Light (Level 2)

The Homemade Rainbow
It's easy to make your own rainbow. On a sunny day, look away from the sun and find the shadow of your head. Then spray some misty water (small droplets) in that direction. The rainbow will always be at an angle of 41 degrees (in a direction about half way between your nose and ear off to the side) away from the shadow of your head. If you could put water there (and see it), the rainbow would make a full circle around the shadow of your head! This is true for all rainbows. If you want to find one in nature, look for water in the sky (rain) at about that same angle when you are looking away from the sun. If there is a rainbow, that's where it will be. You can see the shadow of my water sprayer and my head at the very bottom of this picture and notice how the rainbow makes a curve around it.

Why is a Rainbow?

The beautiful colors of a rainbow are perceptions, like all colors. However, the process that creates the light we see as a rainbow is actually based on some fairly simple math.

That math is what we call geometry. The light from the sun is reflected by the surface of raindrops in the sky (just like it reflects off a window or surface of a lake) and its direction also changes when it enters and leaves the raindrop. The combination of all those changes in direction means that most of the light coming out of the raindrops is at an angle of about 41-42 degrees away from the shadow of our head (or in the direction half way between your nose and your ear when you look directly away from the sun). That alone would make a bright white circle in the sky. The reason the rainbow has its colors, is that the different wavelengths of light, producing different color perceptions, are reflected back to our eyes at slightly different angles. This makes the white light of the sun smear across the sky with different colors produce as the light is smeared and then we have a beautiful rainbow to see.

You might also see that the sky inside the curve of a rainbow is always brighter than the sky outside. This is also because of the way light reflects through the raindrops. You can even see that in the rainbow I made with my garden hose.

You can find a more detailed explanation of rainbows at this website from Dartmouth. And remember that rainbows are rarely seen since you need to have rain drops in the sky off in the distance in front of you and the sun low in the sky behind you; all at the same time!

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Updated: Apr. 19, 2011