Iridescence part II – Hummingbirds

I've already discussed the iridescence of my little fish and how that vivid colouring originates from interference and phase-shifts within a thin layer of material that is as deep as the light's wavelength. Now, I'm going to move on to the iridescence in a male hummingbird's gorget. In a medieval world, a gorget was armor designed to protect one's throat – a term that has been extended to include the metallic-like iridescent feathers on the hummingbird's throat, the purpose of which is likely to attract lady hummingbirds. It takes a combination of optical effects at different scales acting together to create iridescence in a feather. Iridescence is considered a 'structural' colour because it is produced by optical effects instead of pigments.

Hummingbirds are fascinating. Their wings beat so fast I can hardly see them and these tiny creatures are the only group of birds that can fly backwards, sideways or straight up. With a revved up metabolism, their little hearts beat around 1200 times a minute and they only live about three to five years. Which leads me to the question: what would happen if a hummer drank caffeine? Would it be able to beat its wings fast enough to fly out of our time-space continuum?

As part of Newton's investigations in the early eighteenth century regarding the nature of light, he hypothesized that the iridescent colours found in bird's feathers were due to the presence of thin films. Newton was right about the thin films, however, he didn't discover the actual mechanism. It took another 100 years before interference was put forward as the mechanism behind iridescent colours in birds and many more years until this theory was accepted. Plenty of birds display iridescence in either small ways (like a pigeon's breast) to the scintillating colours of a peacock's display. It's feathers that provide the light-reflecting layers that create iridescence.

Feathers are complex in structure and fill a wide variety of roles. They keep a bird warm, prevent it from getting a sun burn, provide a streamlined shape, and aid in waterproofing. More importantly, tail and wing feathers allow a bird to fly (with the exception being the birds that don't fly, where feathers provide buoyancy, insulation and even formal wear for penguins). Feathers supply colour for camouflage and for the males these colours help with attracting mates. Birds have evolved to use many different techniques to produce the colours in their plumage including pigments, structural colours and a combination of the two. Through pigments they can produce yellows, reds, browns, and blacks. Whites and blues are often formed by selectively scattering incoming light in tiny air pockets within the feather barbs, while greens are produced by combining yellow pigment with air-pocket scattering effects. Iridescence is the most intensely striking and vivid structural colour birds produce.

On a hummingbird, the small-scale external structure of the feather is not flat, but raised in a v-shape as a series of ridges and troughs. This microscopic structure preferentially reflects light towards an observer directly facing the bird. At all the other angles almost no light gets reflected, which makes the feathers appear black.

On an even smaller scale, within the hummingbird's feathers there is a stack of three (usually) thin films. Each film has a thickness equal to approximately half the wavelength of the light it intensifies most (visible light wavelengths range from 380 to 740 nanometers, and a nanometer is one billionth of a meter). The films themselves are constructed from eight to ten layers of an irregular mosaic of thin elliptical discs and little pockets of air. The elliptical disks are about 2.5 microns long by one micron wide – about the size of a yeast cell. Different hummingbird feather colours are produced through a combination of the above structural effects. The elliptical disk thickness tends to decrease as the colours pass from red towards blue while at the same time the size of the air pockets increases. For more complexity, iridescent feathers often also contains pigment which absorbs the background light, allowing the structural colours to be even more vivid.

The iridescence-producing feather structure I've described is a complex phenomenon which is actively being researched. There are even on-going attempts to simulate feather iridescence. If this technology pans out, someday I may be able to wear a coat with reds as vivid as an Anna's Hummingbird's gorget.

thanks to G. Hanke for the photo

Iridescence part I – little shiny fish

I keep an aquarium on my desk – just a little community tank with an ordinary grouping of fish: angelfish, corys, guppies and cardinal tetras. All have fascinating aspect that I enjoy watching but, from across the room the cardinal tetras, always catch my eye. Cardinal tetras are little, peaceful, schooling fish about 3 cm long, with colourless fins and two stripes running the length of their bodies; the top stripe is an iridescent blue and the bottom one is a vivid red. It's the iridescent blue stripe that grabs my attention. Like the deep greens in a rooster's tail or the rainbow colours in an oily puddle, the shimmering blue in my cardinal tetras originates from the optical phenomenon of iridescence.

