Dr Charles W Clark, 26 May 2019
ROBERT BUNZLI: Well, welcome to the National Museum of Australia everybody. On this very sunlit end-of-autumn day. Very appropriate because we’re going to be talking about light.
My name is Robert Bunzli. I work in the Lifelong Learning Section of the National Museum. We organise all the lectures and public programs here. I’d like to welcome you to the Museum, but first up, I would like to acknowledge the Ngunnawal and Ngambri peoples who are the traditional custodians of the land on which we’re meeting today. I’d like to pay my respects to their elders past and present and their new leaders emerging. I’d like to extend this respect to all Aboriginal and Torres Strait Islander people who are in attendance today.
Dr Charles W Clark joins us today to talk about the other world as seen by animals. As someone who worked at Questacon in the public awareness of science for many years, it always gives me a great joy to welcome a scientist to the National Museum, because science and technology have played such an important part in our history and in our contemporary culture.
Dr Clark is an atomic molecular and optical physicist at the Joint Quantum Institute and the National Institute of Standard and Technology and the University of Maryland. His main research is in the areas of matter wave interferometry, quantum gases, neutron physics and quantum computing. He’s currently a physicist and a fellow at the National Institute of Standards and Technology in the United States of America.
A specialist in making, measuring and using light, he is particularly fascinated by the invisible light that’s just outside the range of human vision. Infrared on the one hand, and ultraviolet on the other.
Recent studies of animal vision reveal amazing applications of invisible light in the plant and animal kingdoms and offer us a richer vision of nature. We may have some latecomers coming in to the auditorium, but we’re going to get started now. So, I’d really like to ask you to welcome Dr Charles W Clark to the podium.
CHARLES CLARK: Thank you. Do we have this? Good. Well, it’s a great pleasure to be here. I’m like you. I’m a frequent visitor to the National Museum of Australia, at least every time when I’m in Australia, which is not that frequently, but I really enjoy the programs here. It’s a real honour to be able to appear.
Let’s just get started. Say good day to your old friend, Bluey, well-known resident of this continent. In the news last summer, here’s a learned report from a scientific journal. Oh no, this is a report of a report from a scientific journal. ‘Blue-tongued skink hides an ultraviolet surprise for angry predators.’ Let’s see, do I have a laser pointer here? I think I do.
This is from a scientific journal and it reveals that here’s a response that this lizard is often seen when surprised. It’s supposed to — it’s a [inaudible]. It’s a reaction supposed to frighten a potential adversary. I looked this word up in the Oxford English Dictionary. It’s not there. I also looked up the word ‘bluey’ in the Oxford English Dictionary. There are about 10 meanings, seven of them come from Australia. So, it refers to this lizard, and evidently to a red-haired person, is that correct?
Another recent news item comes from — well, this is a part of Australia. Anyone ever been? Hello? Did someone up there raise their hand? Quite an interesting place. It’s actually a municipality of Tasmania, Macquarie Island.
Well, let’s see. Within the past several years, there was quite an astonishing study for those who find such things astonishing. Macquarie Island, apparently, is one of the few places in the world that has flowering plants but no birds or bees. Well, it does have birds. It has penguins, but it doesn’t have birds that serve to pollinate the plants.
The question is how do these flowing plants pollinate? It turns out that they’re pollinated by — there’s a breed of flies there that pollinates the flowers. This is attributed to long-standing mystery as to why the colours of the flowering plants on Macquarie Island are so different from those on the mainland. As we’ll see, this is correlated with things we know from the vision of birds and bees.
Okay. My talk has three parts. First of all, what is ultraviolet? Who can tell me what ultraviolet is? All you kids in the audience, you’re responsible for answering the questions. What’s ultraviolet? I can wait. We’ll find that out soon enough. Basically, if you think ...
This is not a physics talk. I’m trying to give you the information from optics by using simple experiential ideas that don’t require a lot of mathematics. The rainbow is a great motif for understanding what’s going on.
The rainbow, which you’re familiar with, shows red, yellow, green, blue and violet. Now, the question is how many colours are there in a rainbow, and if we have a stripe like this, what happens if we go outside this stripe and inside this one? What do we see there?
It will turn out that from what we can understand about that, it shows a length with modern ideas of quantum physics. It’s very essential. Then it gives guidance as to where to look for new results in plants and animals. I will say as far as this issue of ultraviolet vision in animals, and then also the interaction with plants and insects, the material has all been accumulated within about the past 20 years. It’s a very new field.
It’s made possible in large part by — this is a mobile phone — by consumer electronics. About 20 years ago, the new generation of low-cost digital cameras came along, and it turns out that these could be easily modified. What I’m saying now is true about the camera inside your mobile phone. They can be easily modified to see infrared and ultraviolet vision.
