Spider Web Light Rings
Jonathan Lansey (site) took this contre-jour image of evening sun soaked grass at Leicester, England
Image©Jonathan Lansey, shown with permission .


The scene teems with bright glints set in sun centred circles. Spider webs made them. The grass is full of webs of busy spiders. They usually go unnoticed by our eyes and unfelt by our heavy feet but here they are revealed.

The circles are called �'light rings'�.  Similar rings show up when a streetlight or the sun is viewed through wet tree branches. Scratches on glass or bowls also form them.

In nearly all cases the glinting objects are more or less randomly oriented even though the rings appear highly organised.   We see rings because only a few especially oriented surfaces can glint light towards our eye. Even then, only a part of a spider silk strand or scratch or twig might do so.

Only strands of particularly oriented spider silk reflect** sunlight to the eye. The glints are from points on the web silk that are perpendicular (tangential) to radial lines extending back to the sun.

At right is a computer simulation. Web strands of random length and orientation show as dim green lines.  Sections of silk �light� where they fulfil the condition for specular reflection towards the eye. The light rings form naturally out of randomness and helped by the wish of our eye/brain to seek order.

**Yet all is not so simple! Nearby glints resolve into structured coloured bands. The spider silk is not only reflecting light like a mirrored cylinder. It is also diffracting sunlight in a complex and poorly understood manner. Close-ups of webs show these complex diffraction colours.

Download the light ring simulator to experiment with different conditions. For it to run you need to have previously installed HaloSim, IRIS or TiltingSun to have the necessary files on your PC.