Generally speaking, showers and thunderstorms move from west to east, thus verifying the old adage: Sailors have long known that rainbows can be used to predict the weather.

But whether he or Snell can be fully credited for that part of the explanation, we may never know. So, while Descartes may have explained what a rainbow is, he really couldn't have done it without those calculations for the refraction of light. The end result was that in the West, especially in the English-speaking countries, the law of the refraction of light became known as Snell's Law, while in France it is referred to as Descartes' Law. Then, about 80 years later, after Snell's notes were discovered, controversy arose when some accused Descartes of having somehow seen Snell's manuscript and taken his findings for his own. Snell, however, failed to publish his findings and died in 1626. As it turned out, Willebrord Snell, a Dutch astronomer and mathematician, had discovered the mathematical law of refraction 16 years prior to Descartes' dissertation on the subject. We can't, with absolute certainty say exactly who was the first person to provide the correct explanation of what causes rainbows, although credit is usually given to Frenchman René Descartes (1596-1650), a philosopher and writer who penned a formal and systematic discussion on the subject in an appendix to his famous work, 'A Discourse On Method,' in 1637.ĭescartes supposedly made an accurate calculation concerning the paths that light rays took at different points through a glass globe of water (simulating a raindrop) thereby determining their angles of refraction it was the solution to a mathematical problem that had eluded scientists for two millennia and was the key to explaining the phenomenon of the rainbow.īut notice that I said that Descartes 'supposedly' made that calculation. There is even evidence for a third or tertiary rainbow that has been seen on rare occasions, and a few observers have even reported seeing quadruple rainbows in which a dim outermost arc had a rippling and pulsating appearance. The region between the two bows appears relatively dark, for it lacks entirely both the once- and the twice-reflected rays. The secondary rainbow forms at a 51-degree angle from your shadow it's always fainter and usually disappears more quickly than the primary. It will be fainter, with the colors reversed: red on the inside, violet on the outside. Sometimes a secondary bow forms outside the primary.

The primary bow is due to light that enters the upper part of the drops and leaves after one internal reflection, so this bow is always brighter than the secondary bow where sunlight is reflected twice within raindrops. When there are showers nearby, simply look in the part of the sky opposite the sun at a 42-degree angle from your shadow if there is a rainbow, that is where it will be. Usually the radius of the arc is equal to about one-fourth of the visible sky, or 42 degrees, to the red. Rainbows can also be seen against the spray of a waterfall.Ī single, or primary, rainbow has red on the outside or top of the bow and blue on the inside. Simply stand with your back to the sun and adjust the hose to a fine spray. You can create an artificial rainbow for yourself with a garden hose. They come when the sunlight breaks through rain clouds. Rainbows are frequently seen in the wake of a rainstorm. The raindrops act like miniature prisms, refracting or breaking sunlight into various colors as well as reflecting it to produce the spectrum. This produces an atmospheric solar spectrum in the sky for all to see: a rainbow.Ī rainbow is simply a group of circular or nearly circular arcs of color that appear as a huge arch in the heavens. Colors go from red, which is bent least, through orange, yellow, green, and blue all the way to violet, which is bent the most.Īnd just as sunlight passing through a prism is bent, so is sunlight passing through drops of water. Light leaving the prism spreads out into a continuous band of colors called a spectrum. When it passes through a glass prism, some of the light is bent, or refracted, more than other portions.

Rainbow raindrops