In fact, the phones contain blockers of the infrared and ultraviolet which, if they were allowed through, they would make the kids look horrible in the photos that you send to grandma. You don’t want that.
Okay. First, let’s talk about light. Don’t be frightened, children. This is an old instrument. I know none of you have seen it, but if you ask some of the very oldest people in the audience, they may have seen it. It’s called an overhead projector. I like it because you came here to see spectacular images, right? Like the Avengers movie.
Well, you can see anything on TV. The electronics can draw all these things, but this is just light. You see this? It’s just light coming through, there’s no electronics. Here’s white light coming through the projector. Can we change the colour of light? Yes? Yes! Like, duh. Here. Duh.
That was white and now it is this colour. Does anyone have a name for this colour? My gosh. A couple of college graduates here. Yes, so it’s like, duh, we can change the colour. I just did it. It looks like we can do it in lots of different ways.
Well, what’s actually going on when we do that? How did we change the colour by what I just did? The first person to really understand this was Isaac Newton. The ancients were familiar with colours. They had seen the rainbow. They saw when light goes through a glass prism, it also spreads out the colours.
There are many ideas. Well, there’s something mysterious in the glass that impresses the colour upon the light. Here’s what Newton did. This is a very nice video on YouTube from the Massachusetts Institute of Technology that I’m just going to skip through the highlights.
Here’s a ray of white light. Sorry. Now, just like I did, a red filter is put in front of it, and you see that you just get red colour going past. When you put a prism here, well, you can sort of see what’s happening. The colours are all spread out and the next ...
This makes it quite clear. In going through a prism, which is a sort of thing that produces the rainbow, you see this white light has this band of colours. Newton did these experiments actually being in a dark room and with a pinhole punched in a window shade so that the beam of sunlight would come through.
He put the prism down and saw this effect, but then he asked the question, ‘What if we put another prism just in the field of the blue light here? Does that cause a further separation of colours?’ It turns out — you can do this experiment — it does not. The pure colours when they go through a prism, they just appear as they were.
This is a sign of a truly great experiment, so I hope that if you can remember two things from this talk, the first is the Newton experiment and the second is an experiment that I’m going to describe conducted by Herschel, which is even simpler and of profound importance.
You can see the colour bands slightly there. Here’s the first prism, then there’s another optical element. That’s a sort of a lens. The first prism spreads the colours out and then the second prism recombines them into white again. In other words, white light is intrinsically a combination of all the colours that we can see. You can separate them out and you can recombine them.
I think they have a slide about this. Every time you use your mobile phone, you’re employing a global telecommunications technology that works on this basis. Literally, the radio waves from the phone are spread out not by a prism but by a time domain technique. The same ideas.
They’re spread out into all their constituent colours, each of which is modulated as a different channel in order to enable you to send a wide variety of communications over a common channel. This is the Newton experiment. I’m going backwards. I’m sorry.
Well, let’s see. Starting about 200 years ago, and then 150 years ago, there was an emerging understanding that the human eye has three elements that are sensitive to different colours — red, green and blue. This is RGB. Do you know this thing for the colours? RGB was a colour code. We’ll see that more in a minute.
This is a schematic. Just think of wavelength here. This means light is an oscillation and has a certain wavelength, but just think of this as a colour index. Blue is at short wavelengths. Red is at longer wavelengths. There are these three separate elements that are sensitive to different parts of the spectrum.
Then with that, our vision emerges. It’s a very complicated thing. Though it’s not just the sensors but how the brain operates on the information from these separate sensors. This idea is now encoded into all display technology. When you’re looking at a video, I think even in the modern 4K or 8K systems, the caller that’s used by the video is one in which there’s a red and green and a blue channel and each of those is modulated or has an intensity on the scale of zero to 255.
Some of you, of when I was making this slide, I was choosing colours. There’s this RGB. This is red. It means red index of 255, which is 256 – 1, and then no green and no blue. Here’s the same thing in a different type of colour picker in the hexadecimal notation.
I’m sure many of you have experience with these things. For example, the red would be — we have RGB, eight binary bits, and this is would be the binary representation of red, and then similarly for the other colours. Black is 000. Red is 25500. Green is 02550. Blue is 00255. These are three primary colours.
Now, you kids, do you have a common name for these colours? This is red, this is green, this is blue. I think we heard a correct answer to a previous question. What was this colour? Magenta. Do you see that magenta is sort of not green? It’s the opposite of green.
In other words, if you put magenta and green together, you get white because you get a 250 — if you add them, 255, 255 and 255. What about this one? Not red. Geez, did someone leak the results from this test? Okay, you’ll never guess what the opposite of blue is.
Very good. You’re ready to do all the math. In fact, for those who are involved in binary — thinking about computation in terms of operations on bits. This bit operation, the logical not, basically it takes red to cyan, green to magenta, and blue to yellow.
