A rainbow's light reaches you from the direction of the raindrops, not from the sun. The sun sits behind you, and the drops spread out in front of you. To see one you need bright sunlight, water drops in the air, and the right viewing angle with the sun behind you. A rainbow shows up opposite the sun, so you must look away from the sun to see one.

Start with the spot exactly opposite the sun. It is called the antisolar point. The main, or primary, rainbow forms where you look about 42 degrees away from that point. Here is why a rainbow is round. Every direction that is 42 degrees from the antisolar point makes a circle around it. 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 height of the sun changes how much of the arch you see. The higher the sun is in the sky, the lower the rainbow sits and the less of its arch you can see. If the sun is more than 42 degrees above the horizon, no rainbow can be seen from the ground. 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.

The best chances to see a rainbow come in the early morning or the late afternoon and evening, when the sun is low. Rainbows often show up after a rain shower, when the sun comes out, and they show up best against a dark storm cloud. You can also see one in the spray from a waterfall, a garden hose or a lawn sprinkler.

Now look inside one drop. Light bends when it goes from air into water. This bending is called refraction. Light moves more slowly in water than in air, which is why it bends when it enters a raindrop. Sunlight enters the drop, bends, bounces off the back of the drop and bends again as it leaves, heading back toward the sun's side of the sky.

Sunlight looks white, but it is made of all the colors of the rainbow mixed together. Different colors bend by different amounts when they enter water, so the colors spread apart. Violet bends the most and red the least. Red has the longest wavelength of visible light and violet the shortest. A rainbow's colors and a prism's colors are the same, in the same order, with red on the outside and blue and violet on the inside.

People usually name six or seven colors in a rainbow. The classic order is red, orange, yellow, green, blue, indigo and violet, remembered as ROYGBIV.

A double rainbow happens when light bounces twice inside the raindrops instead of once. In a double rainbow the second bow is outside the first, and its colors are in reverse order. Sometimes two rainbows show at once: a bright one and a fainter one above it with its colors flipped.

Rainbows have cousins. Big raindrops make bright, vivid rainbows, and small drops make paler ones. 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. Moonbows are dimmer than rainbows because moonlight is much fainter than sunlight, and they usually look white to our eyes.

A rainbow is not a thing in one place in the sky. It is light that reaches your eyes only when you stand in the right spot. Your rainbow is yours alone, because each person has a different shadow point and horizon, and a different set of raindrops makes the rainbow each person sees.

Words to know

• antisolar: sun's opposite
• degrees: angle units
• horizon: sky's bottom edge
• vivid: bright, strong