What Makes a Rainbow?

Meteorologist Eric Snitil explains how water and light interact to create beautiful rainbows.

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By Eric Snitil

Story Created: Aug 8, 2011 at 10:08 AM AKDT

Story Updated: Aug 8, 2011 at 10:34 AM AKDT

Just when you thought you've had enough of Anchorage weather, Mother Nature throws you a curveball, or more like a curved arc. Sunday night's multi-colored rainbow display was enough to make even the double rainbow YouTube guy proud.

A rainbow is a phenomenon that requires a special set of ingredients to create. In fact, the two main ingredients tend to be difficult to come by at the same time—you need sunshine and rain.

Now technically you can still get the same effect with any kind of white light and any kind of droplet of water (think fountain spray rainbows), but for the good stuff, we'll stick to sun and rain.

Anchorage tends to offer this set of ingredients at a pretty high rate, especially during the evening hours. Our mountain range to the east acts as a catalyst for showers in the summer, while skies farther west can remain mostly clear.

Because the sun sets in the west, during the evening we occasionally get the scenario when the setting sun illuminates the falling raindrops.

And voila, you have a rainbow!

But it's not that simple. There's a lot going on behind the scenes. That sunlight we've all been craving has a lot more to it than what meets the eye.

That white light is a conglomeration of tons of different colors at different wavelengths just waiting to be pulled apart—that's what the raindrops do.

Light traveling through a different medium, in this case from air to water, initiates a process called refraction. Refraction acts to literally bend light at varying angles depending on the wavelength.

Red light comes out at 42 degrees while blue light comes out at 40 degrees. This variation allows for the separation of colors within that initial beam of white light into the ROYGBIV range perceivable to the human eye.

The refracted light then reflects off the backside of the raindrop and refracts once again as it exits the drop. The end result is a beautiful arc of red, orange, yellow, green, blue, indigo and violet...which by the way isn't actually an arc, it's a full circle.

Kind of like your newfound expertise in rainbows. Happy viewing!

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jabbawabba said on Monday, Aug 8 at 1:41 PM

Really? Don't you learn this stuff in like sixth grade? Everyone knows rainsbows are with by a pot of gold at the end of it...with a little elf that you just have to drop kick out of the way to steal his gold MMWAAHAHAHAHA!!! *clears throat* sorry.

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