In fact, from a physics standpoint, red, green and blue are the primary colours used in video displays. Can someone suggest to me an application of the use of the primary colours cyan, magenta and yellow? Printing. Yes, printing. Because here, you’re using an active emitter of RGB. Here in printing, typically when you’re looking at printed material you’re seeing white light that’s reflected from the material. It is the subtractive effect of this that has the strongest influence on white light.
Well, there’s even more which we’re going to start on now. Here is an example of — this is light from a synchrotron radiation source at the place where I work, National Institute of Science and Technology. It’s coming through this port. It’s reflected off a diffraction grating. It’s on this stand and you see this spread of colours on the far screen.
What’s going on there? This spreading out of colours is accomplished in that case by an effect called the interference of waves. It’s manifested in a diffraction grating by a very simple concept. You have a set of scatters that are spaced. Let me just do it on the overhead projector.
This is usually the point in which the bulb burns out. There’s an incandescent bulb in here. That’s maybe why they’ve been banned in places like Australia. This is a cartoon of a set of wave motions. Now, if I put another wave motion right on top of it, you see I get the same thing. But now as I displace the two, do you see that you get these places of interference? The two things overlapping the same way.
You see, they’re sort of spaced at equal angles. That angle decreases as you change the distance between the centres. Then if you put another one on — this is still a cartoon — you can see there’s even a more intense concentration of light in certain directions.
This principle was first, well, it was used in the Second World War to set up — so you’d take radio towers, radio stations separated by fixed distances along the landscape. Then they could broadcast a signal that would be strong in certain angles. They could guide bombers to their destination or they could provide a homing beam for a returning aircraft.
For example, here’s an application of this type of thing to a very high resolution spectrum of the sun, which shows — this is what you see if you have ordinary sunlight, but you put it through either a prism or diffraction grating to spread the spectrum out. You’re now going from red to violet. If you like by going from red ... It’s like you take the spectrum and then you cut it into little strips and pile them on top of each other so you can see the whole thing.
There are many features in the sun where there’s no light coming through it all. This is something that was quite astonishing when it was first discovered. It provides us evidence for the basic precepts of quantum mechanics.
If you see a figure like this, which is the spectrum of the sun or the rainbow and you see nothing beyond the red and nothing beyond the violet, it’s of interest to ask if there’s something there that, for whatever reason, you cannot see. I mean, don’t you think that you’d want to — it’s like, ‘Is that all there is?’
Very important first step in this was made by a man named William Herschel. He began his career as a military bandsman. He played flute in a German military band. These are Hessians. [points to slide] They were employed by King George III. His father was a sergeant in this regiment.
At some point in 1760s, 1770s, things got hot in Europe and the family moved to North East England and he became a musician. He developed a sideline in astronomy. He actually built the most powerful telescopes that anyone had ever built. With one of them, in 1781, he discovered a new planet which he named the Georgian planet, after King George III of England.
Now, King George III had an interest in astronomy and he invited Herschel to come down for an interview. Herschel did so and I’m sure that in the audience with the king, he addressed the king as your highness. Some of my ancestors were engaged in the American Revolutionary War on the other side in those years, and I think they would have preferred to address King George III by another title. Uranus, not your highness. Uranus, which is what that planet eventually became known as.
Nowadays, Uranus and the further partner, Neptune, is a source of very great interest in astronomy. At the time, they were also of great interest because they were a new planet. Ever since antiquity, people had believed in the existence of seven and only seven planets. They were known to everyone.
In fact, the first peoples here made a lot of accounts that account for every one of those seven planets. Indeed, the language for the days of the week that we use even to this day, the names of the days of the week, are the names of the seven planets.
This is a little bit difficult to see in English, but in English, we have Saturday, Sunday and Monday. Those refer to the three planets, the Saturn, the sun and the moon. Do you agree that the sun is a planet? You disagree with all these people that made up the days of the week that we still use?
What is a planet, you kids? Okay, let’s say at night you see the stars and the planets, what’s the difference between them? How come the ancient people all knew exactly what the planets — yes?
COMMENT: Stars give off light and planets reflect light.
CHARLES CLARK: Okay. The statement is, ‘stars give off light and planets reflect light’. I would say with all due respect to the first peoples, and in fact the Romans and the Greeks and all the Chinese people, they didn’t know that. What’s the basis for your knowledge of that statement?
It’s not untrue, but it’s not the basis upon which these two things were distinguished. The first peoples knew they had exactly the same knowledge from looking up at the sky all the time. Let’s just say maybe it’s been cloudy for a while, you haven’t had a chance to have this experience. The stars move in the sky, but they move together.
There are these constellations, like Orion or the Plough. They appear in the sky and they appear in a different place, but it’s as if they all rotate rigidly together. Then there are the planets. The planet is like a Greek word for wanderer. They appear on a regular basis, but with a very different pattern with respect to the stars. They move with respect.
