Every time it rains, I find myself looking at the sky in search of a rainbow. It's amazing to see something so wonderful and beautiful and to realize that it's merely an optical illusion! Science explains the beauty and magic of rainbows in a simple way. There are two components that are necessary for a rainbow to occur, sunlight and water.
II. Refraction
The first step in understanding a rainbow is refraction, the "bending" of light. Light bends because it travels at different speeds when it is in different mediums. So when it goes from one medium to another, the change in speeds causes the light to change direction. To help understand this, the cart example from How Rainbows Work might help. If you are pushing a shopping cart down the street, the road being one medium, and you go at a constant speed, if the right side of the shopping cart hits grass, that side will slow down while the other side on the pavement will still go at the original speed. The cart travels at different speeds for different mediums (pavement to grass) and when it does, the cart turns to the right.
Image 1- cart example
This is the same way with light as in goes through a glass prism. One side of the light waves hits the prism first and is slowed down, this makes the light bend at the collision of the air and the glass prism. Some of the light does reflect off of the glass prism, but most of it goes through it. Then, as the light goes out of the prism, one side of the light speeds up quicker than the other causing the light to bend again.
III. Dispersion
The light not only goes through refraction, but dispersion as well when it goes through the glass prism. Before we talk about dispersion, it is important to know that different colors have different frequencies. When different colors go through matter, they travel at different speeds. Dispersion happens because slow-moving colors when they hit the glass prism bend sharply, where as fast-moving colors don't bend as sharply. This creates the dispersion of colors, the colors are separated by their speed. If the light bends them twice like in a prism, it is easier to see the dispersion of the colors. Rain water disperses the colors the same way as the glass prism.
Image 2- light refraction and dispersion
that takes place with a glass prism.
IV. Rainbows in Nature
Rainbows in nature go through a similar process as the glass prism. When many rain drops are together and the sun hits them a low angle, it creates a
rainbow (red, orange, yellow, green, blue, indigo, and purple). When the sunlight from the air passes into a rain drop, just like in the glass prism, the
different colors slow to different speeds (determined by their frequency). These different colors then bend at different degrees (violet, the closest to the
ground bends at the sharpest degree). Some of the light is passed through the rain drop back into the air and the rest of the light is reflected backwards.
So every raindrop disperses this white light (from the sun) into the colors of the rainbow. There are different color bands because each raindrop from
the group of raindrops that were reflecting the suns light, reflect one color. This means that each color that we see is coming from a raindrop and every
raindrop is reflecting one color.
Image 3- The process a raindrop goes through when
it is hit by sunlight to create a rainbow
The different heights of the raindrops determine what color will be reflected and that will be seen by the observer. The angle that the raindrop is reflected down to your eye determines the color that you will see.
V. Cool facts about rainbows
- Next time you are looking at a rainbow, look where the sun is. Have you ever noticed that the sun is in the opposite direction of the rainbow (behind you if you are looking at a rainbow) and that the raindrops are falling in the direction of the rainbow.
- Did you know that two people can never look at the same rainbow? (excluding a picture) Actually, each eye sees a different rainbow than the other. This is due to the fact that a rainbow is a, "distribution of colors with reference to a definite point...," Humphreys.
- How far away a rainbow is can be determined by the distance of the raindrops, the closest and the furthest raindrop.
VII. Journal Article Review: Metamagnetics with rainbow colors
This journal article talks about the mathematics involved in finding the width of the bands of the rainbow and the atoms that make up a rainbow. It talks about
the wave lengths and how it corresponds to the width of the bands of the rainbow. It shows graphs on the amount of light that is reflected compared to the
amount of light that just passes through the rain drops in comparison to the wave length. Lastly, it talks about the importance of magnetics and geometry in
understanding the details of the formation of a rainbow.
VIII. References
About Rainbows. (n.d.). NCAR Education & Outreach Homepage. Retrieved December 12, 2010, from http://eo.ucar.edu/rainbows/
Cai, W., Chettiar, U., Yuan, H., Silva, V. d., Kildishev, A., Drachev, V., et al. (2007). Metamagnetics with rainbow colors. Birck Nanotechnology Center, 11.
Harris, T. (n.d.). HowStuffWorks "How Rainbows Work". Howstuffworks "Science". Retrieved December 12, 2010, from http://science.howstuffworks.com/nature/climate-weather/storms/rainbow1.htm
Kansas Tornado Chasers WHY PAGE. (n.d.). New Page 1. Retrieved December 12, 2010, from http://www.tornadochaser.com/to
XI. Images
Image 1, 2, 3:
http://science.howstuffworks.com/nature/climate-weather/storms/rainbow1.htm
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