Two people can stand side by side and look at the same rainbow. But each of them sees a different one. A rainbow is not a thing hanging 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. Each person has a different shadow point and horizon, and a different set of raindrops makes the rainbow each person sees.

To see a rainbow you need three things: bright sunlight, water drops in the air, and the right viewing angle with the sun behind you. Picture the spot exactly opposite the sun. It is called the antisolar point, and it is the center of the rainbow's circle. Sometimes you can see your own shadow while it rains. If the sun is low enough, the rainbow circles around the shadow of your head.

Sunlight looks white, but it is made of all the colors of the rainbow mixed together. A prism is a piece of glass that splits a beam of sunlight into rainbow colors. A rainbow's colors and the colors that come out of a prism are the same, in the same order. Each raindrop acts a little like a tiny prism, splitting sunlight into its colors. So a rainbow is sunlight spread out into its colors.

Light bends when it goes from air into water. This bending is called refraction. Refraction is the bending of a wave as it passes from one material to another. One example is light going from air to water. So the light changes direction as it goes into the drop. 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. It enters the drop and bends. It bounces off the back of the drop. Then it 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. Violet bends the most and red the least.

Red has the longest wavelength of visible light and violet the shortest. The colors of a rainbow always come in the same order, with red on the outside and blue and violet on the inside. People usually name six or seven colors. The classic order is red, orange, yellow, green, blue, indigo and violet, or ROYGBIV.

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

The best chances to see one are 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. You can also see one in the spray from a waterfall, a garden hose or a lawn sprinkler. A rainbow also shows up best against a dark storm cloud.

Rainbows can show up in other ways. 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.

A rainbow is really a full circle, but the ground usually hides the lower part, so we see only an arc. From an airplane, in the right conditions, one can see an entire circular rainbow. Next time you see a rainbow, remember that your rainbow is yours alone.

Words to know

• wavelength: wave size
• antisolar: sun's opposite
• refraction: light bending
• horizon: sky's bottom edge