Flag "Corona"
An image by Glenn E Johnson. Circles of pastel colours, like those of a corona, adorn the flag. The colours were produced by the flag, there was no corona in the sky. Some tiny iridescent clouds at lower right echo the flag's colours. How do fabrics do this?
Fine filament diffraction
Yarn is usually composed of twisted strands of much finer material. Some of these finer filaments can stick out from the surface rather like hairs. It is likely that light shining through the gaps in the fabric is scattered by these filaments. Their alignment is more or less random and a corona would indeed be formed. We need in this instance filaments 14 micron in diameter which is plausible. By comparison, human hair is 20 – 200 μm diameter. Next time you see a corona in a flag or curtain, examine it under a microscope!
Diffraction pattern from a wire mesh window screen. Alan Clarke pictured Venus shining through the screen.
A fabric diffraction grating?
Light shines through the array of holes that might act as a diffraction grating. There is an immediate problem. The diffraction pattern would be a rectangular arrangement of coloured patches rather than rings. We can also compute the thread spacing that would produce the pattern. Unfortunately a fabric of some 150 threads per mm is needed – finer than anything we have. Arachne might have woven it, but we know her fate.
Yarn diffraction
The flag colours are definitely corona like. Perhaps there is diffraction from the threads of the fabric that may be pseudo randomly oriented as the flag flaps? Imagine diffraction from the white elements at left.
Calculations quickly destroy this hypothesis. Measurements on the uncropped image together with the camera EXIF data give the diameter of the first red ring as 8.9°. An IRIS Mie scattering calculation says this comes from objects only 14 micron (0.014mm) across, objects far smaller than the main threads of the cloth.
A corona from the fine filaments of a knitted glove. An image by Monika Landy-Gyebnar.
