Butterflies are one of the most colourful creatures. But then not all butterflies have pigments.
Some of these colours are attributed to "photonic crystals". If you see a red colour it means, red colour is being reflected while the other colours may either go through or get absorbed. In the picture, at certain regions no colour is reflected.
As explained in the earlier post we can obtain a low refractive index materials by making voids in a solid. But this is true only when the spacing of these voids is random. Instead if the voids form a particular pattern then we find that the mixture shows wavelength dependent optical properties. Such unique materials containing a mixture in a definite pattern are called photonic crystals.
More fascinating property being, sometimes these materials allow all colours to pass through except a specific colour also know as the photonic bandgap. This colour is thus reflected as it cant pass through.
How does this help the butterfly? But having well spaced voids, the butterfly has stronger yet lighter wings.
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