What do a soap bubble, an oil sheen on a mud puddle, and the eye of a peacock’s tail feather have in common? The answer is iridescence.
The magic of iridescence causes a hummingbird’s throat to be nearly black one moment and then, with a slight turn of the head, the gorget (throat) explodes in red or green or any one of the colors of the rainbow, depending upon the species.
Iridescence gives us the sheen on a starling, the pinkish-purple neck patch on a mourning dove and much of the beauty of a wood duck. These brilliant colors are produced without the benefit of pigments. Grind up an iridescent feather and the dazzling colors disappear.
The magic that produces the striking colors is rather complicated, but let me try to explain. It’s easier to understand if we first consider the film of oil on a mud puddle. The slick shows many different colors from purple and green to orange and red. The oil film is only a few molecules thick, varying ever so slightly in thickness from place to place on the puddle.
Some light reflects off the top surface of the oil. More light penetrates the oil film and reflects off the bottom where it rests on the water. It’s how the light waves match up when the two reflected light beams meet up again that produces iridescence — or not.
If the crests and troughs of the light waves don’t match up just so, the light looks normal or may even cancel each other out. However, if they match up just right, the light is reinforced, producing brilliant color.
Exactly what color of light is reinforced depends on the thickness of the film. A thicker film reinforces light from the red end of the rainbow, where the wave lengths are longer. A thinner film reinforces light from the blue end of the rainbow, where wave lengths are shorter.
The colors of a soap bubble are produced the same way as the oil film. Light reflects off both the top and bottom of the soap film.
A similar thing happens in the feathers on the throat of a rufous hummingbird. Buried in the feathers are millions of extremely tiny bubbles, all of one very precise size and laid down in layers. Light reflects off the top and bottom of the layers of bubbles much the same way it reflects off the top and bottom of an oil slick or soap film producing the brilliant red color. The more layers of bubbles within the feather, the brighter the iridescence.
Complex colors, such as bronze, are created by a combination of two or more colors. Different layers of bubbles in a feather can be made of bubbles of different size, each layer producing a different color.
Now, examine a peacock tail feather. The “eye” is composed of several rings, each a different iridescent color. The pattern is laid down carefully in the growing feather through the precise placement of millions of extraordinarily tiny bubbles of precise size in layers in just the right way.
And is there anything like this in the hair of mammals? Not even close.
— Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.