burns with an almost invisible fire.

I actually don't want any light from the fire itself,

because now I add a bit of salt water.

The heat from the fire vaporizes the sodium in the salt

and excites it at the same time.

It causes the valence electron to jump up.

And when it drops back down, it releases that orange photon,

which is why the flame is orange.

So all the time in that flame,

you've got a mixture of excited sodium

and sodium in the ground state,

and it's those ground-state sodium atoms

that make the trick work.

Because remember, those ground-state sodium atoms

are really good at absorbing 590-nanometer photons,

which is the exact photons

that are being produced by the sodium streetlight.

And of course, if the flame is absorbing photons,

it's going to look dark.

And that's normally the end of the explanation,

but I think it's important to clear up two things.

The first question you might have is like,

okay, the flame is absorbing photons from the lamp,

but it's also producing photons of its own.

In normal lighting conditions,

you can see that this flame is producing lots of light.

So don't these two things cancel out?

Well, the answer to that is that my sodium streetlight

is just a lot brighter than the flame,

so the tiny amount of light produced by the flame itself

is pretty negligible.

But the more important question that you might have is this;

after a sodium atom has absorbed

one of the orange photons from the streetlight,

it's just going to remit it again almost straight away.

So if one photon is emitted

for every photon that's absorbed,

it shouldn't look black at all.

But let's think about what you're actually seeing here.

To the left of the flame,

you can see the white wall behind it

is well illuminated with that orange light.

The fact that you can see the wall

means that those orange photons

are traveling from the wall into your eyes,

or in this case into the lens of my camera.

The bit of wall behind the flame

is also well illuminated by the sodium streetlight.

But when the photons leaving the wall

begin their journey into my camera lens,

they have to pass through the flame,

and when they do,

they get absorbed by the sodium atoms in the flame.

And yes, for every photon that's absorbed,

a new photon is emitted.

But crucially, the newly-emitted photon

will be traveling in a completely random direction.

The chance that it will be pointing

towards the camera lens is very low.

In other words, the light from the wall behind the flame

is scattered in all directions,

leaving a dark patch behind.

All this reminds me of something really cool

that my friend Andrea Sella showed me;

these special glasses worn by Glassblowers.

They do something amazing and I wanted to see it in action,

so I went to visit the glassblower

at University College London.

John here is working a piece of glass with a blowtorch,

and because the glass has a bit of sodium in it,

as the temperature goes up,

eventually some of that sodium starts to vaporize

and we see that characteristic orange light.

Eventually it gets so bright

that it's hard for John to see what he's doing.

But look what happens

when I put this special pair of glasses

in front of my camera lens.

Isn't that amazing?

It completely blocks that orange light

and suddenly John can see what he's working with.

While I was running tests

on the glassblower glasses for this video,

I realized that I now have the equipment that I need

to run tests on these EnChroma glasses

that I've had lying around for years.

I've wanted to run the tests for a long time

because when I heard about them, I was skeptical.

Like they claim to enhance color vision

for people with red-green colorblindness,

but how can something that removes light

help someone to see colors

that they don't have the anatomy for?

So we're gonna test whether the way these work

matches the way EnChroma says they work.

Wait, if EnChroma can sell these,

I bet I could sell these.

Are there still some old sodium streetlights where you live?

Do you hate the color?

Would you rather see nothing at all?

Then you should try EnMouldia glasses.

They'll completely cut out that horrible orange glow.

Don't use while driving at night.

Don't use while walking at night.

That's the advert sorted.

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Right, so to figure out

what these pairs of glasses are doing to the spectrum,

we need to make some improvements

to this setup that I showed you earlier

where I was putting a diffraction grating

in front of my camera.

And that's exactly what this is for.

There's some kind of webcam at the end of this tube

and at the other end you've got the slit,

and there's a diffraction grating in there somewhere too.

And you can point that slit

at anything you want to find the spectrum of.

And it really is just a webcam.

Look, I can switch

from my built-in laptop webcam to this one.

I might actually use this for Zoom meetings, you know.

But with some special software

I can focus in on just the spectrum part of the image,

and from that it spits out a graph.

When I point it at the sodium streetlight,

you can see there's the 590 nanometers.

This is much more precise than I was doing before,

but you still can't see two individual peaks.

This device has a resolution

of about plus or minus two nanometers of wavelength,

but the two sodium peaks are about 0.6 nanometers apart.

There's another little peak in the infrared,

which I believe is also coming from sodium,

but it's weirdly hard to fact-check that.

There are also little peaks either side of the main peak,

which I believe are also coming from sodium,

but people don't talk about them because,

well, they're so much weaker than the main peak.

The reason the webcam inside this device

can see infrared and a bit of ultraviolet

is because, well, all webcams can,

it's just most of them have a filter

to block those parts out,

but that filter was removed

from the webcam inside this device.

Look, this is Chris Wesley removing the filter

before putting it into one of his devices.

It's a great bit of kit, by the way.

Like you could spend maybe 1,000 pounds

on a similarly capable spectrometer,

but this one made from 3D printed parts

and a modified webcam and a cardboard tube

is less than 200 pounds.

If you're interested,

there's a link to Chris' website in the description.

I did have a go with this spectrometer at UCL,

and you can kind of see a doublet there.

The reason there are two lines instead of one, by the way,

is because of electron spin.

So you might know that electrons

have this property called spin.

It's not like spin in the classical sense

of something spinning around,

because an electron is a point-like particle.

How can something that's one-dimensional have spin?

Doesn't make sense.

But it is like spin in the classical sense

of it gives the electron angular momentum.

And by the way, that's my new favorite

profound and weird fact about the universe.

How can a point-like particle have angular momentum?

This angular momentum turns the electron into a magnet.

And because it's quantized,

that magnet is either pointing upwards or downwards.

That's the spin up and spin down

that you might have heard of.

And that causes them to have subtly different energy levels.

But that never made sense to me,

because, like, they're just the mirror image of each other.

How can one have more energy than the other?

And it turns out it's to do with the orbitals.

Like you know how a moon

can be going around a planet like that,

and the moon can be spinning either in the same direction,

so they're both spinning clockwise.

That's prograde spin.

Or it can be spinning

in the opposite direction to the orbit.

That's retrograde spin.

And the same goes for atoms.

An orbital can have angular momentum

in one direction or the other,

which creates another magnet.

And that magnet is either in the same direction

as the electron spin magnet

or it's in the opposite direction.

If it's in the opposite direction,

they're attracted to each other more strongly

and that changes the energy levels

and that's how you get the split.

Okay, so now let's shine sunlight into this thing.

That dip in the infrared, by the way,

is the signature of oxygen.

So we know that the oxygen in the atmosphere

must be absorbing some of that wavelength

from the sunlight that's hitting the Earth.

I wouldn't see that dip

if I was doing this experiment in the vacuum of space,

because I'd be dead.

And look, if I put the glassblower glasses in front,

there's that dip in just exactly the right spot

for the sodium line.

That notch is achieved by the addition

of neodymium and praseodymium to the glasses.

The absorption spectrum of those elements

nicely cut out the sodium line.

It's also cutting out some of the green there.

I think that's just some different transitions

of neodymium and praseodymium.

Here's something I don't understand, though.

Like in the black flame experiment,

we're seeing that sodium will absorb the sodium line.

So why do glassblowers use neodymium and praseodymium?

Why not just put a load of sodium in the glasses?

Shouldn't that be good at absorbing the orange light?

Well, it turns out that sodium only emits

590-nanometer light when it's in gas form.

When sodium is bound up in glass,

its emission and absorption spectrums are different,

which makes sense because those valence electrons

are going to be involved in the bonding somehow.

But neodymium and praseodymium are different.

They're lanthanides,

and it turns out that the orbitals

of the valence electrons in lanthanides

are actually relatively small

compared to the orbitals that are already full.

In other words, they're kind of hidden away

from what's going on around them.

So the absorption spectrum of neodymium and praseodymium

are unaffected by whether it's in a gas form

or whether it's bonded to something else

or whether it's part of a larger solid.

So finally we get to the question of EnChroma glasses.

What do these glasses do

to the spectrum of light entering them?

So if I point my spectrometer at sunlight

and then put the EnChroma glasses in front,

this is what we see.

So this is interesting.

You've got this notch here,

and actually that does correspond

with the claims of EnChroma.

The logic goes like this.

The most common type of colorblindness

comes from the sensitivity

of the red cones and the green cones overlapping too much.

So if most of the confusion happens

around the peak of those absorptions

where they're really overlapping,

you can cut out that peak with special glasses.

Then maybe your red and green cones

can distinguish between what's left on either side.

This particular pair are for outdoor use,

so they're also lowering everything,

like a pair of shades would.

So the glasses are doing what they claim to be doing

in terms of their mode of operation,

but whether they help colorblind people

to see colors that they couldn't before is another question.

I actually don't wanna comment on that too much,

and that's for two reasons.

The first is it's a weirdly polarized topic now

and I don't like being shouted at,

but the other reason is, like,

the published literature on the subject

doesn't seem particularly conclusive.

If I had to summarize the literature,

it would probably be that there is no evidence

that they help with colorblind tests.

Though it does seem like that's because certain tests

it helps you to be able to perceive,

but at the same time, there are other tests

that it stops you being able to perceive.

So overall, you don't have an improvement on those tests.

Part of the reason

for the polarization around EnChroma glasses

I think is because of their dubious marketing practices.

Like when these arrived,

there was a little note with them

suggesting that I should film the reaction

of the person they were intended for

and then post it to social media.

But then of course,

you're gonna have a publication bias, aren't you?

Like I filmed my nephew using these

and they didn't really have an effect,

so of course I didn't post the video anywhere.

That was about four years ago, by the way,

and that particular nephew

now has a YouTube channel of his own.

If you're even vaguely interested in "Gorilla Tag,"

I highly recommend it.

Link in the card in the description.

EnChroma don't claim that these glasses

work for all colorblind people.

And of course, it's not just about

passing colorblindness tests,

which the literature seems to be focused on.

It's about your experience.

Like, of course it's worth the money

if when you put them on they make you happier

by that amount of money.

So you could go to an optician,

some opticians sell these, you could try them,

and if they pass the happiness versus money test

and you don't care about their dubious marketing practices

and you don't care whether they make you happier

than a placebo pair of shades

under clinical testing conditions,

then maybe EnChroma glasses are for you.

But EnMouldia glasses,

well they're for everyone.

God, that's a good ending to the video, isn't it?

It's a shame that I'm gonna carry on going,

'cause there's one more thing that I wanna tell you.

It's this really cool fact about Bunsen burners.

What we've been doing in this video is spectroscopy,

and it's the method by which loads of new elements

were discovered back in the day.

Robert Bunsen was one of the scientists

using that technique,

and he would heat his elements with a flame.

So like me, he needed a flame

that produced hardly any light of its own,

because that obscures the result.

To do that, he needed a burner

that would mix oxygen into the gas supply before it was lit.

In other words, that twiddly thing

at the bottom of a Bunsen burner

was put there to help us find new elements.

(upbeat jazzy music)