Everyone knows what a rainbow is when they see one in the sky. But not everyone realizes the role that the Sun plays in making rainbows. In many old cultures rainbows were viewed as bridges to the heavens. The first known scientific writing about rainbows was in 1310 by Theodoric of Freiberg.
A rainbow is the result of dispersion; which is the separation of white light into the colors of the spectrum. As sunlight shines through rain, the individual raindrops act as both a prism and the inside of a concave mirror. This means that the sunlight is not only divided into the different colors of the spectrum but that it is also reflected down to the ground.
When you look up and see a rainbow what you are seeing is different than what anyone else can see. Because of the reflection cast back to Earth by the concave mirror effect of the raindrop, no two people will ever see a rainbow in the same way. The dispersed rays of light reach Earth differently. At times rainbows seem to have more or less of the colors which they are made of than at other times. The colors are always present but how you view them is entirely dependent upon how the colors are reflected down to you.
There are two types of rainbows. Primary rainbows are very bright because only one reflection is taking place inside a raindrop. Secondary rainbows are dimmer and are caused by two reflections happening inside a raindrop.
A little known fact by many people is that the colors of a rainbow aren't always in the same order. You may wonder why this is true. The differences in the index of refraction are the cause for the order of the colors. The outer edge of a primary rainbow will be red and the inner edge will be violet. A secondary rainbow will be just the opposite with the outer edge violet and the inner edge red. So the next time you observe a rainbow you can impress everyone by informing them whether they are looking at a primary or a secondary rainbow.
For those of you who may be interested you can prove this is true to your doubting friends by looking at the following table:
|Violet||1.343||40.5 degrees||53.5 degrees|
|Green||1.335||41.8 degrees||51.7 degrees|
|Yellow||1.334||42.0 degrees||51.3 degrees|
|Red||1.332||42.3 degrees||50.6 degrees|
Did you know that it is possible to see a rainbow in another shape than the well known arch? If you are ever in a plane and see a rainbow you may actually have the opportunity to see a "round" rainbow.
Some people believe that it is impossible to see a rainbow in the wintertime. Robert Greenler has proven that it is possible to see rainbows in the wintertime. In fact, the cover of his book "Rainbow, Halos, and Glories", convincingly illustrates this fact by displaying a photograph of a rainbow arching over a wintery, snow covered landscape.
By careful observation there are times where you may spot a green light near the upper rim of the setting or rising Sun. This unusual phenomenon is known as the "green flash". An old Scottish legend says that if you have ever seen a green flash you will always be able to see into your own heart and be able to read the thoughts of others.
A green flash is caused by refraction differences between the wavelengths of light. Longer wavelengths of light are refracted less than the shorter ones. This means that near the horizon there is a difference in the bending of the red and blue wavelengths. The difference can be between 20' and 40'. This difference makes you see two Sun discs that are partially covering each other. The violet-to-green arch is just a little bit higher than the red one. This means the violet and blue rays are disrupted coming through the atmosphere and we can't see them. What we do see is the "green flash".
Green flashes are usually only visible for seconds; however, Commodore Byrd saw one for a much longer time. While on an expedition to Antarctica in the winter of 1934-1935 he observed a green flash for 35 minutes.
Rainbows are a beauty of nature and the Sun's interaction with water creates them. This is just another example of how the Sun is man's best friend.
©Copyright 1998 Elizabeth
Beckett, Holly Bernitt, and Vishwa Chandra.