A traditional rainbow is all different colors of the spectrum spread out and then diverted into the eye by small drops of water. Each raindrop contributes only one color to the rainbow that you see! Where is the sun when you see a rainbow? It was probably behind you while the rain was probably somewhere in front of you. Because the “sun is behind you and the rain is in front of you the sunlight must have been bouncing off the raindrops and reflecting into your eyes.” This is confusing because sunlight is primarily white light and that is what is reflecting off the raindrops, how do we see all of the colors of the rainbow? I found out that “these colors are present because the sunlight is not only reflecting off of the raindrops, but it is also refracting and dispersing in the raindrops.”
Rene Descartes was the first person to clearly discuss the formation of a rainbow by raindrops. In his studies he simplified the studies on rainbows by “reducing it to one rain droplet and how it interacts with the light falling upon it.”
He writes:"Considering that this bow appears not only in the sky, but also in the air near us, whenever there are drops of water illuminated by the sun, as we can see in certain fountains, I readily decided that it arose only from the way in which the rays of light act on these drops and pass from them to our eyes. Further, knowing that the drops are round, as has been formerly proved, and seeing that whether they are larger or smaller, the appearance of the bow is not changed in any way, I had the idea of making a very large one, so that I could examine it better.
Descartes describes how he held up a large sphere in the sunlight and looked at the sunlight reflected in it. He wrote "I found that if the sunlight came, for example, from the part of the sky which is marked AFZ
and my eye was at the point E, when I put the globe in position BCD, its part D appeared all red, and much more brilliant than the rest of it; and that whether I approached it or receded from it, or put it on my right or my left, or even turned it round about my head, provided that the line DE always made an angle of about forty-two degrees with the line EM, which we are to think of as drawn from the center of the sun to the eye, the part D appeared always similarly red; but that as soon as I made this angle DEM even a little larger, the red color disappeared; and if I made the angle a little smaller, the color did not disappear all at once, but divided itself first as if into two parts, less brilliant, and in which I could see yellow, blue, and other colors ... When I examined more particularly, in the globe BCD, what it was which made the part D appear red, I found that it was the rays of the sun which, coming from A to B, bend on entering the water at the point B, and to pass to C, where they are reflected to D, and bending there again as they pass out of the water, proceed to the point.”
This quote shows how the shape of the rainbow is explained. You can simplify this analysis by considering the path of monochromatic light through one raindrop. If you imagine how light is refracted through the raindrop, reflected inside the raindrop, curved, mirror-like surface of the raindrop, and last but not least how it was refracted as it emerges from the drop. We can apply the results for a single raindrop to a whole collection of raindrops in the sky. By applying these results we can visualize the shape of the bow.
What makes the colors in the rainbow? A traditional rainbow is made up of seven colors. ROY G. BIV (red, orange, yellow, green, blue, indigo, and violet). But despite that these seven colors are the ones visual to our eyes, a rainbow is actually a continuum of colors from red to violet and beyond these are colors that we can see. A rainbows color comes from two basic facts:
· “Sunlight is made up of the whole range of colors that they eye can detect. The range of sunlight colors, when combined, looks white to the eye. This property of sunlight was first demonstrated by Sir Isaac Newton in 1666.”
· Light of different colors is refracted by different amounts when is passes from one medium (air, for example) into another (water or glass, for example).
Two scientists, Descartes and Willebrord Snell, made a discovery. This discovery was how a ray of light is bent, this is also known as refraction, as the waves travel through different densities or materials. When light paths are traced through a raindrop, for example red and blue light, a person would find that the angles of deviation would differ. The blue light is refracted more than the red light.
The Picture below shows a faint secondary rainbow above then a bright primary rainbow. There are also several pastel-shaded rainbows inside the primary rainbow. While the additional rainbows can not be explained by geometric optics, they have their own term which is called “supernumerary.” But we do have explanations for the primary rainbow and the secondary rainbow. The primary rainbow comes from a “single internal reflection of refracted light inside a raindrop.” While the secondary rainbow results from “a double internal reflection (About Rainbows)”
In order to see a rainbow we need sunshine and falling rain. Rainbows are very rare. Rainbows are much rarer than we think. Halos are seen more frequently. The best times to see rainbows are in the late afternoon and the early morning because in order for us to see rainbows the sun cannot be too high in the sky. The lower the sun is in the sky the higher the bow is. A rainbow is not just a set of colored rings, the sky inside of the rainbow is bright as well because raindrops direct the light there as well (Bianco).
Works Cited
Bianco, Carl. "how stuff works." How Vision Works 02/21/2008
Freudenrich, Craig. "How Light Works." How Stuff works 02/24/2008
The National Centor for Atmospheric Research & the UCAR Office of Programs. "About Rainbows." 2/29/2008
No comments:
Post a Comment