Physics Photo of the Week

August 26, 2011

Crepuscular Rays - Dawn of a New Day
This magnificent sunrise on August 17, 2011 is enhanced by the crepuscular rays, a fan-like pattern emanating from the Sun's position.  The Sun normally lights the sky, but the intermittent clouds cast shadows into the sky.  The fan-like appearance of the shadow bands is simply an illusion.  Because the Sun is a great distance from the Earth, the shadow bands are actually parallel.  The apparent convergence of the paralel rays is analogous to the apparent convergence of parallel railroad tracks in a perspective view (see a good student discussion on PPOW for Dec. 15, 2006).  In this case these rays are not only parallel, but they are also horizontal because the Sun is actually on the horizon.  The true horizon is a bit difficult to determine in the mountains, however

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 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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