Iridescence is the result of light striking a thin layer similar in width to the incoming light's wavelength, also referred to as the distance from crest to trough. Because the layer depth is close to the light's wavelength, light is reflected off both the top and bottom of the layer. As light bounces around, it interferes with itself. Interference is when two or more waves, alternately a wave and its reflected self, encounter each other and their amplitudes combine. Colour is simply light of a specific wavelength, so to amplify one colour, the two waves meet 'in phase' (aligned trough to trough and crest to crest). To negate or reduce another colour, the two waves meet 'out of phase' (aligned trough to crest). Because the wave alignment changes based on your viewpoint and/or changes to the layer itself, when you change your angle of view the colours change as well. An observer would see a colour of a certain hue – however if the angle of view to the layer is changed, by moving ones head or moving the layer as examples, the colour can appear different. This explains why patterns within the surface of an oil slick appear to swirl and shift as wind and water move the surface. Sometimes, interference can be the result of many layers of semi-transparent surfaces, where phase shifts may also occur in conjunction with many reflections and interference opportunities, creating multiple layers of iridescence.

If I took a picture of my fish, assuming they would stay still long enough for me to do so, the iridescence would not be reproduced – ordinary pigments and computer screens can't do it. Instead I would need to make a hologram to capture the effect. A hologram also works through tricks with light, this time by interference and diffraction.

Back to my iridescent fish. The pearly blue stripe results from the presence of Guanine crystals in the cardinal tetra's skin. These tiny multi-layered crystals are grown in the shape of a dinner plate within individual, almost dry chambers in the fish's skin. Somehow the fish can control the shape of the growing crystals, because when these crystals are grown in a lab, they form a much more three dimensional shape. The fish's crystals form a layer compatible with the wavelength for the colour blue, giving the fish a shiny blue stripe. So why do fish need this iridescent shininess? For some fish, the iridescence provides camouflage against the shimmering water surface when a predator is looking up from below or as a large school, little fish could produce a display so visually stunning that their predator gets confused. For my cardinal tetras, since they originate from tannin-stained, blackwater rivers in South America, most likely the iridescent strip allows them to find each other in the murky water and stick together in a school.

Someone, not me, could isolate these crystals and use them to add iridescence into lipstick, nail polishes and other cosmetic items (which is done). It takes about a ton of fish to make only 250 g of Guanine crystals. As I only have four cardinal tetras and no need of fancy lipstick or nail polish, I'll continue to enjoy just watching my fish swim.

thanks to G. Hanke for the photo as I didn't have the patience to wait long enough to get one.

The day math nearly killed me – a cautionary tale about checking your calculations

I was once a soldier, what seems like an eon ago now. As a newly trained junior officer I was given my first command, a troop (platoon) of about 25 soldiers. A week after I took command, I was deployed along with my troop for an exercise on the demolition range. I had run demolition ranges in training, but always someone was there to watch me and catch my mistakes. This day I was in charge, I knew what to do and I was naive.

We were cutting metal with C4 explosives – it was a bit more complex than that, but cutting metal was essentially what we were doing. It was early spring, not yet warm enough to want to spend the day dilly-dallying in the sun, so I wanted to get what I had to be done completed without unnecessary delay. As soon as we arrived, I briefed my soldiers on how the day would go and assigned tasks. I put Sergeant 1 in charge of cutting the time fuse, while Sergeant 2 and myself supervised the soldiers laying out the C4 to cut the metal.

Time fuse is a tricky thing, because a roll of time fuse can't be counted on to burn at the same rate as any other role. So, for every roll someone needs to time how long it takes to burn over a known distance, let's say 1 meter. Then that someone needs to figure out how long it takes to walk from where the charge was to be detonated to the safe area. On this day we had a lovely concrete bunker to hide in for safety, about 200 meters from our detonation spot.

It's important to get this right because shrapnel from cutting metal with C4 can travel up to a kilometer.

Sergeant 1 carefully measured the time it took the burn 1 meter of time fuse. He then took out a stopwatch and walked at a brisk but not hurried pace from where we were working on the charges to the bunker, adding about 30 seconds to his result as an extra safety measure. We wanted the explosives to go off shortly after we got into the bunker. In addition to wanting a nice count down for dramatic effect, it is important that we know precisely when the detonation was to occur because a detonation that doesn't occur when it is supposed to is the kind of thing that can wreck a day.

