Dandelion Diffraction

The seed head hairs of a dandelion gleam and display multicoloured bands like those of out-of-focus spider silk. Images by Daniela Rapava of KHaP M. Hella Observatory, Slovakia.


©Daniela Rapava, shown with permission
The common dandelion, thought of mostly as a garden nuisance, is the most marvellous device. Here are components of the seed head or pappus. They open into the familiar puff-ball. Each hair carries a seed to be borne by winds and breezes. The individual hairs are constructed from many small hollow pipes giving a strong but lightweight structure. The hairs have semi-regularly spaced protuberances that maybe help them stick where they land or perhaps have some other function. One thing they will do they diffract light to form the colour bands.

Scanning electron microscope image by London Natural History Museum - science photo library.
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A regular diffraction grating. Each equally spaced scratch, opening or protuberance scatters light into an outgoing wave. The waves overlap and interfere. In-phase waves persist, out-of-phase waves destruct. Because the structure and therefore wave spacing is regular there are directions where the overlaps are in-phase and give brightness for a particular colour.
               
Multi-coloured bands are best seen when the camera is deliberately defocused. They are seen on spider webs and sometimes even animal hair. Quasi-regular structures of some kind must form them by diffracting the incident sunlight. The diffracting structures of the dandelion seed heads are likely to be protuberances on the individual seed hairs.