Hummingbirds and peacocks, butterflies and beetles, pearls and opals, soap bubbles and oil slicks after a rainstorm all have something in common. Their unique colour. Or is it colour…?
By Alan Samons
Who hasn’t been mesmerised by the iridescent shimmer of a peacock’s tail. Or the cerulean blue of a butterfly’s wing. Jewels too – especially the types (not all) that fall into the category named ‘phenomenal’ gems – some pearls, opals, rainbow moonstone and labradorite, to name a few… and there are very few in this illustrious category.
But what would you say if I tell you those wondrous colours aren’t always ‘real’. They are perceived as such by the human eye, but are, mostly, a product of light refraction called ‘interference’.
About a year ago I was doing some retail therapy and my heart stopped a beat as I spotted a pair of shoes in the window at Monsieur Pricé (not my preferred clothing shop!). They were reflective purple and depending on the light, changed colour to dull bottle green. Of course, I bought them. How could I not? I’d just finished making a piece of jewellery using Thai jewel beetle elytra (the hard casings that protect the delicate wings of a beetle). And no, I didn’t pull the wings of bugs, in case you might be wondering, they are shed naturally as the insect grows and moults. These beetles had fascinated me for years, as they can be mostly a metallic green, but can also have a dominant blue, gold or red colour. I use the word ‘colour’ quite loosely, since what we perceive as colour is actually reflected light from small prism-like scales, similar to those on a butterfly wing, or a peacock’s tail. Even our humble hadeda has a touch of purple on its wing and the beautiful blue starlings all over Johannesburg also have their shimmering plumage because of refractivity.
I’ll start by focusing on peacock feathers since that’s probably what most of my readers are best acquainted with. In 1634, Sir Theodore de Mayerne, physician to Charles I, observed that the ‘eyes’ on the wings of the peacock butterfly “shine curiously like stars, and do cast about them sparks of the colour of the Rainbow; by these marks is it so known that it would be needless to describe the rest of the body though painted with a variety of colours.”
In general, the diversity of bird feathers’ colours can be explained by just two factors: pigments, and simple structures in the feathers that interfere with incident light. Pigment particles are embedded into the newly grown feathers during the moulting season. They absorb light of certain wavelengths, or disperse the reflected light, and so contribute to the colour of the plumage. I will concentrate on structural colours caused by interference, which can be seen in certain butterflies and moths, and most spectacularly in peacocks.
Each feather consists of thousands of flat branches and when light shines on the feather, we see thousands of glimmering-coloured spots, each caused by minuscule bowl-shaped indentations. Stronger magnification reveals microscopic lamellae (thin plate-like layers) at the bottom of the indentations. As with butterfly wings, the regular pattern of the lamellae leads to interference phenomena and iridescent colours. The feathers of pheasants, birds of paradise, and hummingbirds create colour using the same mechanism.
Although we often think of beetles as garden pests or garden protectors, many cultures use parts of dazzling, colourful beetles as ornamentation for ceremonial costumes and headdresses, as well as in prized jewellery. Just as structural colour is responsible for the beautiful shimmering blue of neotropical Morpho butterflies, it is also responsible for the glistening iridescent colours of many tropical beetles.
The epicuticle, or outermost surface, of iridescent beetles is made of many stacks of slanting, plate-like layers, which are oriented in different directions. These layers bend, and then reflect the incoming light in the same way as the ridges of iridescent butterfly and moth scales. Similarly, they produce structural colours by interference in the same way as butterfly wings. A layer of pigment below the refractive plates of beetles and the ridges of iridescent butterfly scales enhances the effect of the iridescence. In some species, the epicuticle acts as a reflection diffraction grating to cause iridescence. The exact mechanism of the structural colour for many species is an open topic of research.
Why are many tropical beetles iridescent? Unlike birds, most insects do not use showy colour to attract mates; instead, they primarily rely on chemical attraction. It might seem that the colourful appearance of these beetles would advertise their presence to predators; in fact, these creatures are surprisingly well camouflaged.
It has further been found that when viewed in the ultraviolet spectrum – which some birds and insects are able to see in – another shimmering world opens up entirely.
And on to one of my favourite topics – gemstones. Opal, which also happens to be my birth stone, is formed from silica-bearing waters and can be found inside any type of rock. Throughout the world, silica gel precipitates at low temperatures to form layers or nodules of opal in fissures, veins, and cavities of volcanic and sedimentary rocks. Opal is an amorphous form of silica but contains 3% to 21% water within its mineral structure. Opals of gemstone quality usually contain 6% to 10% water.
There are three main types of opal, but only precious opal is identified by the defining “play of colour,” or the way in which colours change within a particular stone as it is rotated and tilted. The play of colour seen in opals is attributed to diffraction. Under suitable conditions, water percolates through the earth and silicates encountered in the soil dissolve into this water to form a silicate-rich solution. When it enters a cavity, the water deposits the silicates as tiny spheres. The layers of precipitated silica spheres form a jelly-like water mass, sometimes producing a diffraction grating when the spheres are even in size and well ordered. The diffraction grating arrangement creates a play of rainbow sparkling light from within the stone.
The play of colour is due entirely to the uniformity of tiny spheres, each in the order of a tenth of a micron in diameter. If the spheres are random in shape and arrangement, common opal is formed and hence no play of colour will be present. If they are uniform in size and shape, they will diffract light, and the play of colour is evident. The colours caused by the regularly packed spheres making up the internal structure in an opal depend on the size of the spheres and the voids between them. If you move the stone, light hits the spheres from different angles, and you perceive a change in colour.
Humans also utilise interference effects of which holograms are quite common. Holograms also rely on interference colouring, and you might be surprised to learn that surrealist Salvador Dali was among the first to incorporate holograms into fine art. The ability of holograms to play with light and create a transparent yet realistic three-dimensional world is both inviting and intriguing. Holograms allow us to reproduce the three-dimensional aspects of an object. Instead of capturing an image directly onto film as a photograph does, a hologram records an interference pattern.
These are but a few examples of this fascinating phenomenon and now that you know what to look for, you’ll be able to spot these shimmering optical effects quite regularly.
Source: Gay Pages Edition 3 of 4, 2023
