Atmospheric
Optics
Thistledown Diffraction
Imaged by James Aikman
A regular diffraction grating
At left, equally spaced scratches (ruled lines on glass or metal) scatter light in an organised way.
Each scratch creates an outgoing wave.
The waves overlap constructively in certain directions. The directions are colour (wavelength) dependent and so the grating generates a set of spectra.
Add a random element to the scratch spacing and there might still be colours but they will shine in pseudo random directions. Waves from small groups of scratches will create waves reinforced in a particular direction. Another group will send them at a different angle. We get colour bands.
The camera looks close at the fluffy seed head of a thistle designed to carry far in the wind.
The deliberately unfocussed strands show the fascinating semi-random banded
colours displayed also by spider webs, dandelion seeds, moth silk and scratched bicycle wheels.
Irregularities, protuberances or refractive index changes along the fibres are diffracting light. They act as a very poorly made diffraction grating to scatter light. The outgoing light waves combine constructively in some directions to create the bands.
That is all very well but we do not know for many fibres - including spider silk - just what are the knots of material scattering the light.
