Physics Photo of the Week
Wednesday, October 27 late in the day created this magnificent
rainbow. Looking carefully, you can see a faint outer rainbow
called the secondary rainbow.
Rainbows are one of the most fascinating meteorological phenomena -
mostly due to the brilliant color. In order to form a rainbow,
bright sunlight as well as rain is needed. The clouds were
clearing letting in the sunshine, and heavy rain was falling towards
the southeast. The
sunlight is refracted and internally reflected by the tens of millions
of raindrops in the line of sight.
In the physics lab we had studied what happens with a
single drop of water. The photo at right
shows a round-bottom flask completely filled with water to simulate a
large raindrop about 10 cm in diameter. The flask is supported
with a lab clamp and
illuminated by a projector lamp. The projector light -
originaing from behind the camera - enters the front of the
raindrop. The total front surface of the raindrop is illuminated
by the lamp. A
small reflection of the projector is visible as the white dot on the
right center of the surface. The light enters the
raindrop, some is internally reflected off the back surface, and exits
the left edge of the drop. Because the light enters and leaves
the surface of water at an angle it is dispersed into the separate
colors. This drop is situated relative to the camera and light source
such that the ray of light leaving the inside of the drop is
tinted red. If the drop were moved further to the left, the light
exiting the drop would appear blue.
A detailed plot of the light path through the spherical raindrop is
shown in Physics Photo
Week on April 4, 2008, which was the last Rainbow featured
on PPOW. If anyone has any photos of rainbows, especially
brilliant ones, feel free to send them to me at the address below for
Photo of the
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 firstname.lastname@example.org.
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.
the Physics Photo
the Week Archive.
Observers are invited to submit
digital photos to: