May
18
2006

How far away are rainbows?


Rainbow: (Photo credit: Mark Ryan)

Living in Minneapolis, I often ride my bike for exercise around the city lakes. The other day, while trying to get a ride in between rain showers, I was presented with a stunning rainbow. Actually, only a small portion of the arc was visible when I first saw it at Lake Harriet, but by the time I reached the west side of Lake Calhoun it had grown into a full blown double arced rainbow.

I stopped to admire it and regretted not having my camera with me because it was truly one of the best rainbows I had witnessed in a long time. A kid next to me, snapping a picture of it with his cell phone, wondered out loud, “How far away do you think that rainbow is?”

It was a good question and made me want to learn more about the atmospheric phenomenon.


Raindrop refraction and reflection: Sunlight is refracted as it enters a raindrop, reflected inside, then refracted again as it exits. An observer sees only one color reflected from a particular raindrop.Graphic by Mark Ryan

Basically, rainbows are the result of sunlight being once reflected and twice refracted by raindrops. Certain conditions are required. First and foremost, the viewer needs to be located between the sun and rain clouds. A ray of sunlight enters an individual drop of water and is refracted (bent) as it enters, then reflected from the back of the drop, and refracted again as it exits the drop. The refractions cause the white sunlight to divide into separate colors. Each color refracts in slightly different amounts, red the least, and violet the most. A particular raindrop will reflect red light because it is positioned at just the right angle from your eye (42°). This is known as the “rainbow ray”. Another droplet located at a slightly different position will reflect blue light to your eye. Now multiply this by the innumerable suspended water droplets that make up a rain cloud, and you have a rainbow.

The main colors in a primary rainbow will have red at the top followed by orange, yellow, green, blue, indigo, and violet. An easy way to remember the order is to note that the first letter of each color spells out the name ROY G. BIV.

The rainbow I witnessed had a second, fainter rainbow just above the first. This is the result of some light being reflected twice, and at a higher angle. The colors in a secondary arc are reversed with red on the bottom and violet on top.

The inside of a rainbow is always brighter than the sky outside the arc. This is because other rays of light are reflected from individual raindrops at angles smaller than the rainbow ray. Since this scattered light is made up of all the other incidental colors the light inside the bow is white.

So, how far away is a rainbow? I’ll let you figure that out for yourselves. The answer may surprise you.

Your Comments, Thoughts, Questions, Ideas

Anonymous's picture
Anonymous says:

rainbows go as far as the rain and the sun are shining through each other and it makes a rainbow

posted on Wed, 05/24/2006 - 1:58pm
Anonymous's picture
Anonymous says:

im hurt you didnt put up my comment

posted on Wed, 05/24/2006 - 1:59pm
bryan kennedy's picture

It is likely you comment wasn't posted because it violated our community guidelines. We also generally don't post questions that are way off-topic in relation to this story.

posted on Wed, 05/24/2006 - 2:43pm
Anonymous's picture
Anonymous says:

Nice article. However:
The secondary rainbow is not explained enough. The same holds for that the inside of rainbow appears brighter than the outside.

posted on Sun, 11/19/2006 - 3:42am
mdr's picture
mdr says:

The purpose of these postings is to get others to engage in the discussion and add too it. Please feel free to research further into secondary rainbows and the brightness of the inside of the rainbow and post them here as a comment. Thanks!

posted on Sun, 11/19/2006 - 9:46am

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