Blue Feathers – almost coherent scattering

A collection of Mountain Bluebirds at the local museum

This year, Stellar’s Jays have been common in my neighbourhood, it’s the only wild blue bird we get here. But, it has a secret, it’s feathers are pigmented to be black. They look blue because of an optical trick that occurs within the feather’s structure. And Stellar’s jays aren’t the only one using this trick, all the birds out there with blue feathers are doing the same thing.

In the late 1800s, naturalists used the recently discovered concept of Rayleigh scattering to explain why blue feathers were blue. Since tools to examine the nanostructure (structure in the order of a billionth of a meter) of a feather hadn’t been invented yet, naturalists assumed that a feather contained tiny transparent cells full of particles the right size to create Rayleigh scattering. Like the sky, blue light would be more efficiently scattered. As a result, to our eyes these feathers would appear blue.

Because Rayleigh scattering is incoherent, it produces the exact same colour irregardless of the observation direction. Since blue feathers in natural light don’t change colour depending on viewing direction, the assumption that their colour was formed through Rayleigh scattering seemed valid — until someone looked closer.

In the 1930’s, scientists examined a a non-iridescent blue feather under a directional light source. Colour variations were observed as the light source was moved – an iridescent characteristic that called into question the hypothesis of Rayleigh scattering making the feather blue.

By the 1940’s, a cool new gadget became available – the electron microscope. Now the internal nanostructure of blue feathers could be directly examined. On the first look, the internal feather structure appeared to contain randomly spaced objects. This meant scattered light would be incoherent resurrecting the hypothesis of Rayleigh scattering being responsible.

It took until the 1970’s for scientists to finally determined that the nanostructures were, in fact, not fully random. Instead they were a quasi-ordered matrix – not quite the perfect order of iridescence but not the full randomness required for Rayleigh scattering. Under natural light from all directions, like sunlight, these feathers appear to be the same colour from all directions. However when a directional light is shone on blue feathers the colour will change depending on the light direction.

A Blue Jay wing (I don’t have a close up of a Steller’s Jay)

Since the colour of a Steller’s Jay’s feather comes from its internal structure on a tiny scale, a damaged feather would lose its blue colour. The dark pigments in the feather, that act to help show off the blue, would make a damaged feather would look almost black. So if you are lucky enough to find a Steller’s Jay feather, take care of it to keep it blue

Thinking of Blue Skys

A blue sky behind some blue flowers

It’s rainy season on my island in the Pacific. The days are grey and not particularly inviting. We’ve been staying inside a lot, which can drive me a little bit stir crazy. So last weekend, when the sun came out we crammed in some time outdoors. With the mid-winter browns and greens that are typical around here (my island is not tropical), the blue sky was the most vibrant colour around.

Although we perceive blue blanketing the sky, in reality, the sky has no colour. Instead, the hue is created from the interaction between our atmosphere and the incoming sunlight. Our atmosphere is made up a bunch of different stuff —  nitrogen (78%) and oxygen (21%) with bits of dust, water vapour and some inert argon, among other things (some of which we’ve put there).

Water vapour and dust are the physically biggest components of the atmosphere, and are relatively large compared to the wavelengths of light. When light hits the water vapour and dust, it’s reflected in different directions, but the light remains white (an example is the clouds). So why does the sky appear blue?

Over time, all sorts of theories have surfaced to explain the blueness, which started heading the right direction with Goethe’s 1810 explanation: “If the darkness of infinite space is seen through atmospheric vapours illuminated by the daylight, the blue colour appears.” His theory said the sky’s colour comes from something within the atmosphere during the light of day — which is true, but vague.

About the same time a more scientific inquiry was being made into the nature of scattering light. John Tyndall showed in an 1869 lab experiment that the blue hues of the sky could be created when white light was scattered by tiny particles. A few years later in 1871, John William Strutt, aka Lord Rayleigh, was the first to describe the actual mechanism that makes the sky appear blue was a result of light interacting with gas molecules in the atmosphere.

These gas molecules are tiny compared to the wavelengths of light – several thousand times smaller. When light strikes one of these molecules, that molecule absorbs a specific wavelength (or colour) of the light’s energy and later re-emits the same colour in all directions; a process called Rayleigh scattering.

Most of the longer wavelengths of light pass through our atmosphere unaffected, resulting in the full spectrum of sunlight with a higher ratio of blue wavelengths from the scattering. For this extra blue light to make the sky appear a brilliant blue, a dark background is required. Fortunately, beyond our atmosphere is the blackness of outer space, which makes an ideal dark background. The combined effect of the extra blue light and the black of outer space results in a sky that appears blue.

