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. T  he
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.
