Iridescence

The idea for an anthropological exhibit about iridescence occurred 25 years ago. The curator, Peter Sutton, has finally achieved it. The exhibit at the South Australian Museum commenced on 12 December 2014 and will conclude on 15 March 2015.

The exhibit brings together content from several collections, but with the common theme of iridescence – “structural colour that produces iridescence, whether that be from refracted light (as with crystals), diffraction gratings (as with feathers or butterfly wings), thin film interference (with coated or oily surfaces), multiple layer interference (as with nacre in shells), or 3-d arrays of spheres (as with opals).”

Refracted light is the bending of light as it moves from air into different media (like liquid drops or prisms). Diffraction is the bending of light at it encounters a tiny obstacle or opening  which creates a distinctive pattern in troughs and crests, which reflect different shades of the same colour, such that it looks like the colour is flashing or moving. Thin film interference occurs when light rays touch and interact with a surface from different angles (oil slicks and soap bubbles are iridescent due to thin film interference where light is reflecting and interacting with the top of the film as well as the bottom). Multi-layered interference is light refracted and reflected through multiple layers of an object (such as pearl shells). 

After the introduction of the science of light, the exhibit is divided into sections, such as: (1) human cultures (humanity and the iridescent), (2) biological sciences (brilliant animals), and (3) earth sciences (treasures of the earth).

The word “iridescence” comes from iris, the Greek word for rainbow. It has been likened to “music for the eyes” because of the many notes and tones of colour. Iridescence creates colour by splitting and reflecting light from different structures – which is why it is often called structural colour.

The exhibit includes peacock feathers, bird of paradise feathers, abalone (paua) shells, a myriad of jewel beetles and beetle wings, scales from a children’s python, chalcopyrite (a copper iron sulphide often called “fool’s gold”), ammolite (a gem from fossilised shelled molluscs), rainbow kelp shells, pearl oyster shells, and soap bubbles.

Animals often use iridescence to communicate. Iridescent patterns often confuse predators or attract mates. Rapid flashes of colour can be achieved often with the smallest of movements. Sometimes iridescence is used to communicate social status. For example, male monkeys that are more dominant in a social group have deeper blue iridescent skin. 

Less is known how plants use iridescence. It may help plants to signal and attract pollinators, such as bees. Leaf iridescence is produced by multi-layered interference and although it has been found in more than 60 species, the underlying structures have only been described in about 7 species. Leaf iridescence might help camouflage plants from herbivores and help plants to regulate light intake and photosynthesis. 

Iridescence can also be found in the sky and in the earth. Clouds reflect and refract sunlight, and rainbows transform the sky after rain, but can also be seen in the steam of kitchen kettles or within ice crystals. 

MARTINA NICOLLS is an international aid and development consultant, and the author of:- The Shortness of Life: A Mongolian Lament (2015), Liberia’s Deadest Ends (2012), Bardot’s Comet (2011), Kashmir on a Knife-Edge (2010) and The Sudan Curse (2009).