Light sucking flames look like magic

- In this video I will show you how to make a black flame. There is no trick here. Like I hadn't inverted the colors or anything like that. The explanation of how it works is really great. And along the way we will learn a strange trick that glass blowers use to make fire invisible. So I said there wasn't any misleading photography in this footage, but actually the fact that it's black and white is kind of a trick. If I put it back to full color and then turn off my camera's automatic white balance, you'll see that everything is very yellow.

That's because I'm

light

ing the scene with one of those old-fashioned street

light

s called sodium streetlights. Sodium streetlights have been used for a long time because they are incredibly efficient. Unfortunately, sodium streetlights make everything seem strange. Like you know there's this number, the CRI of a light bulb, and it tells you, how good is this light source at making colors

look

like they naturally are? Sunlight has a CRI of 100, and a light source that cannot 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 reproducing color is also what makes this flame black.

You may already know this, but if you excite an atom the right way, an electron will jump to a higher orbital, and when it comes back down, it will release a photon of light. The amount of energy the photon has is simply the difference between the two energy levels. The electron is giving up part of the energy it had up here and is wasting it in the form of a photon. But because these orbitals are quantized, their energy levels are fixed, meaning that 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 really only one possible transition within the visible spectrum, which emits photons with about a third of a femtojoule of energy, and they have this yellow-orange color. By the way, we don't normally talk about the femtojoules of a photon. We usually talk about 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 have some xenon gas in this vial, and when I excite it with electricity through this little Tesla coil, you get this beautiful color.

To see what that color is made of, you could use a prism in the same way Newton used a prism to discover that sunlight was made up of the entire spectrum of colors. However, instead of a prism, I will use a diffraction grating. You see how it splits the sunlight into a rainbow like a prism when I hold it in front of my camera lens. Placing the diffraction grating in front of the xenon shows us that although it appears white, it is actually made up of many discrete lines rather than the entire spectrum. Compare that to sodium, which has only one line.

I keep saying that sodium only has one line, but that's not strictly true. In fact, there are two emission lines very close together. We just can't tell it apart with this setup. But I'll get to that later, because I really want to explain this black flame thing first. So a sodium atom emits a very specific photon color when an electron falls 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 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 will not absorb it, but furthermore, if it has too much energy, the sodium atom will not absorb it either. So, in addition to 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 to emit orange light. The sodium street light uses electricity, which is actually about collisions. Electrons flowing from the anode to the cathode collide with those sodium atoms, and that is what causes the jump in energy levels.

The other way to do it is with heat. Then I start with a methanol flame. The reason I choose methanol is because it burns with an almost invisible fire. I don't really want any light from the fire, because now I add a little bit of salt water. The heat of the fire vaporizes the sodium in the salt and at the same time excites it. It causes the valence electron to jump. And when it falls again, it releases that orange photon, which is why the flame is orange. So all the time in that flame, you have a mixture of excited sodium and sodium in the ground state, and it's those sodium atoms in the ground state that make the trick work.

Because remember, those ground state sodium atoms are really good at absorbing 590 nanometer photons, which are the exact photons that the sodium streetlight produces. And of course, if the flame absorbs photons, it will

look

dark. And normally that's the end of the explanation, but I think it's important to clarify two things. The first question you might have is, okay, the flame is absorbing photons from the lamp, but it is also producing its own photons. Under normal lighting conditions, you can see that this flame produces a lot of light. So don't these two things cancel each other out?

Well, the answer to this is that my sodium street lamp is much brighter than the flame, so the small amount of light produced by the flame itself is quite insignificant. But the most important question you might have is this; After a sodium atom has absorbed one of the orange photons from the streetlight, it will send it back almost immediately. So if one photon is emitted for every photon absorbed, it shouldn't look black at all. But let's think about what you're really seeing here. To the left of the flame, you can see the white wall behind it that is well lit with that orange light.

The fact that you can see the wall means that those orange photons travel from the wall to your eyes or, in this case, to the lens of my camera. The piece of wall behind the flame is also well lit by the sodium street lamp. But when the photons leaving the wall begin their journey toward my camera lens, they have to pass through the flame, and when they do, they are absorbed by the sodium atoms in the flame. And yes, for every photon absorbed, a new photon is emitted. But the most important thing is that the newly emitted photon will travel in a completely random direction.

