Iridescence of a Morpho Butterfly Wing
A surface that displays colors due to thin-film interference is said to be iridescent because the tints of the colors change as you change your view of the surface. The iridescence of the top surface of a Morpho butterfly wing is due to thin-film interference of light reflected by thin terraces of transparent cuticle-like material on thewing. These terraces are arranged like wide, flat branches on a tree-like structure that extends perpendicular to the wing.0 Suppose you look directly down on these terraces as white light shines directly down on the wing. Then the light reflected back up to you from the terraces under- . goes fully constructive interference in the blue-green region of the visible spectrum. Light in the yellow and red regions, at the opposite end of the spectrum. is weaker because it undergoes only intermediate interference. Thus. the top surface of the wing looks blue-green to you. If you intercept light that reflects from the wing in some other direction. the light has traveled along a slanted path through the terraces. Then the wavelength at which there is fully constructive interference is somewhat different roll that for light reflected directly upward. Thus, if the wing moves in your view so that the angle at which you view it changes. the color at which the wing is brightest changes somewhat, producing the iridescence of the wing.