Posted in Education, Spectroscopy, UV-VIS-NIR

What makes a rainbow?

All of us love a rainbow, and a double rainbow is even more exciting. This is pchem* in action!

350px-Double-Rainbow
A double rainbow photographed in Karlsruhe on July 22, 2011. Leonardo Weiss http://commons.wikimedia.org/wiki/File:Double-Rainbow.jpg

You may have noticed that rainbows only appear with the sun at your back. Why?

Rainbows are angle-dependent. The light coming from behind you hits water drops of a particular size and at a particular angle so that they are diffracted (bent) inside the drop. This can occur in a clockwise or counter clockwise manner. The clockwise path through the drop to your eye and the counter-clockwise path occur at slightly different angles. This creates the two rainbows.

The different wavelengths of light from red to blue also travel at slightly different angles (just like through a prism), and this creates the spread of colors in each rainbow.

Have you ever seen a triple or higher-numbered set of rainbows?

Some will brag and say they have. But this is extremely improbable. If their eyes were sensitive enough to see this higher-order diffraction, then they would be blinded by the bright sunlight needed to produce the rainbow in the first place. The probability of light traveling multiple circles within the water drops to create second- (and higher-) order diffraction effects is very slim compared to the single pass rainbows that we are all familiar with.

Post links to your favorite rainbow images in the comment section.

Subscribe to this blog for more Pchem* topics.

*Pchem (Physical Chemistry) is the study of the physical properties of the universe.

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Author:

Dr. Darren L. Williams (pchem4u) is an Associate Professor of Physical Chemistry at Sam Houston State University. When he is not blending solvents, he is tinkering with contact angle measurements, FTIR microspectroscopy, etc. http://www.shsu.edu/~chm_dlw/ . He welcomes your comments and questions by phone (936-294-1529) and by email (williams “at” shsu.edu).

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