The stars don’t move with respect to each other. They stay in the fixed position. Now, that’s not entirely true, but it’s true on the basis of human experience because they’re so far away that no human being ever sees the relative movement of stars, except with very powerful telescope.
The planets also behave regularly but on a completely different cycle. From antiquity up to the time of Herschel, there were seven that were known. The sun and the moon are planets because they are positioned in the sky, varies in a way that’s different from the stars. Earth is not a planet. Earth is not a planet. The sun is a planet.
Okay, that’s not the story. We now understand things in a different way, but in the standpoint of experience, that’s what it looks like. Then there’s Venus. What? Mercury, Venus, Mars, Jupiter and Saturn, and that was it. You might say Herschel was the first person to have actually discovered a planet.
Who discovered the moon, or the sun, or Mars? They were all known. For people who believed in astrology at the time, the appearance of a new planet was either an attack on the foundations of the system or an opportunity to make new money from another crazy theory. I’m getting diverted a little bit.
The other thing that Herschel did, which in some sense is more important for us today was he discovered something called the infrared, which was found in the spectrum of the sun.
I said, ‘If there’s only two things you can remember from this talk, it is my suggestion you remember the Newton experiment and how it was done, because at least I hope I described it right.’ It was based on very simple ideas that you could replicate with just prisms. You could do that in a school kid’s science fair project.
The experiment on the infrared is even simpler, also the subject of schools fair science project and it’s amazing in what it shows. Here’s a diagram at the science Museum in London of a Herschel experiment. It’s very simple. It involves just a prism and three thermometers.
I’ll give you a better example which does come from a school science fair. You can do it yourself. Oh, this has four components—a prism, three thermometers and a cardboard box. I don’t know where we’re going to get the cardboard box, but in principle you could use something else.
Take it out in the back garden, set it up like this. There’s sunlight comes through the prism here. You see the violet to red. Then you have these three thermometers and you now position the thermometers, as we say, in the field of the light. I hope this makes it clear.
You’ve noticed like when light shines on you, it’s warmer than when it doesn’t. Now what you do is you put the thermometers — here’s the three of them — you put them at different positions in the field of the sun’s rays and you see that the temperature goes up as you go from blue to the red.
In fact, maybe it’s not so clear from this, but here the ball with the thermometer is in the dark patch beyond the red, yet it shows a higher temperature. Herschel found just by using a prism that there was something that was refracted by the prism just as if it were light. It could be detected by thermometer, but it wasn’t visible.
There’s this invisible radiation as seen with a simple optical device just on the other side of the red. He didn’t call it the infrared radiation. That’s a later name but that’s the basic idea.
Now, as I was saying before, because of the revolutionary advances in digital cameras. In fact, it uses a complementary metal oxide semiconductor. It’s a semiconductor sensor that’s used in your phone. By appropriate modifications, anyone can take infrared or ultraviolet photographs.
Here’s an example. It shows a suit of armour from Japan, mediaeval Japan seen in the visible. Here’s the same thing seen in the infrared. This means that you can pick up a very — you can understand in more detail what is the composition of this thing based on analysis of these two types of image.
This is leading to the hint about applications and animal vision. As we’ll see, animals can discern light that we can’t see, mostly in the ultraviolet. With that, they can understand things in a different way than we do. We can beat them now with our mobile phones, but until then, no one knew what was going on.
Okay. What Herschel showed — here’s the spectrum of light. Then on this side of it, he showed — did I give a clear enough explanation of that experiment? You have a prism and in a place where there’s no light, you can see that the heat radiation is there and it acts like light in the sense that it’s deflected in the same way by a prism. That’s the signature that it’s light.
Without knowing anything about it, we could just put down some imaginary scale. Let’s just say that’s here’s a rule, right? Out here, there’s the spectrum of light. Then behind here, we have infrared and we can actually — without really knowing what this stands for, except that in the optical spectrum it represents the progression from red to violet, we can suggest that there’s something similar for the infrared.
What does this simple idea imply to you as the next question? Oops, I gave away the answer. Well, okay, what about over here? Is there something that we don’t understand here? The answer to this was first given by a man named Ritter, a German scientist, who discovered what we now call ultraviolet radiation. He had a different word for it. He called it chemical radiation.
Well, this is for the scientists in the audience. People like me, we spend like a month writing some paper in order to have it rejected by Nature, usually because it’s too long. It exceeds the word limit. These are usually papers about some incremental discovery that can’t be explained to civilians.
Here’s this foundational discovery of a new form of radiation. Here’s Ritter’s report in its entirety which says, ‘On February 22, using horn silver.’ I mean, that’s actually the word he used. It refers to a silver halide compound. More about that later. ‘I found rays of the sun on the side of the violet in the colour spectrum but outside it, this radiation reduces drawing the violet light itself and the field of these rays is very wide.’ More about this later.