If you shift your gaze towards the horizon, the brilliant blues give way to paler colours and perhaps even white. The light reaching you from near the horizon passes through much more atmosphere, so the scattered blue light is scattered again and again, reducing its intensity. Preferential scattering of blue light by our atmosphere occurs everywhere, not just above us. For example, light reflected from your hand to your eye is affected by this scattering, but the effect is so minuscule we can’t detect it. Over a longer distance, like to a range of distant mountains, there is enough atmosphere to superimpose a blueish haze on our view of the mountains.

Since the creation of a blue sky overhead is entirely depended on the preferred wavelengths the molecules in the atmosphere absorb — in our case, molecules in Earth’s atmosphere absorb energetic light (blues) at a much greater rate than less energetic light (reds). So the blue we see above us is an Earth thing, on another planet, the sky could look dramatically different. Check out the possibilities here.

Electric blue icing

A glacier dripping down the cliff in Scott Inlet

Some time ago I promised I would write about the icebergs and glaciers I saw this fall along the Baffin Island coast and in Scott Inlet.

In every gap between cliff faces in Scott Inlet, tongues of glaciers dripped in slow motion like icing on a cake of grey cliff. These weren’t glaciers that could be accessed by fancy-big-wheeled buses like Columbia Glacier or even easily reached to trek across like these folks did. I wouldn’t want to try to get up to the main part of the glaciers as even without the dripping ice, the cliff-gaps would have required us to use ropes to successfully scramble up.

Glaciers flow under their own weight, a direction that is obviously downhill. I have no idea how long it took, but a few of the glacier tongues had made it to the ocean calving bergy bits into the water. The icebergs in the inlet were tiny compared to the big icebergs moving south along the coast of Baffin Island.

Along the coast, the iceberg that sank the titanic once passed more than a century ago. Even knowing before I arrived that I would be heading into iceberg waters, I was surprised at how many there were. Some looked large enough to dwarf an aircraft carrier (as a tangent: there was a proposal in World War II to make aircraft carriers out of ice). Some that had grounded could easily be mistaken for an island. Many had wave-rounded forms reminiscent of modern sculpture, or ancient weathered architecture.

An iceberg glowing blue

What fascinated me about the glaciers and icebergs was their colour. Even under the grey skies parts glowed electric blue – almost like they were generating their own light deep within. Glaciers and icebergs don’t actually glow, but under the right light it looks that way.

Snow looks white because of all the reflective edges from the layers of snowflakes. Once the snow is compressed into glacier, the edges merge and air is pushed out. However, any ice can look blue in time. Like the reflective snowflake edges, air bubbles scatter all the wavelength of light making young ice look white. Older ice looks bluer because air bubbles and other impurities have been pushed out.

Like water, ice absorbs the longer wavelengths of light as it passes through. That is, the red end of the spectrum is absorbed first, which is why a short distance underwater the seascape is dominated by blues and greens. Ice has the same effect on light, it filters colours as light passes through leaving blues. And it appears to glow because those blues have passed the whole way through – so the ice looks bluest from the inside.

Water on the road?

Desert travel stories take a dramatic turn for the worse when the hero rushes towards what appears to be an oasis. When she arrives, despair sets in as the inviting waters vanish, revealing more hot, dry sand. I’ve never seen a desert mirage, but on hot days, I’ve seen what appears to be shimmering pools of water on the road – only to drive closer to find the road is dry.

Mirages aren’t a hallucination of dehydrated desert travelers, instead they arise from atmospheric optics. Remember how light bends when it passes from one medium to another? The refracted light is bent if the mediums are of different densities – mirages occur when light passes through many layers of air with different densities.

On a hot, sunny day, sunlight heats up the ground. This heat radiates, heating a layer of air right next to the ground. The next layer up also heats up – but not as much. The result is a gradient of heat with hottest air next to the ground and cooler air further away. Since the density of air depends on its temperature, hotter air is less dense than cooler air. So, in our sunny day example, the least dense air is closest to the ground (an unstable situation only persisting as long as the ground is being heated up). Which means the refractiveness of the air is less at the bottom than the top, so the light bends towards the cooler air.

Sunlight entering this temperature gradient at a shallow angle to the horizon is bent slightly differently by the different density layers. At first, it successively bends into shallower angles because each layer was less dense. At some point, the angle becomes so shallow light reflects, turning upwards, but still at a shallow angle. As this light travels back through the now progressively denser layers it’s bent the opposite direction and the angle to the horizon would increase. Eventually, an observer’s eye is reached – the poor hero in the desert or me driving my car.

So, a mirage is simply light refracted and reflected from the sky. Since sky reflections on the ground are typically indicative of water, our brains interpret what we see as a body of water.

I’ve described a static scenario, but in the real world hot, less-dense air rises, heating of the the ground is uneven and turbulence will form – all acting to make the mirage shimmer.

Why is glass transparent?

