It looks like an arch built across the sky, but a rainbow is not a thing in one place; it is light that reaches your eyes only when you stand in the right spot. A rainbow is a curved band of colors in the sky, and it needs two ingredients to begin, which are sunshine and water drops in the air, such as rain.

Where you stand matters a great deal. To see a rainbow, the sun must be behind you and the water drops in front of you. A rainbow shows up opposite the sun, so you must look away from the sun to find one. Rainbows often show up after a rain shower, when the sun comes out. The best chances are in the early morning or the late afternoon and evening, when the sun is low. You can even see a rainbow in the spray from a waterfall, a garden hose or a lawn sprinkler.

A rainbow is really a full circle, but the ground usually hides the lower part, so we see only an arc. From a plane you can sometimes see a whole circle rainbow. There is a handy way to find the middle of the circle. If you can see your own shadow while it rains, and the sun is low enough, the rainbow circles around the shadow of your head. The spot exactly opposite the sun is called the antisolar point, and it is the center of the rainbow's circle.

Now consider the light itself. Sunlight looks white, but it is made of all the colors of the rainbow mixed together, and a prism is a piece of glass that splits a beam of sunlight into rainbow colors. Light bends when it goes from air into water, and this bending is called refraction. Light moves more slowly in water than in air, and that is why it bends when it enters a raindrop.

Follow one ray of sunlight into a raindrop. The light enters the drop and bends, bounces off the back of the drop, and then bends again as it leaves, heading back toward the sun's side of the sky. Different colors of light bend by different amounts when they enter water, so the colors spread apart, with violet bending the most and red the least. Red has the longest wavelength of visible light and violet the shortest.

Why does the bow sit where it does? The main rainbow forms where you look about 42 degrees away from the antisolar point. A rainbow is round because every direction that is 42 degrees from that point makes a circle around it. The light you see reaches you from the direction of the raindrops, not from the sun. The higher the sun is in the sky, the lower the rainbow sits. If the sun is more than 42 degrees above the horizon, no rainbow can be seen from the ground. At sunset the arch is a half circle with its top about 42 degrees above the horizon.

A single drop does not hold the whole rainbow. Every raindrop sends out all the colors, but each drop sends different colors in different directions, so your eye catches just one color from each drop. Red light comes out of the drops at a steeper angle than blue light, so you see red from drops higher up, at the top and outer edge of the bow, and blue from lower drops. Red is on the outside and blue and violet are on the inside.

Sometimes two rainbows show at once: a bright one and a fainter one above it with its colors flipped. A double rainbow happens when light bounces twice inside the raindrops instead of once. The second bow is outside the first, and its colors are in reverse order. The secondary rainbow comes from light that reflects twice inside the drop and leaves at about 50 to 51 degrees, wider than the primary bow. The dark strip of sky between the two bows is called Alexander's band.

Rainbows have cousins. Fogbows are pale, nearly white rainbows that form in fog, mist or cloud. Rainbows can also be made by moonlight. These moonbows, or lunar rainbows, are rare, and they are dimmer than rainbows because moonlight is much fainter than sunlight. They usually look white to our eyes. Closer to home, you may spot faint extra pastel arcs just inside a bright bow. These are called supernumerary bows. The sky inside a rainbow is brighter than the sky outside it. Big raindrops make bright, vivid rainbows, and small drops make paler ones.

How many colors are there in a rainbow? The usual list is red, orange, yellow, green, blue, indigo and violet, which is remembered as ROYGBIV, and people usually name six or seven of them. But a rainbow is really a smooth, continuous range of colors, and Isaac Newton popularised the idea of seven.

Words to know

• refraction: light bending
• prism: light-splitting glass
• antisolar: opposite the sun
• wavelength: wave length
• fogbows: white rainbows