Colour can be generated in a couple of ways. We are very familiar with colours produced by pigments or dies.
Some insects and birds, compact disks, oil slicks and other things however, produce their bright, pure colour through the process of iridescence.
A pigment absorbs light in most parts of the spectrum and reflects a small range of wavelengths to give the impression of the pigmented surface being ‘coloured’.
Clothing, hair, paint, plant tissue and skin colours are all pigment based effects. It is a chemical effect, in fact the colour of something can tell you a lot about its chemical make-up. Hemoglobin makes blood red, carotene, from carrots is yellow-orange, and melanin gives you your tan and hair colour. The effect relies on electrons in the pigment molecules behaving in certain ways to achieve the result of absorbing particular wavelengths of light and reflecting others.
Iridescence is a very different process. It relies on some of the weirder, quantum aspects of light to achieve the effect of making a surface seem to reflect bright light of a very pure colour. It relies on the microscopic, three dimensional structure of the iridescent surface and as a result it is sometimes called structural colour.
Light can be thought of as traveling as a wave. Waves can be thought of as fluctuations of a particular value over time or space. As such, if you can cause a couple of waves to overlap and interact you can get some interesting things to happen.
Wave peaks are ‘high’ energy values and wave troughs are ‘low’ energy values. The wavelength is the distance between the wave peaks. If waves of a particular wavelength can be lined up with each other, the peaks and troughs can be superimposed in different ways to either amplify or cancel the waveform. This is known as interference. Amplification of a wave is constructive interference and cancellation of a wave is destructive interference.
Iridescent surfaces use microscopic, layered, reflective structures to superimpose multiple waves of light over each other as they are reflected. The exact dimensions of the layered reflectors determine which wavelengths of light are constructively or destructively interfered with.
The important thing to realize is that all of the light is reflected by the reflector arrays on an iridescent surface, it’s just that after reflection, on their way back to your eyes, the waves themselves work together to either amplify or cancel each other. Light really is extremely weird stuff.
On the wings of a butterfly like a Ulysses, or on the surfaces of various other creatures with similarly vivid colour displays, are arrays of these layered reflectors which the animals use to generate spectacular iridescence effects. The exact reflector structure varies between species but the theory behind the effect is the same.
Ulysses Butterfly wing scales are covered by arrays of what look like microscopic library shelves. In cross-section these arrays looks like pine trees, each successively lower shelf is longer than the one above it. From above, the distance between the shelves is an exact multiple of the wavelength of the bright blue light you see from that angle. All other colours cancel as their wavelengths are not exact fractions of the array dimensions.
However, as you move to a more oblique viewing angle, the relative spacing of the reflector arrays change for light bouncing off at that angle. The iridescence effect shifts to purple as that wavelength becomes an exact fraction of the reflector array at that angle, and blue joins the other colours in being cancelled.
Most iridescence in animals is at the blue end of the spectrum, with green, blue and purple iridescence being most common. The red end colours have long wavelengths, so the structures needed to play with those colours would need to be much larger in scale, they could pose problems like getting particles trapped in the larger arrays. The arrays would also be more prone to damage through abrasion and impacts.
Most iridescence effects are probably used for the obvious advertising functions that antlers, squawks and Lamborghinis serve in other organisms. “I’M GORGEOUS, AND I’M OVER HERE, BABY!!!”
Giant Clams though, use iridescence for something else entirely.
Recently it was noticed that the black sections of Some butterfly wings are blacker than they have any right to be if pigment was being used to produce the black. No pigment known is able to achieve perfect black and Ulysses Black is very close to perfect black.
Scientists looked into it and discovered that the Ulysses butterfly has developed structural colour devices that achieve an almost complete cancellation of the entire visible spectrum!!! That’s a first, and it has generated quite some excitement in a number of fields where the blackening of surfaces is important. Astronomers want it to reduce light contamination in telescope barrels and the military naturally want to put it to good use in stealth applications so that they can kill more people more effectively.
Leave it to Homo to name a creature a symbol of peace and then use it to develop weapons of war.