Physics Photo of the Week

October 29, 2010

The rain on 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 drop near 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 of the 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 possible display.

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: