Science Of Butterfly Colours

As winter leaves, spring arrives with blossoming flowers and plenty of butterflies dancing on them. Seeing their colourful wings we wonder how these colours were created by nature.

We all know about the presence of scales on the wings of a butterfly. These scales arranged in an intricate pattern give rise to bright colours by a combination of various phenomenon of light (absorption, reflection, interference, diffraction and refraction). Scales on butterfly wings are made of chitin, a semitransparent polymer of glucose derivative, assembled on a pigment layer (eg. melanin) which gives the wing its base colour.

Scales of Polyommatus daphnis (Lycaenidae)

reference: Z. Vértesy et. al. 2004

Absorption and Reflection: Pigments of various colours are present below the chitin layer. These pigments absorb some colours from incident light and reflect remaining colours. A combination of reflected colours forms the base colour of butterfly wing.

Interference and Diffraction: Scales on butterfly wing, act like a grating, cause dispersion of light (A grating is a surface with many parallel grooves that splits light into its constituent colours). The constituent colours of the incident light then undergo interference and diffraction. Interference is the superposition of two or more waves that results in a new wave pattern. Interference of colours can be constructive or destructive in nature. Constructive interference raises the intensity of colour where as destructive interference lowers the intensity of colour. Diffraction is the bending of waves around small obstacles and the spreading of the waves past small openings.

Refraction: As mentioned above the chitin forms a semitransparent grating (also referred as photonic crystals) which is also capable of causing total internal reflection. Certain constituent colours of white light undergo total internal reflection in these photonic crystals. The remaining constituent colours that do not undergo total internal reflection gives rise to brightest colours of butterfly as they emerge from the wing. Same phenomenon is also responsible for brilliance of a diamond.

All the colours of butterfly wings are not only due to the above-mentioned phenomenon but also due their combined effect. Colours emerging due to refraction may further undergo interference and get brighter. Some butterflies have two layers of chitin, which may further give rise to intricate combination of colours. Next time when we see a butterfly, just ponder how amazing is nature. The beauty of butterfly is because of a chitin layer that has no prominent colour of its own but gives rise to so many colours due to its translucent nature.