Physics Photo of the Week

December 15 2006

Crepuscular Rays - Discussion by Kopano Mmalane

Photo by Donald Collins
Crepuscular rays, also known as sun rays or God's rays,  are rays of sunlight, which stream through gaps in clouds. The rays are parallel columns of sunlit air partially shadowed by cloud regions or mountain peaks. They are usually seen at twilight and hence the name, crepuscular, which means, of or like twilight. 

The picture at left is a beautiful example of the crepuscular rays. The rays seem to diverge but are in fact nearly - parallel.

The reason that the crepuscular rays seem to spread out is because the sun is so far away that our perspective makes it seem like they meet at a point, just like our perspective makes us see railroad tracks meet at a point in the distance.  See the picture at right.

The same phenomenon of shafts of light is seen in the midday photograph at left.  With the sun high in the sky the shafts of light appear to be parallel because they are perpendicular to the viewer's line of sight.

A complimentary phenomenon called "Anti-Crepuscular Rays" was featured recently in PPOW on November 10, 2006.

Next week's Physics Photo of the Week will be published on the last day of classes in the semester: Wednesday December 20, 2006.  Look for more students' work.



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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 dcollins@warren-wilson.edu.


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