The wonder of blue: the rarity and complexity behind the colour.
Writer: Sermila Ispartaligil
Editor: Gracie Enticknap
Artist: Lucie Gourmet
Blue is everywhere, but is it really? The colour blue has a long history. Starting with azurite in the graves of Catalhoyuk (Turkey) in the sixth century, it extends to the genetically engineered chrysanthemums developed in 2017. However, the colour blue is quite rare in nature, and blue pigmentation is seldom seen in animals.
What appears as blue to the human eye is most often made possible by the ways in which animals and plants “perform tricks of the light to appear blue”, as Professor Andy Lowe describes it. This is explained by the difference between pigments and structural colour. Pigments are molecules that create colour by selectively absorbing and reflecting specific wavelengths of electromagnetic radiation. Structural colours, on the other hand, are the result of the shape and structure of a material instead of its chemical properties.
The appearance of the colour blue in nature is a wonderful example of the difference between pigments and structural colours. Structural colours can be classified according to the way that they scatter light: incoherent or coherent scattering. Incoherent light scatter occurs when individual light-scattering objects are randomly separated by a larger average distance than the wavelength. Conversely, coherent scattering of light is when the light-scattering elements are dispersed in an ordered way which prevents the phases of scattered light waves from being random. The light waves that are out of phase cancel each other, and waves that are in phase enhance each other. Some of the common examples of incoherent light scatter are blue sky, blue smoke, blue ice, and blue snow, while coherent scattering manifests as soap bubbles, iridescent oil slick on pavement, and the bright colours of insects and bird feathers.
The blue morpho butterfly is an example of the phenomenon of structural colours. It gets its colour from its wing scales that are shaped in ridges which bend light so that only blue wavelengths are reflected. Another explanation is that the nanostructures of their scales cancel all wavelengths except for blue ones, by the unique way in which they reflect light. Measurements from a paper by the Royal Society show that some morpho butterflies’ wing microstructures reflect up to 75% of the incident blue light. Lowe also suggests that the feathers of blue birds get their colour by their microscopic beads which are ordered in a manner that lets every wavelength of light be cancelled out except for blue. However, there is one exception to blue pigmentation in animals. The obrina olivewing butterfly is often cited as the only known animal to produce a true blue pigment.
When it comes to plants, they produce the colour blue by mixing their palette of pigments. Anthocyanins are prominent pigments that appear red at acidic pH and deepen into blue and purple hues with increasing alkalinity. These changes, together with reflected light, help create such flowers as plumbago, cornflowers, bluebells, and dayflowers. As Beverley Glover, a botanist at Cambridge explained, “Flowers are doing crazy chemistry to generate that blue.”
On the other hand, the reason why blue leaves only exist on the floor of tropical rainforests relates to the physics of light. The apparent colours result from the wavelengths of light that are reflected, not absorbed, by the pigments. Green chlorophyll doesn’t absorb, but reflects green light. Given that blue light has the highest energy in the visible spectrum, it is very rarely reflected by plants. The pigments of plants include chlorophylls, which give the green colour to leaves; carotenoids which produce orange (carrots), red (tomatoes), and yellow (maize); betalains that create the red colour of beetroot; and anthocyanins.
So why do these animals and plants have such complex structures to obtain blue in all these challenging ways? The answers are reproduction and survival. The various colours of plants can attract pollinators, while animals might use it by either attracting mates or warning predators such as the poison dart frog.
Thus, even though blue is highly prominent in our lives, it is not as prominent as we may think in nature. It is not as simply created or frequently found as it might seem. What appears as a simple blue pigment to us might be an ingenious play of light instead. Therefore, blue is not everywhere, at least not in the most apparent and simple sense of the word. Wherever it is, it is there to make us wonder.