Assistant Professor of Chemistry Babak Sanii on Butterfly Wings
A wondrous thing about the wings of the blue morpho butterfly is that their blue color is not a chemical pigment—it comes from nanoscale shapes on the wings’ surface. The shapes, which are so small that we need electron microscopes to even see them, reflect light waves that interfere with each other so that only blue light reflects off of the wing. How and why does humanity know this trivial fact?
The mystery of how likely began with the observation that the wings are not simply blue—they are shimmering blue. The shimmering is visible up to half a kilometer away (roughly one-third of a mile), which suggests a biological utility in communication. It was also determined that the chemical composition of the wings—they are largely made of chitin, a long molecule that vaguely resembles a string of sugars—only deepened the mystery, as this chemical is typically brown, not blue.
As far back as the 15th century, Leonardo da Vinci also noted the shimmering quality of certain reflections, and he inadvertently helped solve the origins of the butterfly’s wings. In a notebook he kept about painting, he observed that light was additive. Building on this, other scientists, such as Thomas Young in the 18th century, discovered that light waves could add together while interfering. Together, this gave us a physics framework of diffraction, which predicted that different colors would reflect off a pattern of structures at different angles.
Scientists who lacked the modern tools necessary to see the structures on the wings hypothesized that the wings somehow structured the chitin to cause the blue color by diffraction, but they could not explain why the wings were blue when viewed at a broad range of angles. Others believed it was due to some reaction in the butterfly’s blood. During the 1940s, nascent electron microscopes were first used to examine biological material, and almost immediately afterward the butterfly wing was also imaged. The structural color hypothesis was supported by observations of nanometer-size treelike features on the wings. Later, computational diffraction models confirmed that the complex treelike shape of these features reflected blue light even at a broad range of angles.
Of all the things scientists across disciplines might have spent our time and money on, focusing on understanding what makes a butterfly’s wings blue might be seen as trivial. I cannot believe it was because we expected to reap the rewards of the wings’ technology, as nature is typically far too complex in structure to reproduce on industrial scales. Our motivation was the nagging of not knowing why and the humbling wonder of being surprised. We learned what the wings were made of, and we were surprised that this did not explain the color. We learned about diffraction from da Vinci and Young and were surprised that these simple ideas were manifested with complexity and beauty in these shimmering blue wings.
This sense of surprise is why learning about science can be so wondrous, if you let it. Further, if you had an electron microscope and a butterfly wing, wouldn’t you try to see for yourself? I would, and did, with glee, even though I already knew that the structure would be there. The motivations of curiosity and surprise are so powerful that we can feel wonder even when we already understand the phenomena. This is why laboratory teaching of science is also powerful. And if in the laboratory one discovers some small new thing—that the wings are made of brown chitin, that light adds and diffracts, that shimmering wings are biologically useful, that the same structures repel water away from the butterfly’s head, etc.—well, then, one is constructively adding to the growing compendium of wondrous knowledge.
Babak Sanii is an assistant professor of chemistry at the W.M. Keck Science Center. His research interests include developing materials that self-heal and creating low-cost and 3-D-printed scientific equipment, such as a fluorescence microscope/imaging ellipsometer, using off-the-shelf and 3-D-printed components.
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