The wings of butterflies and moths consist of a colorless translucent membrane covered by a layer of scales (the name of the order is Lepidoptera, meaning “scaly wings”). Each scale is a flattened outgrowth of a single cell and is about 100 µm long and 50 µm wide. The scales overlap like roof tiles and completely cover the membrane, appearing as dust to the naked eye.
The iridescence is caused by multiple slit interference. Sunlight contains a full range of light wavelengths. “Interference” occurs when light hitting the wing interacts with light reflected off the wing.
Light is a wave. If the crests and the troughs of the waves are aligned, or in phase, they will cause constructive interference, and iridescence is the result. One light wave hits the first groove, and a second light wave travels half of a wavelength to another groove, and is then reflected back in phase with the first.
If the crest of one wave meets the trough of another wave (out of phase), they will cancel each other out, as destructive interference occurs.
Why does the Morpho appear blue?
Blue light has a wavelength range from 400 to 480 nm. The slits in the scales of the Morpho are 200 nm apart. Because the distance between slits corresponds to half of the wavelength of blue light, this is the wavelength that undergoes constructive interference. The slits are attached to a base of melanin, a material that absorbs light, further strengthening the blue image.