The next step is to figure out how much time fuse is needed to set off the charge at the appropriate time. So this is what we have:

time taken to burn one meter of time fuse = time taken to walk to bunker / length of time fuse

Which can be rearranged to give:

length of time fuse = time taken to walk to bunker / time taken to burn one meter of time fuse

Sergeant 1 figured out how much time fuse was needed, cut and delivered it to where we were setting up the charges. I should have checked his math – math with times can be tricky.

Sergeant 2 and I waited until everyone was safely in the bunker before we lit the time fuse. We walked at a brisk but not hurried pace towards the safety of the bunker. The bunker door was situated so it was facing away from the explosions so we would have to walk around the building to get inside. When we were about 5 m from the bunker door at the edge of the bunker, the explosives detonated.

We ran into the bunker and slammed the door shut. We could have been riddled with little slivers of flying metal, but neither of us were. And those injuries which didn't happen, they would have been fully my responsibility. Sergeant 1 was very apologetic, I think he expected me to punish him profusely – but I didn't. My squadron (company) commander and the squadron sergeant major had shown up just before we detonated, so I had a little chat with them. Since I accepted responsibility and would never ever make that mistake again, I was allowed to continue. From that day on, I always checked the time fuse calculations. If I did them myself, I had someone check my work. I never had a mistimed explosion again – instead I had to deal with a whole whack of new problems.

Tangent Ramblings

I've always been fascinated with scientific ideas that are new to me, it is amazing how complex and diverse the world is. I've chosen to follow a scientific path. Science is a way of thinking and viewing the world that can provide profound insights into how the universe works, but it has some limits. Specifically, science doesn't deal well with the interconnectedness between seemingly unconnected ideas and by its nature it looses the non-linear path taken to gain knowledge in the first place. It is possible that the path taken to new ideas may have impacted what those ideas turned out to be and how long it took for them to be accepted – for example the probabilistic nature of quantum particles was deemed impossible by many notable scientists at the time including Einstein.

My own research is fascinating but, sometimes I want to explore the world with a broader view. I want to consider how we came up with ideas that are now considered as commonplace and obvious. I want to follow the path of how an idea came to be and the seemingly random ideas that diverge from this path.

A decade ago I was working in Europe and my employer gave me the option of flying home or to somewhere else in between. For a week's holiday I chose to go to Florence, kinda randomly as I had no idea what I would be doing when I got there. For that week I walked everywhere, eventually stumbling on the Institute and Museum of the History of Science located somewhat off the main track in a Renaissance era building. I went in. It was a weekday and late fall, so the museum was virtually deserted.

I love the old scientific instruments, how they are crafted out of polished wood and custom made brass fittings – as though someone really cared for the aesthetics as well as the function. I work in a field where our instruments are a combination of high tech electronics and ordinary plumbing supplies that all gets slapped together and tossed into salt water. The craftsmanship and beauty of old instruments captivated my imagination.

That day I strolled from case to case looking at models showing the concentric circles of the old view of the heavens, gruesome diagrams of early human dissections beside the grim looking tools used, then I came to a room of telescopes, ramps and microscopes – the original instruments used by Galileo. I looked at each one in turn until I came to a cabinet against the back wall, on the centre shelf was a human finger. The caption said it was Galileo's, removed when his body was exhumed in 1737 from the unconsecrated ground where he was buried after his death in 1642. His body was then transferred to a mausoleum in a Florentine church.

I'm not particularly squeamish, but seeing that finger startled me. It felt out of place in a room full of crafted instruments and I was instantly curious why it would be there.

I often come across ideas, topics and concepts that are out of place in my research, like a finger in a room full of instruments, but none the less I feel compelled to investigate further. I will collect some of these 'tangents' here and include my thoughts on them. My aim is to broaden my own knowledge of science while adding a splash of history and perhaps a dash of math. I love to sift through ideas and explore a smattering of topics drawing connections between them, especially when at first glance the topics hold nothing in common. I have no intention of providing a precise summary of science and my own views are bound to be flawed.