He did the same experiment but using a chemical. We’ll get to that in a minute. So exactly what was he doing? Why was the chemical aspect important?
I’m now going to tell you a story which seems to be impossible, but if you ask your great-grandparents, you’ll find that the substance of what I have to say is true, as incredible it may seem. Once there were no camera-enabled mobile phones. In fact, there’s more extreme version of this statement to which I do not subscribe, but some people believe it. They say that at one time there were no mobile phones.
Now, I’m not going to stretch your credulity. This is bad enough. How can this be? Because for example, we have plenty of examples of photographs from many, many years ago. Here’s the first King of England, Alfred the Great, captured in a photograph. How is that even possible?
Well, let’s see. I’ve used the word ‘camera’. Here’s an example of the camera. There’s a beautifully written book, about 150 years old, explaining various sorts of gadgets. This a gadget that was prepared for use of artists or draughtsmen to make accurate drawings of things that could be seen in light.
It’s a dark chamber. That’s where the camera comes from. Latin ‘camera’ means ‘chamber’ or ‘room’. The artist would sit in this chamber and then there was a mirror and a focusing lens and it would see something outside and it would project the image down onto a piece of paper. Then the artist could trace over it and make an accurate drawing. If you look, you’ll find books from even the early 19th century in which this is used for things like architectural drawings where accuracy was very important.
The modern camera basically has the chamber in place but it replaces the artist by — okay, I hate to say it — the camera used to have something called film. In order to take a picture — I know, I’m just sort of making this up, but some will be — you put the film in the camera, then you’d open a shutter, the light would come in and this film was a plastic type material coated with some chemicals, and the light would cause chemical reactions to take place. Then by chemical processing, you could make that image fixed.
Okay, so how does light cause chemical action? It actually causes chemical changes in matter. I’m going to do a few demonstrations here. Here are two bottles of water that I brought from America. Here’s green light. if I pass green light through the water, you can see a little bit of green, can’t you, in the water. It’s just basically from reflection of the green light through the water.
If I pass red light through these two bottles of water, once again, you don’t really see much. This is an ultraviolet laser. This emits colour. If you own an Xbox or PlayStation, it has a laser of this type in it. It’s in the ultraviolet. Now, what you can see, it goes through one of these bottles. You can’t really see anything, but from the other, it makes this bright blue streak.
Here’s another example. They’re the same thing. I don’t know if you can see it here. This is a strip of paper that says ‘exit salida’. This means ‘exit’ and the Spanish word for ‘exit’. This is a strip that glows when the lights are turned out. If I illuminate it with a bright green laser, nothing happens. I illuminate it with a red laser, again nothing happens. But then when we use the violet, it causes it to glow wildly. How about that?
Why does this purple light cause green to appear? Or cause blue to appear in the second of these bottles? By the way, this is a club soda or ordinary sparkling water. This is tonic water which has quinine in it. Quinine, when exposed to ultraviolet, gives this bright blue fluorescence.
Let’s see if I can do this without injuring anybody. I wonder if this will be shown. Here are these two things. No, you can’t really see it there. Let’s see if you can see it here. No, you can’t really see it here. I’ll do it without the light.
Here’s this magenta piece of plastic. When I put a red light onto it, nothing really happens. When I put green light onto it, you see if you look up there, you’ll see the green light is still reflected. Do you see it up there in the corner? That’s reflection, but then there’s this bright orange glowing spot that appears on the surface. Whereas with red you can see the same thing — the red light is reflected there and also transmitted but nothing happens.
Now on the green. The green piece of plexiglass. Once again, you can see the red light reflected up there and transmitted but nothing happens. The green light transmitted and reflected up there but nothing happens. Now with the ultraviolet, if you look up there you can see that there’s part that’s reflected, it’s unchanged, it’s up there on the ceiling, but then it also causes this glow that’s not purple and it’s not really the colour of the green either.
These are examples of how this more energetic radiation can cause the creation of things, by what we call chemistry, that weren’t there previously. How many hours left do I have in this talk?
This phenomenon is a very important element of quantum mechanics. The explanation of that phenomena that I just showed you — like the violet causes these different types of fluorescence but the red and the green do not — that was all explained by Albert Einstein in a famous paper published in 1905, which led him to get the Nobel Prize in physics.
The basic idea is that the colour of the light is an index of its basic energy, of its energy at the quantum level. The more it goes into the blue, the higher the energy it has. Here’s sort of a site for the physics audience. The manifestation of the importance to that idea of Einstein’s was made most pronounced by Niels Bohr who gave an account of how that was reflected in the properties of matter as exposed to radiation.