The possibility of it 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 spot behind. This all reminds me of something really cool that my friend Andrea Sella showed me; These special glasses that glass blowers wear. They make something incredible 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 glass has some sodium in it, as the temperature increases, eventually some of that sodium starts to vaporize and we see that characteristic orange light.

Eventually it becomes so bright that it is difficult for John to see what he is doing. But he looks what happens when I place this special pair of glasses in front of my camera lens. Isn't it amazing? It completely blocks out that orange light and suddenly John can see what he's working with. While testing glassblower lenses for this video, I realized that I now have the equipment I need to test these EnChroma lenses that I've had for years. I have wanted to take the tests for a long time because when I heard about them, I was skeptical.

Like they claim to improve color vision in people with red and green color blindness, but how can something that eliminates light help someone see colors they have no anatomy for? So let's test if the way they work matches the way EnChroma says they work. Wait, if EnChroma can sell them, I bet I could sell them. Are there still some old sodium street lights where you live? Do you hate color? Would you rather not see anything at all? Then you should try EnMouldia glasses. They will completely eliminate that horrible orange glow. Do not use it while driving at night.

Do not wear it while walking at night. That is the ordered announcement. Oh, I'm going to need a website. I know, I'll use the sponsor of this video, Odoo. This way the website will be free for life, including e-commerce, with a free custom domain name for one year. However, Odoo is not just about websites and e-commerce. It is an all-in-one management software for entrepreneurs, which includes business management tools such as billing, accounting, project management, inventory, etc., etc. So the first app, like eCommerce plus website, is free for life and that includes unlimited hosting and support. And if you want to add other standalone apps, switch to the paid plan.

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Okay, so to figure out what these pairs of lenses 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 sort of webcam at the end of this tube and at the other end you have the slit, and there's also a diffraction grating somewhere. And you can point that slit at anything whose specter you want to find. And it's really just a webcam. Look, I can switch from my laptop's built-in webcam to this one.

I could actually use this for Zoom meetings, you know? But with some special software I can focus on just the spectrum part of the image, and a graph is generated from there. When I point it at the sodium streetlight, you can see it's 590 nanometers. This is much more precise than what I did before, but you still can't see two individual peaks. This device has a resolution of about plus or minus two nanometers in wavelength, but the two sodium peaks are about 0.6 nanometers apart. There is another small peak in the infrared, which I think also comes from sodium, but it's strangely difficult to verify.

There are also small peaks on either side of the main peak, which I think also come from sodium, but people don't talk about them because, well, they're much weaker than the main peak. The reason the webcam inside this device can see infrared and a little bit of ultraviolet is because, well, all webcams can, but most have a filter to block those parts, but that filter was removed from the webcam inside this device. Look, this is Chris Wesley removing the filter before putting it on one of his devices. By the way, it's a great team. You could spend perhaps £1,000 on a spectrometer of similar capacity, but this one, made from 3D-printed parts, a modified webcam, and a cardboard tube, weighs less than 200 pounds.

If you're interested, there's a link to Chris's website in the description. I tested this spectrometer at UCL and you can see a doublet there. By the way, the reason there are two lines instead of one is the electron spin. You may know that electrons have this property called spin. It's not like spin in the classical sense of something that spins, because an electron is a point particle. How can something that is one-dimensional have spin? Has no sense. But it's like spin in the classical sense: it gives the electron angular momentum. And by the way, that's my new favorite deep, weird fact about the universe.

How can a point particle have angular momentum? This angular momentum turns the electron into a magnet. And because it's quantized, that magnet points up or down. That's the up and down spin you may have heard about. And that causes them to have subtly different energy levels. But that never made sense to me, because, like, they're just reflections of each other. How can one have more energy than the other? And it turns out it has to do with orbitals. Like you know how a moon can rotate around a planet like that, and the moon can rotate in the same direction, so they both rotate clockwise.

