Spiderweb colors violate the rainbow rule

Spiderweb (overview). Backlight [1]

According to the laws of physics, rainbows caused by sunlight always appear in the section of sky directly opposite the sun [2]. Here I show that spiderwebs violate this rule by pictures taken of spiderwebs with backlight resulting in rainbow-like colors as well.

Kruisspin / Garden Cross Spider / Araneus diadematus 

Detail. length ~ 1 cm / 0,4 inches

Straight lines demarcate a sector of the spiderweb circle
that lights up selectively. The rest is faintly visible and colorless.
The lines meet at the center of the web,
and seem to point towards the sun.

detail of previous picture. Click to enlarge.

So the part of the spider web circle that lights up seems to be a segment which points to the sun. Furthermore, as shown in the previous blog, the threads of the web consist of many small segments with different colors. Contrary to the previous blog, these pictures are taken towards the sun. The fact that colors are seen at all contradict the physical rules for the formation of a rainbow by the sun. A possible explanation could be that the spider web threads are not made up of water droplets but of proteins...   

Notes

1. The main light source, the sun, is behind the spider, facing the camera and photographer. Note: in Dutch this is called 'een tegenlicht opname'.
2. Thanks Rolie Barth and Matt Young for pointing this out. See wikipedia article Rainbow.  

Previous blog

• Accidental discovery of the ultra finestructure of the web of the Garden Cross Spider [+additional pictures].  29 June 2026