These bubbles reflect trees,
But the eye also sees
A shimmering iridescence.
Colors from a reflected essence;
Bouncing from the sides
Of the bubbles’ thin hides.

Structural colors

This section is about colors that do not come from pigments, but are the result of light interacting with physical  structures.  Very often, such colors look metallic or glisten. They are the iridescent colors.

The different colors that make up the spectrum of light have different wavelengths.  The wavelengths range from about half to one thousandth of a millimeter.  If light encounters structures, especially layers, with this kind of size, light with wavelengths matching the dimensions in the structure are more likely to be reflected or strengthened. 

Reflections from two nearby surfaces interact with each other and create different colors – these are interference colors. In physics, this is called thin film interference. Soap bubbles demonstrate that colors can be seen in something that has no color of  its own.  The ‘skin’ of the bubbles has an outer and inner surface. Each surface can reflect light.  The thickness between the layers varies. Especially when it is about the same thickness as the wavelength of light, colors will be reflected differently. As a result, some colors are stronger, some weaker – forming a pattern of colors. They swirl as the bubble thins.   The same kinds of colors are also formed when oil spreads on water, and thins out to about the same thickness as the wavelength of light.

Layers in feathers and scales
Of glossy starlings and swallowtails
Cause iridescence too.
By this, bird and butterfly make their blue. 

Butterflies, as a kind of insect, have a hard external skeleton.  Also, feathers of birds such as this African superb glossy starling (Lamprotornis superbus), are made from a tough protein. In both cases, the hard material can be shaped to form layers and ridges that are about the same dimensions of light waves.  Depending on its wavelength (= color), some light will be be reflected or refracted. The presence of some dark pigments will help to create a black background. 

If you look at a feather that is blue or green (blue with yellow), you will almostly certainly find that the blue/green color disappears when the light comes from behind.  That shows that the blue /green is not caused by pigments, but is due to structural colors.  Like this …

More about blues in animals here

If you have any reflections, comments or examples, please let us know below.

Nature Reader colors web Iridescence

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