Physics Photo of the Week
Rainbow in the Valley - Photo by Miranda Hipple
Four weeks ago (March 4, 2008) we had a hard rainstorm in the late
afternoon at Warren Wilson College. Whe the rain cleared we were
treated to a miraculous rainbow that Miranda Hipple captured on her
cell phone camera. Notice that the rainbow "ends" just beyond the
WWC farm in the Swannanoa Valley. We can also see the faint
bow to the right of the bright main rainbow.
The physics of the rainbow consists of sunlight entering a myriad of
raindrops. The light is refracted as it enters any raindrop, and
dispersed into component colors both as it enters the raindrop and as
it leaves the raindrop. See the drawing below.
Only the raindrops which lie at an angle of 42 degrees ralative to the
sunlight angle emit the "bow light" toward the observer. The
angle of 42 degrees is governed by the fact that raindrops are
spherical and have a refractive index of about 1.33. That means
that the ratio of the speed of light in air is 1.33 times faster than
it is in water. The
fainter secondary rainbow is caused by the light making two internal
reflections in a raindrop. Because with each partial internal
reflection, a portion of the light is lost. This makes the
secondary rainbow much fainter than the primary bow as Miranda's
Many of us have heard the myth that a "pot of gold" exists at the end
a rainbow, but no one ever has found this mysterious pot of gold.
The end of a rainbow, quite evident in Miranda's picture, proves to be
quite elusive. In fact the distance of a rainbow from an observer
is impossible to define. Any raindrops that are illuminated by
the Sun and are located at the correct 42 degree angle from the Sun
contribute light to the rainbow arch. The distance of the
raindrops from the observer does not matter. That is why the
"end" of the rainbow is so elusive. Water droplets from a garden
hose create rainbows right in front of our eyes. Distant rain
showers also create a rainbow. One may say that the "location" of
the rainbow itself is located at infinity.
Photo of the
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 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.
here to see the Physics Photo
the Week Archive.
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