An illusion of color
The brilliant, metallic blue of the Morpho butterfly, a common sight in the rainforests of Panama, is not what it seems. The wings contain no blue pigment. The color is "structural," an effect created by the microscopic architecture of the wing scales interacting with light. Each wing is covered in thousands of individual scales, which are themselves covered by tiny, overlapping ridges. These ridges, made of chitin, have a cross-section that resembles a Christmas tree, with multiple layers of horizontal branches called lamellae.
The structure is a naturally occurring photonic crystal. The lamellae are incredibly small, with their thickness and the air gap between them measuring in the range of 100 to 200 nanometers. This precise spacing is perfectly tuned to interfere with light waves. When sunlight hits the wing, the layered structure causes light waves to reflect off multiple surfaces. Blue light, with a wavelength around 450 nanometers, reflects in a way that its waves align and reinforce each other, a phenomenon called constructive interference. Other wavelengths of light are cancelled out through destructive interference. The brown pigment on the back of the wing absorbs any light that isn't reflected, making the blue appear even more intense.