Confined blue iridescence by a diffracting microstructure: an optical investigation of the Cynandra opis butterfly

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Abstract
When illuminated and viewed along certain well-defined directions, segments on the wings of the butterfly Cynandra opis shows a striking violet-blue to blue-green. We quantify the spectral and the directional properties of these areas of the wings of the insect. Electron microscopy shows that wing scales from these iridescent regions of the wings contain two gratinglike microstructures crossed at right angles. Application of the diffraction theory, as formulated by the Stratton–Silver–Chu integral, to the microstructure can explain all the important features observed experimentally.