What Causes Iridescence?
Iridescence is one of the most visually striking optical effects found in nature. This optical phenomenon causes colours on a surface to change depending on the angle from which they're viewed and the direction of light.
Materials that commonly exhibit iridescence include soap bubbles and oil on water, where colours form in thin surface films, as well as butterfly wings, peacock feathers, and certain crystals and minerals, where colour changes are produced by fine internal or surface structures.
How Iridescence Works
In most naturally occurring objects, iridescence can be seen because light reflects and bends as it passes through and reflects off many super-thin layers either on or within a material. Some colours become more visible, while others fade or disappear, which is why they change depending on the viewing angle and the angle of the light source.
Natural light is made up of red, orange, yellow, green, blue, indigo, and violet. When these colours combine, the human eye perceives them as white. Daylight is an example of this type of white light.
When light encounters layered or structured surfaces, such as those found in butterfly wings and some crystals and minerals, the different colours of light behave slightly differently. This interaction produces the effect known as iridescence.
This photo shows intense iridescence caused by extremely thin iron-oxide surface coatings, commonly referred to as turgite. These coatings form on iron-rich rocks during oxidation and vary slightly in thickness across the surface.
As light reflects from these layers, it interferes with itself, producing exceptionally vivid, layered rainbow colours that shift with viewing angle.
Iridescence, Refraction, and Dispersion
Iridescence is closely related to other optical phenomena, including refraction and dispersion.
Refraction occurs when light changes speed as it passes from one substance to another, such as from air into water, glass, or a gemstone. When light changes speed, it also changes direction, or bends.
Dispersion happens when refraction causes light to separate into its individual colours. This effect can be clearly seen when white light passes through a prism, where each colour bends by a different amount depending on its wavelength.
Rainbows form through a combination of refraction, internal reflection, and dispersion. Each raindrop acts as a tiny prism, refracting and dispersing sunlight and sending different colours back toward the viewer at slightly different angles.
Everyday Examples of Iridescence
In crystals and minerals, iridescence is often caused by very thin internal layers, fractures, or inclusions. These fine structures interfere with light as it passes through or reflects off them, producing flashes or bands of colour.
Clear quartz, mother of pearl, and labradorite all exhibit iridescent effects. The colours are not within the material itself, but are produced by the way light interacts with its internal structure.
The colours depend on the thickness of each layer and occur when light is either weakened or amplified as it passes through transparent or semi-transparent layers of varying thickness. It happens because light waves reflected from extremely thin layers interfere with one another.
Iridescence can also be seen when a thin film of oil spreads across water, such as on wet roads or a petrol spill.
Light reflects off multiple layers of oil and water, each with a slightly different thickness. As the light passes through these layers, a spectrum of colours appears. The colours we see depend on the film's thickness and the angle of light.
The Word Iridescence
The word iridescence comes from the Greek word iridos, meaning 'rainbow,' which is also the origin of the word iris. In Greek mythology, Iris was the goddess of the rainbow and a messenger of the gods.
The suffix -escence means 'a process or state of being,' describing the effect rather than a material or substance.
The scientific term goniochromism is also used to describe iridescence. It comes from the Greek gonia, meaning 'angle' and chroma, meaning 'colour', referring to colour changes that depend on viewing angle.
A Simple Explanation
Iridescence is not a colour in itself. It's a visual effect created by light, structure, and angle, all working together. As either the observer or the light source moves, the colours appear to shift and change, creating a curious and fascinating optical effect.
Article Photos
The butterflies are on display in the Harvard Museum of Natural History, Massachusetts, USA. Photo: Stone Mania.
Turgite: Courtesy of James St. John. Double rainbow: Courtesy of Stan Celestian.
The final photo shows iridescent colours created by reflected light on a polished granite kitchen worktop.
Pop-up photos
Soap bubble: Courtesy of Mike Ward. Peacock feather: Courtesy of James St. John. Labradorite: Stone Mania
