Iridescence in Butterfly Wings
The same principle behind soap bubbles applies to butterfly wings. Their wings, however, amplify the effects of iridescence because they have many more layers for the light to pass through and thus many more opportunities for the light waves to reflect and magnify one another.
As small as they are, butterfly wings are covered by thousands of microscopic scales, split into two to three layers -- thus their Greek order name, Lepidoptera, meaning scaled wings. In turn, each scale has multiple layers separated by air. Rather than having just the constructive interference from the top and bottom layer that you have in a bubble, the many, equally spaced layers of butterfly wings create multiple instances of constructive interference.
When light hits the different layers of the butterfly wing, it is reflected numerous times, and the combination of all these reflections causes the very intense colors that you see in many species. Some butterfly displays even extend into the ultraviolet spectrum, which is visible to butterflies but not to humans. This ability to detect ultraviolet light guides monarch butterflies on their annual migration from North America to Mexico.
The combination of a butterfly's structural and pigmented color can create interesting effects. For example, if you saw a butterfly with yellow pigment underneath a structure that creates a blue iridescent color, you might see a green shade, made by the merging of the two colors. Or depending on your viewpoint, you might see blue, yellow, green or a combination of the three. Your view would change as the butterfly moves its wings and the light enters at different angles.
Whether they serve as camouflage or communication, the brilliant, complex wings of painted ladies, red-spotted purples and the thousands of other butterfly species owe their beauty to iridescence and structural color.
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