Moth Silk Diffraction

Peter Rosen imaged this geometer moth tent in a forest near Stockholm, Sweden. "Nature seems to love iridescence as we can find it everywhere.. ..I used my EoS5D MkII with a Canon 100mm f/2.8 macro lens almost wide open giving a very short depth of field. But instead of getting blurred, as expected when out of focus, the threads display sharp lines."

All images ©Peter Rosen, shown with permission

Diffraction colours and patterns occur when light waves scattered by separate parts of an object combine and interfere.   Directions where outgoing overlapping waves are in phase are strengthened while those where wave crests are out of phase are weakened.   Light and dark patterns result and, as the effects are wavelength dependent, the patterns are coloured.

Some diffraction, that from coronae or glories is from individual particles or droplets (although one university meteorology department that should know better claims otherwise!).    Other diffraction is from periodic structures, butterfly wings, DVDs  and fly screens.

A regular diffraction grating

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Spider & Moth Silk

Spider silk is best known for producing diffraction colour bands. Periodic structures on spider webs (but not moth tents) give diffraction colours although there is much that is speculative or not understood.   Silks are complex and varied materials.

Sticky spider capture threads (right) have quasi regular glue globules containing bunched up thread.    The thread unreels when an insect impacts the web, confers elasticity and prevents breakage.    Some spider web out of focus colours are diffraction by these globules.

But non-sticky threads and presumably those of geometer moths can show the complex colour bands pictured here.    Their origin is less easy to explain.  

The non-sticky threads of spiders are known to have rough surfaces, presumably to allow the spider a firmer footing.   Diffraction by these surface irregularities is possible.   There can be an internal structure of pleated protein fibrils.   They partially unfold when stressed and make the thread more elastic.   They too could produce colours by diffraction from their quasi-periodic structure.






Outgoing waves interfere to give coloured light in particular directions