Physics Photo of the Week

October 30, 2009

Spider's Rainbow
Tunnel spiders are very common in the late summer.  These spiders spin matted webs on vegetation near the ground.  The spider's lair is at the end of a tunnel woven in the sheet.  These layers of sticky silk presumably serve to catch insects similar to the spoke and spiral spider webs we are familiar with.  These traps apparently use much more silk.

The tunnel spider traps/nests show up very clearly in the morning as they accumulate much dew.  If the Sun is at the correct angle, a rainbow of colors is quite noticeable.  The rainbow effect is caused by the myriads of dew drops that have collected on the web fibers.  The fibers and dew drops are so small that the drops are spherical - the same requirement for a typical rainbow (see PPOW, April 4, 2008 for a rain rainbow and explanation).  The photo at right shows the whole web for the tunnel spider.  The dew droplets on the
silk fibers are not resolvable, but one still sees the rainbow of colors as in the close-up photo above.

When examining these rainbow colors in the dew, if one moves one's eyes slightly to one side, one is no longer at the optimum position to see the spectral colors.  The photo below shows the same tunnel spider's web, but from a slightly different angle.  No colors are seen.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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