Sitting at my desk, I can look out onto my backyard through sliding glass doors. So why can I see my backyard at all? That is, why is glass transparent? We take the clearness of glass (and plastics) for granted, but this property is incredibly important. Seeing the birds in my backyard may not be critical, however, seeing oncoming traffic when I’m driving my car is. Allowing light into my home through windows saves the energy required to illuminate my home so I wouldn’t walk into things. Think of the deli case at your local supermarket – the glass allows you to see the goodies inside, but protects them from the other customer’s germs.

I wrote about the history of glass here, however, the fact that glass is clear likely kept us using it for so long. For example, my house would be a lot more secure from break-ins if I replaced all the glass windows with steel plates. Two physical properties play a role in making something transparent, the object itself and its sub-atomic makeup.

Transparency to visible light is common in the stuff around us — For example, air and water. In fact, many gases and liquids are transparent because their structure isn’t rigid, leaving plenty of room for light to pass through. However, solids don’t tend to be transparent because they have a tighter, more orderly structure, making it harder for light to pass through. Glass (and clear plastics) are made by heating their components, mostly silica sand, into liquid form and then allowing it to cool. As a result, glass is rigid like a solid with a random structure like a liquid making it possible for light to pass through.

Light acts both as a wave and a particle. If we consider light as a particle, which is called a photon and contains a certain amount of energy, it can interact with the electrons in the matter around us. When a photon encounters an electron the following may occur:

1.The electron absorbs the photon’s energy and vibrates a little faster – that is, the photon’s energy has been converted to heat.

2.Again the electron absorbs the photon, but this time it stores the energy and re-emits it later, a phenomenon called luminescence. Think of an analog wrist watch (remember the ones with a two arms and a circle of numbers?). Often the numbers were painted with a substance that would absorb light and glow, allowing you to see the time in the dark.

3.The electron can absorb the photon then re-emit it back in the direction it came from. This is reflection and is why you can see your image in a mirror.

4.Finally, the electron may not be able to absorb the photon at all, so the photon just passes by.

These electron/photon interactions can all occur within a single substance, or some combination of them. If only case 4 occurs, that is the electron’s within an object can’t absorb a photon in the visible light spectrum, that object will appear transparent. Glass has this property, which is why it makes great windows.

As a tangent, glass absorbs much of the UV spectrum which is why you can’t get a tan behind glass.

Let me be clear… a bit about glass

I found a photo I took of a rainbow last summer when we drove across Canada. I took it from a moving car (I wasn’t driving) – so it isn’t as fantastic as it could be.

I’ve been thinking about Theodoric of Freiberg’s rainbow experiment (I wrote initially about it here). He used a spherical glass filled with water to approximate a rain drop and a piece of parchment with a pin hole in it. By shining light through the parchment hole and onto the glass sphere, he was able to observe the result of raising and lowering the sphere. From this he explained all the colours of a rainbow. So his glass sphere must have been essentially perfect for this to work, and he wasn’t the only one using these sphere’s for optics experiments. So how did we get so good at making glass? (the extremely short version)

Glass making is an old art, by about 1500 BC the Egyptians were making glass vessels and soon after the Phoenicians mastered the art and began exporting glass goods all over. However, the Romans with glass blowing (likely invented by the Phoenicians), put cheap glass vessels into their citizen’s homes. Romans went on to adapt glassblowing for making glass windows for some of their buildings – not widely done because they lived in a warm environment. Roman windows were made by blowing glass into a bulb shape, then manipulating it into a cylinder shape. The cylinder was split open lengthwise before being re-heated and forced flat. One of the largest windows made of this method was found in Pompeii measuring just over a metre wide.

So, the Romans weren’t hugely into glass windows, but after they were gone, those who lived to the north took up interest in them. The technique used became simplified to blowing glass into spheres and then cutting them while still hot into the shallow bowls of ‘crown-glass’ windows. Additives of different minerals result in brilliant colours for stain glass windows. Since, churches were one of the few places rich enough to afford glass windows and they wanted to tell stories through cut coloured glass put back together, large sheets of uniform glass wasn’t necessary.

Throughout medieval times, rich folks were drinking out of blown glass and keeping the weather at bay with blown glass windows. So by, Theodoric’s time in the fourteenth century, glassblowing had been around a long time. If an artisan can make a nice wine glass, certainly that skill could be put to use for scientific instruments.


As a continuation of my optics theme, I thought I’d take a look at rainbows. I usually see them when I’m driving (which is why I have no photos of them). The half-sunny, half-rainy days rainbows need usually are threatening to soak me, so I do indoor activities instead.

Since antiquity, people have wondered about rainbows. Why did they form? What did rainbows mean? Some believed they were an omen of some sort, as in “should we look at the end for a pot of gold?” On the flip side, reasonable scientific explanations have been around to explain rainbows for quite some time. Theodoric of Freiberg (1250-1310), is one of the first Europeans to have come up with an explanation for why rainbows form based on his experiments (his work was based on that of an earlier Arab scholar). He managed to explain, before a solid theory of refraction was published, the rainbow’s colours, its position, and how it forms from multiple rain drops. Since then, others have refined his explanation.

Rainbows form from refraction and reflection within millions of raindrops. And size does matter for the rain drops, optimum results occur for drops in the range of 0.3 to 1 mm in diameter – this is why rainbows formed on mist are so much more subdued, the raindrops aren’t big enough to generate brilliant colours. Along with the rain, a strong light is needed – usually sunlight, but a bright moon can also form a rainbow (something I’ve never seen but sounds cool).

As sunlight hits a rain-drop, it’s bent slightly (refracted) and the colours spread out. Against the back surface of the rain drop, the light is reflected then it passes out the front surface, again bent slightly. So, to an observer, the resulting light will appear a certain colour based on what angle the drop is viewed at. Violet light emerges from a drop at 40 degrees to the incoming light and red at 42 degrees, with the rest of the spectrum ranging between.

From this same effect occurring in millions of different drops simultaneously an entire spectrum of colours can be seen, remembering that each drop only produces one colour for a stationary observer.

As a tangent, rainbows may be able to form on Saturn’s moon Titan.

Good stripes, bad stripes

Stripes have been viewed in a variety of ways through time. I would have thought that as soon as people invented the loom, stripes would have followed. Stripes must be one of the easiest patterns to make – yet medieval western Europe shunned them.

Stripped clothing was considered at best demeaning and at worst downright diabolical. On the other hand, dots, discs, stars, rings and other simple repeating patterns were good – even viewed as expressing something majestic. This distinction between good and bad patterns was even applied to the animal world; horses were good and zebras were bad. Fortunately, our views about stripes has morphed with time and I can sleep in striped pajamas without worrying about my soul.

Although stripes can’t tell us anything about the wearer’s moral character, they can tell you what something is made of – even from a distance. Here is a rough idea how it is done (yes it’s another optical trick).

Remember Newton’s classic experiment where he shone light through a prism and got a rainbow coloured spectrum? If you look really, really closely at the spectrum you can see hundreds of irregularly spaced, thin, dark stripes, which is exactly what the German scientist, Joseph von Fraunhofer, did in 1814. Today, we know more than 30,000 of these lines exist in the sun’s spectrum – but what are they?

Elements, like oxygen, helium and the others on the periodic table, are fundamental. They can’t be broken down into smaller parts without taking extreme measures like using a super-colliders. If you shine a light (assuming this light gives off a perfectly continuous spectrum) through a gas of an element, then let the light go through a prism the resulting rainbow will have dark stripes in it. These stripes are called absorption lines and are unique to the element. So the stripes from helium will look different that the stripes from nitrogen. This means that, an element can be identified from its stripes alone.

So all those stripes in the spectrum of the sun tell us what the sun is made of – without having to go there.

Universe, 5th edition by William Kaufmann and Roger Freedman, W.H. Freeman and Company, New York, 2000.
The Devil’s Cloth: a history of stripes and striped fabric, by Michel Pastoureau, Columbia University Press, 1991

Buckets of water

I was asked why a bucket full of water looks shallower than an empty one, so I pulled out an old physics book to find the answer. It’s been many years since I’ve taken optics, although recently I’ve developed a new interest for it.

Refraction occurs because the speed of light changes based on the density – something I discussed here. The refractive index is the ratio of the speed of light in a vacuum to the speed in the medium. If we think about water with its refractive index of 1.33, we find that light travels 1.33 times faster in a vacuum than the water. The denser the medium, the greater the difference in speed of light and the bigger the refractive index.

Not only does light slow down, it also bends. When a ray of light hits a surface at an angle (angle of incidence) it gets bent to a new angle (angle of refraction) inside the surface. With a little trigonometry applied to these angles, we find that their ratio is also the refractive index, a trick discovered by Willebrod Snellius (of Snell’s law fame) in 1621 – although an Arab scientist figured this out almost 500 years earlier.

So, what fun can we have with the refractive index? Ever looked into a still pool of water? Due to light rays bending in the water, the pool will look ¾ the depth it actually is. If a post sticks up through the water, it will look oddly disjointed at the surface – appearing to extend at one angle above the water and another below the surface even through the pole is straight.

From another point of view, what does a fish see when it looks up? A fish sees a lot more than expected. By looking up in a cone of 98 degrees, a fish gets a 180 degree view above the water due to refraction. The view above the water would be strange – someone fishing on the shore would look excessively squat, standing at an odd angle and probably distorted due to ripples on the surface. But, the fish would see the fisherman, making it much more difficult to be successful at fishing (spear fishing is even more complex due to refraction). By the way, if you put on your goggles and hopped into the local swimming pool, you would see what the fish sees.