That is a prograde turn. Or it may be spinning in the opposite direction to the orbit. That's a retrograde turn. And the same thing happens with atoms. An orbital can have angular momentum in one direction or another, which creates another magnet. And that magnet is either in the same direction as the electron spin magnet or in the opposite direction. If it is in the opposite direction, they attract each other more strongly and thatit changes the energy levels and this is how division occurs. Okay, now let's shine sunlight on this thing. That immersion 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 of sunlight reaching Earth. You wouldn't see that fall if you were doing this experiment in the vacuum of space, because you'd be dead. And look, if I put the glassblower cups in front, there's that dip in exactly the right place for the sodium line. That notch is achieved by adding neodymium and praseodymium to the glasses. The absorption spectrum of these elements cuts the sodium line very well. It's also removing some of the green there. I think it's just a few different neodymium and praseodymium transitions.

But here's something I don't understand. As in the black flame experiment, we are seeing that sodium will absorb the sodium line. So why do glassblowers use neodymium and praseodymium? Why not just put a lot of sodium in the glasses? Shouldn't it be good at absorbing orange light? Well, it turns out that sodium only emits 590 nanometer light when it is in gaseous form. When sodium is bonded to a glass, its emission and absorption spectra are different, which makes sense because those valence electrons will be involved in the bond in some way. But neodymium and praseodymium are different.

They're lanthanides, and it turns out that the valence electron orbitals in lanthanides are actually relatively small compared to the orbitals that are already filled. In other words, they are hidden from what is happening around them. So the absorption spectrum of neodymium and praseodymium is not affected by whether it is in gas form, whether it is bound to something else, or whether it is part of a larger solid. Finally we come to the question of EnChroma glasses. What do these glasses do to the spectrum of light that enters them? So if I point my spectrometer at sunlight and then place the EnChroma glasses in front of it, this is what we see.

So this is interesting. You've got this notch here and it actually corresponds with EnChroma's claims. The logic is the following. The most common type of color blindness comes from sensitivity of the red cones and excessive overlap of the green cones. So if most of the confusion happens around the peak of those absorptions where they actually overlap, you can eliminate that peak with special glasses. Then maybe your red and green cones can distinguish between what's left on either side. This particular pair is for outdoor use, so they also pull everything down, like a pair of curtains would.

So the glasses are doing what they claim to do in terms of how they operate, but whether they help colorblind people see colors they couldn't before is another question. In fact, I don't want to comment too much on that, and that's for two reasons. The first is that it's a strangely polarized topic now and I don't like being yelled at, but the other reason is that the published literature on the topic 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 testing.

Although it seems that this is because certain tests help you to be able to perceive, but at the same time, there are other tests that prevent you from being able to perceive. So, overall, you don't get any improvement on those tests. I think part of the reason for the polarization around EnChroma glasses is due to their dubious marketing practices. For example, when they arrived, there was a small note with them suggesting that I should film the reaction of the person they were intended for and then post it on social media. But of course, you will have a publication bias, right?

I filmed my nephew wearing them and they really had no effect, so of course I didn't post the video anywhere. By the way, that was about four years ago, and that particular nephew now has his own YouTube channel. If you are even vaguely interested in "Gorilla Tag", I highly recommend it. Link on card in description. EnChroma does not claim that these glasses work for all color blind people. And of course, it's not just about passing color blindness tests, which the literature seems to focus on. It's about your experience. Of course it's worth it if when you put them on they make you happier for that amount of money.

So you could go to an optician, some sell them, 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 if they make you happier than a pair of placebo sunglasses in clinical testing conditions, then maybe EnChroma glasses are for you. But EnMouldia glasses, well, they are for everyone. Gosh, that's a good ending to the video, right? It's a shame I'm going to keep going, because there's one more thing I want to tell you. This is a really interesting fact about Bunsen burners.

What we've been doing in this video is spectroscopy, and it's the method by which many new elements were discovered in the past. Robert Bunsen was one of the scientists who used this technique and heated his elements with a flame. Like me, he needed a flame that hardly produced any light of its own, because that obscured the result. To do this, he needed a burner that would mix oxygen with the gas supply before lighting it. In other words, that trivial object at the bottom of a Bunsen burner was put there to help us find new items. (upbeat jazz music)