I arrived at today’s topic by a strange combination of thought processes. Delving into the wonderful world of jellyfish got me thinking about the often under appreciated invertebrates around us. Meanwhile, I’ve been reading a biography of Max Planck, and his work on resolving the particle/wave debate in light. What do you get when you combine invertebrates with optics? Butterfly wings, of course!
How I wish I could say I’d taken that photo. Anyway, the two kinds of color are reflected in that photo. You heard that right. The two kinds of color. Normally, when we think of color, we think of pigment color. A given pigment absorbs most of the visible light wavelengths it encounters. A small segment of the visible light spectrum is not absorbed, and the human eye perceives the corresponding color. This kind of color is an intrinsic property. For example, if you take a lump of red clay and grind it, you will get red powder. Add water and you will get red mud. Fold, spindle, mutilate - the stuff will stay red. Now, it may change color if you apply sufficient heat to change the chemical properties of the clay, but the color remains independent of any physical structure.
The other kind of color is structural color. Rather than what wavelengths are not absorbed, structural color comes from the diffraction, reflection, scattering or interference of incoming light radiation. These effects are achieved by nanoscale structures on the surface of the object in question. If you take an object with structural color, say, a peacock feather, and grind it, you will not get iridescent dust. Destroying the physical structure destroys the color.
Butterfly wings, as mentioned before, contain both kinds of color. For example, the wing scales of the blue Morpho butterfly are covered with complex, microscopic structures that interfere with incoming light in such a way as to transmit the characteristic blue iridescence of the genus. Check these things out.
These structures are tiny. That picture was taken with an electron microscope - note the micron scale of the image. In Morpho butterflies, the structures are in turn underlain by a blue pigment that enhances the wing color. It is hypothesized that the color combination evolved to allow members of the same species to better recognize one another. Cabbage white butterflies exhibit a similar pigment/structure combination - here, the patterns are different between males and females of the same species, and are assumed to aid in potential mate recognition.
Butterfly wings represent an intersection of the biological and the optical, and have helped to inspire a branch of science called biophotonics. A recent innovation is an arrangement of carbon nanotubes layered onto Morpho butterfly scales. At even very low levels of incoming infrared radiation, the nanotubes are able to detect interference brought about microstructures on the scales (the same structures responsible for iridescence). This makes for an extremely sensitive thermal imaging system, capable of detecting small, short-lived temperature changes - a subject of great interest to developers of medical scanners, night-vision sensors and portable thermal imagers. Other areas of research in biophotonics include the use of single electrons to track neural impulses within the brain, a combination of carbon nanotubes and a sophisticated imaging system to track the movement of a cell or drug through the body, and a drug that binds only to cancer cells, and fluoresces under certain imaging wavelengths.
I’m not going to say that biophotonics will cure all of our problems, or even cure most of them. But, like many other emerging fields that combine elements of existing disciplines, biophotonics is inspired by the wide world of nature, where biology, physics and engineering are in no way separated.
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