Just a mnemonic, what’s happening here? Red is the lowest energy light. Green is the next and violet is the highest energy of the three that I’ve shown you today. If you were in Hobart, you might say, ‘Well, let’s pretend that taking something up into Australia requires an energy that’s proportional to distance.’
The red light might take an electron from Hobart to Adelaide. A green light take it to Townsville, and the violet to Darwin. Then when you put a lot of energy into the system, you might get some of it back by a re-radiation at the last energetic rate wavelength. This, what I showed you with the violet laser and the red, is basically really just like this. The violet light is absorbed and then it also produces this fluorescence in the red.
Okay. Now, I guess you’re wondering where do the animals come in. About 20 years ago, there was a solution of a longstanding problem in ornithology, the study of birds. As a prelude, I’ll just say this. There are many birds in which the females and the males have different colouration, very much different.
On your left here, lower left, there’s a brown and black bird. That’s the female of the Baltimore oriole, which is the yellow and black bird. I think originally, people moving into new continents would see birds of a different colour and it would take a while to see if they were the males and females of the same species.
Another example from Virginia. The Virginia state bird is the cardinal. The male has bright red plumage and then the female has distinctly different coloration. There were some anomalies. Perhaps, the one that really made the headlines was called the blue tit. This is a passerine bird quite common in Europe.
In physics, we can’t write papers with titles like this, but biology, even the proceedings of the Royal Society of London will publish them. Blue tits are ultraviolet tits. When you’re writing a scientific paper, you have to make the matter sound like the most important question of all time.
That’s done pretty effectively in this paper, which says that even the most experienced ornithologists didn’t have a way of distinguishing male from female blue tits by visual inspection. Then it was found, due to the availability of accessible ultraviolet sources and cameras, that if you look at the birds in the ultraviolet, if you use ultraviolet illumination and ultraviolet camera, you could see there’s a huge difference between male and female.
This suggested the ultraviolet, for some reason — this is related to sexual differentiation which, you can imagine, that’s an important part of the ability of the species to propagate. It’s of some significance in the lives of birds. It turned out that it was soon thereafter discovered that the vision of most birds, maybe all of them, I don’t know if the whole spectrum has been uncovered. The vision of most birds — birds have four sensors in their eye, a red, a green and a blue like ours, and then a third one in the ultraviolet which we can’t see. These birds, if they had cameras, would need to have 32-bit colour rather than 24. We have a much more limited system.
Some of you may ask, ‘Why do the birds who seem to live much simpler lives, why do they have a much richer visual system?’ There are answers to that question, but you might say, ‘Why do we have red, green and blue?’ Well, in fact, there are people who are colourblind who are still able to exist without very much difficulty.
The story doesn’t end with birds. Here’s something called the mantis shrimp, which is neither a mantis nor a shrimp, hence the name. Beautiful creature. It also has an incredibly complex visual system. I said birds have four different colour receptors. The shrimp mantis has 12 that extend from near infrared right down into the ultraviolet.
I think no one really understands why it’s 12 rather than 11 or 13, but it’s something that provides a huge capability for differentiating, for processing signals in different regions of the spectrum from its own environment.
So, here’s a very nice video by Richard Hammond that will show you — has anyone seen this video before? It will show you an example of how the ultraviolet properties of flowers are well matched to the ultraviolet properties of bees.
RICHARD HAMMOND: [video plays] Just beyond violet in the rainbow lies ultraviolet. It’s completely invisible to us but not to certain animals. The invisible world of ultraviolet has many other things to reveal about the life of animals. Alongside the …
CHARLES CLARK: These you’ve all seen before. Bees landing on flowers. Here’s what the flower looks like in the ultraviolet.
RICHARD HAMMOND: [crosstalk] hidden signs and secret codes all designed to attract the interest of passing insects. That’s because insects can’t see our world clearly at all but they can see ultraviolet.
CHARLES CLARK: The flowers actually provide a target landing zone around the source of the pollen.
RICHARD HAMMOND: [crosstalk] no reproduction, so it’s actually a matter of survival, and flowers have had to learn to advertise themselves to bees in a way that bees can understand, because to the bee, this garden looks very different. For the first time, high-definition cameras can give us a bee’s view of the garden.
CHARLES CLARK: There’s a correlation of the reflecting power of the flowers with the visual apparatus of the bee. Here’s another one. This has some unfortunate scenes of violence in it, but it makes a similar point.
DAVID ATTENBOROUGH: [video plays] pollinators to and from flowers is so heavy, and in particular so predictable, that it’s not surprising that some invertebrates have learned to exploit it.
CHARLES CLARK: This is the bad guy.
DAVID ATTENBOROUGH: [crosstalk] sits almost invisible on a white flower, waiting in ambush, and it catches a bee.
CHARLES CLARK: It looks like it’s camouflaged against the flower. Just wait.
