Rainbows are a pretty spectacular sight. Since almost everyone will agree with that statement, have you ever wondered what causes a rainbow to appear in the first place? The explanation is really quite simple.sca
First, the weather conditions must be right. A rainbow can only form and be seen when it is raining and the sun is shining at the same time. This doesn't happen very often, for when it rains, the sky is usually blanketed in a complete cloud cover over the affected area. Yet, and on occasion, that cloud producing the rain directly overhead may simply be small or be located at the edge of a rain system. Think of it this way: Rain has to start and end somewhere over the ground. When this happens to be the case in your location during daylight hours, the sun will be visible because the cloud producing the rain is no longer blocking its light.
Now, a simple lesson in optics is in order.
When a ray of sunlight passes through a raindrop, it is bent, or refracted at the point where the light leaves the air and enters the water droplet. In the meantime, the raindrop acts as a prism. Thus, the white light of the sun is broken up into its component colors of the spectrum. The color that will appear depends on its wavelength. Wavelengths of light will be refracted at different angles, and this is what causes the observer to see different colors. While the sunlight is inside the water droplet, it will in turn be reflected in yet another direction. Finally, when the reflected light inside the raindrop exits the other side and re-enters the air, it is bent, or refracted a second time.
Raindrops are all more or less the same shape, so when the sun shines into millions of these similarly-shaped droplets, the pattern of the light is altered identically. When you face the rain with the sun at your back, red light will appear at 42 degrees, violet light will appear at 40 degrees, and the other primary colors will fall between these two angles. This spectacle is a rainbow! Now here is something you may not have known: Rainbows will never appear the same to two people at different vantage points and are not usually visible if the sun is higher than 42 degrees, so in addition to those optimum weather conditions described above, rainbows generally can only be seen a few hours after sunrise or before sunset. Moreover, the higher your altitude, the better the viewing. What's really cool is the fact that entire circular rainbows can be seen from an airplane!
In regard to double rainbows, this occurs when sunlight is reflected through the raindrops twice. Interestingly, the order of colors will also be reversed. The secondary rainbow will also always be lighter than the primary one because the refraction has taken place at a greater distance and is essentially a mirrored image of the first rainbow. One way you can remember the order of colors in the light spectrum is to use the mnemonic Roy G. Biv. Red, orange, yellow, green, blue, indigo, and violet! in the case of a double rainbow, however, you'd have Vib G. Yor! If you live in an arid climate and have never seen a rainbow put on by Mother Nature, you can make your own by spraying water out of a garden hose on a sunny day. The concept is the same, but on a much smaller scale. It's a bit peculiar to realize, but regardless of how one sees a rainbow, it is no more than an optical illusion.