Bike Wheel Diffraction ~Jir� Srba in Czech Republic noticed unusual coloured stripes radiating from the sunlit wheel rim of his bike. 
©Jir� Srba, shown with permission.


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Spider Web Diffraction imaged by Ágnes Őri in Hungary. The web silk was deliberately defocused to better show the colours. The coloured bands are likely produced by bulges of viscous fluid that confer extreme elasticity by coiling and uncoiling the silk and, by being sticky, trap insects. ©Ágnes Őri , shown with permission.

Parallel machining marks and scratches made by the brake pads run around the wheel rim.   They form a pseudo diffraction grating that scatters sunlight.  

A perfect grating (below)  would produce ordered spectral bands each passing from red through green to blue and violet.    These bike rim bands are disordered and of mixed hues.    The scratch spacing is non uniform and probably forms several groups with different separations.   In effect there is a multiplicity of gratings and their scattering overlaps.

The colour bands have echoes of those seen in dry spider webs. Unknown scatterers in the silk structure are probably acting like the wheel scratches.

Diffraction by a grating of equally spaced scratches (or grooves or wires) illuminated by parallel light. Scattered cylindrical waves radiate out from each scratch. Some distance from the grating the individual outgoing waves overlap. In certain directions the wave crests are in phase and they eventually combine to an outgoing plane wavefront.   Elsewhere the waves destructively interfere. The outgoing wave direction depends on wavelength and thus with incident white light the grating produces a spectrum.

The scattering is different from that producing a corona or glory. In those the complete interference pattern is produced by waves radiating from just one drop. The light from millions of randomly positioned drops give a corona by simple addition rather than any interference. Grating diffraction is interference between waves scattered by a macro scale structured array of individual scatterers..