Abstract
Using Jamin-Lebedeff interference microscopy, we measured the wavelength dependence of the refractive index of butterfly wing scales and bird feathers. The refractive index values of the glass scales of the butterfly Graphium sarpedon are, at wavelengths 400, 500 and 600 nm, 1.572, 1.552 and 1.541, and those of the feather barbules of the white goose Anas anas domestica are 1.569, 1.556 and 1.548, respectively. The dispersion spectra of the chitin in the butterfly scales and the keratin in the bird barbules are well described by the Cauchy equation n(λ) = A + B/λ(2), with A = 1.517 and B = 8.80·10(3) nm(2) for the butterfly chitin and A = 1.532 and B = 5.89·10(3) nm(2) for the bird keratin.
Highlights
Structural coloration is a widespread, well-recognized phenomenon in the animal kingdom [1,2,3,4,5]
The dispersion spectra of the chitin in the butterfly scales and the keratin in the bird barbules are well described by the Cauchy equation n(λ) = A + B/λ2, with A = 1.517 and B = 8.80·103 nm2 for the butterfly chitin and A = 1.532 and B = 5.89·103 nm2 for the bird keratin
They approximate plane-parallel plates [16], making them ideal for interferometric investigation
Summary
Structural coloration is a widespread, well-recognized phenomenon in the animal kingdom [1,2,3,4,5]. Many butterfly species feature beautifully colored wings due to a cover of nanostructured scales [6,7]. For a detailed understanding of the reflection properties of the scales and feathers their three-dimensional structure as well as the refractive index of the composing material has to be known. For butterfly wing scales the material in question is the biopolymer chitin and for bird feathers this is keratin [11,12]. Mason [13] extensively investigated the structural colors of insects by using immersion fluids. He estimated the refractive index of Morpho butterfly scales at about 1.55. Vukusic et al [14] found for Morpho scales a complex refractive index, with real part 1.56 ± 0.01 and imaginary part 0.06 ± 0.01, via measurements of the scale reflectance and transmittance in various immersion fluids
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