Synopsis

Why Blue Dominates Red in Bird Feathers

Experiments explain why certain birds, beetles, and photonic glasses, which derive their colors from interference effects, can be blue but not red.

The stunning blue color of birds like the Indigo Bunting comes not from pigments but from interference of light scattering off of randomly arranged, light-scattering structures in the feathers. This so-called “structural coloration” can be mimicked with artificial materials, but neither industry nor nature has managed to produce the color red through this mechanism. A new study explains this long-wavelength exclusion and proposes material designs that could have us all seeing red.

Structural colors are those that arise from microstructures in a material. Light bouncing off these structures interferes in such a way that particular wavelengths (related to the separation between elements of the structure) dominate the scattering. If such structures are ordered like a crystal, the material can be “iridescent”: their color changes depending on the viewing angle. By contrast, when scattering elements are disordered, as in blue bird feathers and some purple beetle scales, the structural coloration is seen from all angles.

Sofia Magkiriadou of Harvard University and her colleagues realized that no reds (or oranges or yellows, for that matter) had ever been produced by angle-independent structural coloration. To explore why, they examined the light scattering from a particular type of “photonic glass,” consisting of randomly packed plastic beads of different sizes. For small beads, the light output was dominated—as expected—by blue wavelengths that corresponded to the average bead-to-bead separation. However, for larger beads, the expected red color was overshadowed by a second peak at shorter wavelengths, which the authors identified as light that enters single beads and reflects off their back surface. This backscattering is typically in the UV but moves to the visible for larger beads. The researchers describe how one could design red, structurally colored materials by suppressing this backscattering with specially designed hollow beads.

This research is published in Physical Review E.

–Michael Schirber