Physics Photo of the Week

May 7, 2010

Crepuscular Rays
Just this Wednesday, May 5, 2010, we had a sunset on a mostly clear day with a few scattered clouds.  The clouds cast their shadows onto the air beyond.  The shadows and light beams from the Sun are clearly visible as scattered light from the air molecules and dust in the air.  There was not much dust nor haze - the air was about as clear as possible!

These rays are quite common.  They are called "crepuscular rays" because they occur most frequently near sunrise and sunset - the transition between daylight and night.  "Crepuscular animals" like deer are active at twilight as opposed to nocturnal and diurnal animals.

Crepuscular rays appear to radiate outward from the position of the Sun.  However, the divergence is just an optical illusion.  The rays and shadows of the clouds are all parallel because the Sun is very far away.  The appearance of the divergence (or convergence towards the Sun) is analogous to the perspective convergence of parallel railroad tracks.  See the PPOW for December 15, 2006 writen by Kopano Mmalane during her first year at WWC.  (Kopano is graduating next week.  Congratulations!)

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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