The Deception of Blue

by Izzy Sambles

In nature you can find a spectrum of colours which are predominantly caused by biological pigments known as biochromes. These include chlorophyll, the green pigment in plants, and carotene, a pigment which produces the colours red or orange and can be found in carrots and other plants and animals. Specifically, for creatures colour can be a huge advantage in terms of survival and passing on of characteristics, as it can help you mate, as well as camouflage you in your natural habitat.

However, not all colours that we see are produced by pigments.

There is another way in which colours can be produced in nature and this is known as structural colour. Structural colour occurs at a nanoscale scale; harnessing the physics of light and causing particular waves to be absorbed and only emitting certain colour waves.

An example of this in the environment is the Morpho butterfly’s inner wings, which appear an iridescent blue. Although their wings are physically brown, because they have tiny scales on the surface of their wings, which come in layers of overlapping rows, their wings look blue. Each scale has ridges on its surface, which in turn have microscopic cross-ribs attatched to them, these diffract the light (the light waves spread out as they pass through the wings’ structure), similar to a prism, and cause constructive interference (when certain colour wave lengths are intensified and reflected) to occur. The constructive interference occurs in the spaces between the ridges whilst at the same time other colour wave lengths meet and cancel each other out (destructive interference).

It is the shape of the structures and the distance between the cross ribs in the scales that determines the specific colour which is reflected. In the Morpho butterfly’s case, the blue light wave, which has a wavelength of 450-495 nm is reinforced (constructive interference) which is why its inner wings appear blue. In fact, the microstructures on the Morpho butterfly’s wings reflect up to 75% of the incident blue light.

Structural colour isn’t just found in the wings of Morpho butterflies. Other examples include peacock feathers and blue eyes, both of which do not naturally contain any blue biochromes but the way which light is reflected off their surfaces depicts an alternate colour to their physical colour.

Many of our everday objects are based on plants and animals in nature. For example, Velcro’s invention was based on the tiny hooks of the cockle-burs. In the future, maybe scientists will develop transparent clothes which you can change the colour of by sending electronic pulses to nanoscales on the surface of the material or for military camouflage to hide planes or weapons.