DAVID ATTENBOROUGH: It looks superbly camouflaged to our eyes, but insect eyes are different to ours and see parts of the light spectrum invisible to us. Under ultraviolet light we can get a better idea of how they see things. And most surprisingly, the spider looks more obvious to them than it does to us.
CHARLES CLARK: It’s black, but perhaps sitting on the centre of the flower blossom makes it look like a target that the flower would have set up. I’m sure you rather hear David Attenborough than me. I would to, but anyway, you get the point that there are these clues to animal behaviour. The coordination of signals by plants and animals that you can’t possibly understand without knowing something about this spectrum.
How does it work? Here’s an early paper about the bee. The bee, these are known to have green, blue and ultraviolet receptors, not the red. This paper pointed out that if you look at the reflectance spectra of — the upper curve is the reflectance spectra of foliage, the leaves and stems of the flower as a function of wavelength, and the lower one is the reflection spectrum of blossoms.
You can see the greatest difference between those two occurs in the ultraviolet. So the ultraviolet is a region where the bee sees a greater contrast between the pollen-bearing part of the flower and the ordinary leaves. I mean, it’s not like this is an explanation, but it’s something that is a consistent story about why these two things are coordinated.
As for the badass lizard, I don’t think anyone has really figured out why it’s important for this exposed part of its mouth to have an ultraviolet signature but that could indicate something about the nature of the threats that it usually sees. Where do the threats come from?
There are many other recent examples. Here’s a zebrafish which has ultraviolet and ordinary colour vision, and then a non-colour vision, all integrated together in order to provide a sort of sensation about its environment that with different signals can be used to process different information.
I guess maybe if you forgot to renew your subscription to the Journal of Molluscan Studies, you didn’t see the spectacular image that shows the different appearance of shells in visible versus ultraviolet light.
Here’s sort of a practical application, intriguing. This was a study that found that sea turtles had a very strong pronounced green sensor, or a UV sensor. The idea is that you could maybe take some of these low-cost ultraviolet emitters developed for the PlayStation and so on and put them on gill nets. Festoon the gill net with a string of these ultraviolet emitters and the sea turtles might learn to use that to avoid being fouled in fishing nets which is a significant problem.
Where was I? Sorry. Here’s an article just published in the beginning of this year involving a study of a forest in Sweden and in Australia, up in Queensland. This shows that the issue is if a bird is in the forest flying around in a dense forest, it would have an advantage of seeing the underside of a leaf which would indicate that it’s within a forest canopy or below a tree versus the front surface of a leaf, which would give other directional information.
This shows that if you look at the reflectance spectrum of leaves on trees in a forest, you can see a big differentiation if you compare the visible and the ultraviolet. This indicates that avian UV vision, the ultraviolet vision of birds, can be used to enhance leaf surface contrast in such an environment.
So we see red, green and blue, it’s well matched to our environment, but we’re not flitting around in forests foraging for food like a bird. The other living things may have very much differentiated visual systems than what we have. This is still an area of active involvement and I think for those kids who are thinking about doing something exciting, Australia is a particularly rich field of inquiry for this type of thing because it has this unique flora and fauna. There are probably many things to be learned here that would give results that aren’t seen in the rest of the world. Macquarie Island is an example.
I hope some of you youngsters will have gotten enough excitement from this talk to maybe think about looking at this in a school science fair, or later in life as a researcher. That’s the end of my presentation and I only have to answer questions the best I can.
ROBERT BUNZLI: Thank you Dr Clark. We’ve got two microphones here. If anyone would like to ask a question — we’re right on three o’clock, so if anyone needs to leave, that’s fine, but we’ll take a few questions. We’ve got one right here.
QUESTION: Thank you very much for your presentation. It was fantastic. My physics is limited to school and early university many, many years ago.
My question is this. On your spectrum there, you had roughly from 300 to 700 as the wavelength with ultra red at the lower end and ultraviolet at the other end. Are we talking here about what goes down beyond 300 and above 700?
CHARLES CLARK: Yes. Let’s go back to one of these. It seem to be jammed. Well, your question has offended the people in Redmond, Washington who make this computer project.
If you just think of blue going to red, the upper limit of human vision in the red corresponds a wavelength of around 700 nanometers. In fact, you can see things further out, but they only trigger the red receptor and they become very dim. Light that has a wavelength greater than 700 nanometers, it all seems to be the same colour of red, because it’s not exciting the green or the blue receptors.
Then below the blue is what we call the ultraviolet. Beyond the violent is what we call the ultraviolet. There, in fact, people who have had cataract operations can see ultraviolet. It’s the lens of the eye that blocks it, that makes it irrelevant to most people’s human vision.
I don’t know if that’s an answer to your question but, I mean, let’s say the transmission properties of the — there’s some optics in your eye that focus the light and they have limitations in the type of light that they will pass without attenuation.
QUESTION: My dog seems to have a really good night vision. Is there something that she’s using beyond the normal colour?
CHARLES CLARK: Yes. I’ve had this question before. I don’t really know the answer, but it’s true. Also, cats have very good night vision as well. It might well be that their — I don’t really know the answer, but it’s presumably correlated with the hunting, ancient role of — they’re hunters and there’s an advantage for them being able to see at night. But I do not know the answer in detail as to how their visual system responds. Or whether your dog is better than — oh, it’s much better than the average, of course, but how much better, I don’t know.
QUESTION: The thing you said about modifying your phone to see ultraviolet light.
CHARLES CLARK: Yes.
QUESTION: Is that something that you could do yourself?
CHARLES CLARK: That’s a really good question. The first people to do it did do it to themselves. I could talk about this separately at length. Basically, if you have an old — there’s old-fashioned web cameras that people would attach to their computer. You could probably buy one on eBay for 50 cents.
If you take it apart, you’ll find that there’s a filter which looks like a little plastic sheet of light and its function is to block the infrared radiation, because in the natural environment there’s lots of radiation, like from the sun. If you don’t block it, then the semiconductor filter that’s used in these cameras gets swamped by the infrared and it hinders its processing of colour, so you can do that yourself.
As is usually the case, sort of an industry developed. You can go on to the Internet and you can search for infrared sensitive digital cameras and there are firms that have — if you’re going to modify a digital camera, you want to do it in a clean room environment. You want to do it in a way that you don’t get contaminants in the environment that affect the result.
In fact, if you had a budget to do such a thing, you’d be better off spending — you can buy a modified —they’ll sell you a Sony or a Pentax or any type of camera that they’ve modified just by removing the infrared filter.
COMMENT: Thanks so much.
CHARLES CLARK: The same applies to the ultraviolet as well. There’s typically an ultraviolet filter in these cameras that’s used to distinguish light. In fact, the story — you might want to just look it up — the story of the colour sensor in digital cameras is just amazing.
I mentioned this 24-bit colour. You’re familiar with that I think, eight bits per channel. The underlying sensor in a modern mobile phone camera has a 48-bit colour system. That’s not just twice as many levels as 24. It’s like 16 million more. In fact, say like the image processing systems that are used in movie studios, they use a much larger colour representation space than the 24-bits that are used on display, but that’s just because they want to retain fidelity through multiple and long processing of colour where if you’d had round off, you might accumulate errors.
The display industry is fanatic about quality control and they’re willing to invest anything to solve their problems. This type of high-end engineering of colour systems is a really pretty interesting subject.
ROBERT BUNZLI: We’ve got time for one last question up there. Thomas, in the fourth row.
QUESTION: Can you please explain a little bit more about how you use absorption and emission spectrums in your particular fields of study?
CHARLES CLARK: Okay. The question is how we use absorption and emission spectra in our field of study. Let’s say if I go back to the — I’m seeing it on my screen, but it’s not appearing up there. Thanks. Let’s go to the spectrum of the sun.
The sun emits, we believe, basically a white light that consists of from the red through the violet. When you look at it in detail, you can see that there are certain colours that aren’t present in the spectrum of the sun. That’s because there are atoms in the atmosphere of the sun that absorb light of those colours in particular.
This type of map gives us an indication of the chemical composition of the sun. In fact, it’s been implied to many stars as well. Then, similarly, when you see an emission spectrum of something that’s hot and you see features in it, that can be used to determine the elemental composition.
Then there are other things like — we call these lines, these dark patches. Sometimes a change in the pressure of the gas will show a shift in the wavelength of absorption or emission line. That can be used to determine the physical characteristics of what caused that shift.
Basically, all the information that we have about the chemical composition of the sun. All the direct information we have on the chemical composition of the sun or all the other stars comes from absorption and emission spectra in the optical, in the ultraviolet light. The same for the chemical composition of planets.
Once again, there are distinct lines that one could see. As the sun passes, there’s an eclipse of a sun by a planet. At the edge of the eclipse, we can see that there are particular chemicals in the atmosphere of the star. Some information about the atmosphere of the moon.
The moon is said to be airless. It’s just got a very thin atmosphere and the information that we have about that comes again from these types of observations. So because there are lines that are at well-defined wavelengths, let’s say, or well-defined colours, shifts in those tells us something, and also the existence or nonexistence of a line tells us whether a certain substance is present in the thing that we’re looking at. That’s a partial answer.
ROBERT BUNZLI: Okay. Well, thank you very much Dr Clark for an absorbing lecture.
CHARLES CLARK: Thank you for coming. Thank you for the questions.
ROBERT BUNZLI: Please show your appreciation to Dr Clark.
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Date published: 09 July 2019