The Rainbow

Q: I know a rainbow is caused by the sun, but I don't understand how or why you can never catch up to one.

A: A rainbow is actually a holographic reflection of the sun which has been refracted by billions of microscopic droplets of water. Rainbows are not only beautiful, they are also rare. We see more than a fair share of them in Hawaii because of our mountains and tradewinds.
The trades produce clouds and rain as they rise over over the windward slopes. Some of it blows over the peaks where it the rain falls in the backs of the valleys. Because of the orientation of the islands is not unusual for the afternoon sun to shine into the back of an amphitheater headed valley and illuminate the rain and mist beneath the clouds. That is when the light show begins.
Each tiny drop of water acts like a microscopic, spherical prism which refracts the light to spread out the rainbow of colors which comprise white light. The light reflects off the back inside surface of the drop and exit the front of the spread out in a cone shaped like the surface of a megaphone. Under ideal conditions, the rainbow appears as a ful circle of light with a radius of forty degrees for violet and forty-two degrees for red.
Although each droplet contains the entire rainbow you see only a portion of it reflected from each droplet, like the dew which flashes colors from the grass in the morning sun. To see a rainbow requires there must be multitude of drops spanning the diameter of a circular cross-section of the cone whose axis lies directly in line with the sun and the observer.
If you could suspend a single water droplet in space and move around it, you would see the entire rainbow. When you see the whole rainbow at one time, it is because the combined light from a multitude of droplets is reflected back to your eyes at the same place.
From different locations you will see light from different droplets so no two observers see exactly the same rainbow. This is why the rainbow follows you as you move, and the reason for the legend of the pot of gold at the end.
You can no more touch a rainbow than you can touch a reflection in a mirror because they are not really there.
Richard Brill is assistant professor of science at Honolulu Community College . Send questions to Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email to rickb@hcc.hawaii.edu. Visit our web site at http://www.hcc.hawaii.edu/~rickb/SciDoc.html


The Rainbow 1996 Richard C. Brill