Photo courtesy Dan Metz

Rainbows are one of the most beautiful spectacles nature has to offer. In fact, their beauty is so incredible that they have inspired countless fairy tales, songs and legends. It's a good bet that most of the artists behind these tales were totally mystified by the rainbow phenomenon -- just like most people are today. But the science of rainbows is really very simple. It's just basic optics!

The fundamental process at work in a rainbow is refraction. Refraction is the "bending" of light. If you're wondering how light bends, that's a good question. It's probably more accurate to say that it changes directions. When light travels from one medium to another, it changes directions - but most people refer to this as "bending." This happens because light travels at different speeds in different mediums.

To understand why light bends, imagine you've just gone grocery shopping with your parents. It's your job to push the cart to the "cart return" area. So you're pushing a shopping cart across the parking lot. The parking lot is one "medium" for the shopping cart. If you're exerting a constant force, the cart's speed depends on the medium it's traveling through -- in this case, the parking lot's paved surface.

Now, let's say you decide to take a little detour and go through a grassy area beside the parking lot. What happens when you push the shopping cart out of the parking lot, onto a grassy area? The grass is a different "medium" for the shopping cart. If you push the cart straight onto the grass, the cart will simply slow down. The grass medium offers more resistance, so it takes more energy to move the shopping cart.

But when you push the cart onto the grass at an angle, something else happens. If the right wheel hits the grass first, the right wheel will slow down while the left wheel is still on the pavement. Because the left wheel is briefly moving more quickly than the right wheel, the shopping cart will turn to the right as it moves onto the grass. If you move at an angle from a grassy area to a paved area, one wheel will speed up before the other and the cart will turn.

Double Rainbow Sometimes you see a double rainbow -- a sharp rainbow with a fainter rainbow on top of it. The fainter rainbow is produced in the same way as the sharper rainbow. Instead of the light reflecting only once inside the raindrop, it's reflected twice. As a result of this double reflection, the light exits the raindrop at a different angle, so we see it higher up. If you look carefully, you'll see that the colors in the second rainbow are in the reverse order of the primary rainbow.

Similarly, a beam of light turns when it enters a glass prism. This is a simplification, but think about it this way: One side of the light wave slows down before the other, so the beam turns at the boundary between the air and the glass. (Some of the light actually reflects off the prism surface, but most passes through.) The light turns again when it exits the prism, because one side of it speeds up before the other.

In addition to bending light as a whole, a prism separates white light into its component colors. Different colors of light have different frequencies, which causes them to travel at different speeds when they move through matter.

A color that travels more slowly in glass will bend more sharply when it passes from air to glass, because the speed difference is more severe. A color that moves more quickly in glass won't slow down as much, so it will bend less sharply. In this way, the colors that make up white light are separated according to frequency when they pass through glass. If the glass bends the light twice, as in a prism, you can see the separated colors more easily. This is called dispersion.

Drops of rainwater can refract and disperse light in the same basic way as a prism. In the right conditions, this refraction forms rainbows.

The Colors of a Rainbow Some people use mnemonic devices to remember the arrangement of colors in a rainbow. For example, if you were to memorize this phrase "Rainbows Over Your Grass Bring Instant Victory," it could help you remember the color order. R=red, O=orange, Y=yellow, G=green, B=blue, I = indigo, and V=violet. Red is at the top/outer edge of the rainbow and violet is at the bottom/inner edge, with the other colors in order in between.