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: each person has a different shadow point and horizon, and a different set of raindrops makes the rainbow each person sees. Two people standing side by side are therefore looking at two different rainbows, even though it feels like one shared arch.

You may have heard that every raindrop is a tiny prism. In fact, raindrops are nearly round, and the light separates because it is bent twice at curved surfaces of the drop. Light moves more slowly in water than in air, and that is why it bends when it enters a raindrop. Sunlight enters the drop and bends, bounces off the back of the drop, and bends again as it leaves, heading back toward the sun's side of the sky. Different colors bend by different amounts when they enter water, so the colors spread apart, with violet bending the most and red the least.

Light leaves a raindrop in many directions, but the most light is concentrated near 42 degrees, so that is where the bright rainbow appears. Red light leaves a raindrop turned about 42 degrees from the sunlight's path and blue-violet light about 40 degrees, so the colors line up in bands instead of mixing together. The spot exactly opposite the sun, called the antisolar point, is the center of the rainbow's circle, and the main bow forms where you look about 42 degrees away from it. The light you see comes from the direction of the raindrops, not from the sun.

Why is red always on the outside? 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 has the longest wavelength of visible light and violet the shortest, and human eyes can see wavelengths from about 380 to 700 nanometers, with violet at the short end and red at the long end.

In the mid-1660s Isaac Newton used glass prisms to show that white light is made of a spectrum of colors. A prism is a piece of glass that splits a beam of sunlight into rainbow colors, and a rainbow's colors and the colors that come out of a prism are the same, in the same order.

How many colors does a rainbow have? A rainbow is really a smooth, continuous range of colors. Isaac Newton popularised the idea of seven rainbow colors. Many scientists think indigo is too close to blue to tell apart, so some modern lists merge indigo into blue and violet.

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.

Fog can make a rainbow too, but a different one. Fogbows are pale, nearly white rainbows that form in fog, mist or cloud. They are broad and pale because fog droplets are far smaller than raindrops, so the light spreads out, or diffracts, and smears the colors. Moonlight can make rainbows as well. 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.

A rainbow can even turn red. A red rainbow shows when the sun is very low, near sunrise or sunset, because scattering in the long path through the air removes the blue and green light and leaves mostly red. Air molecules scatter short wavelengths such as blue and violet more strongly than long wavelengths such as red, which is also why sunsets look red. Low sun matters for ordinary rainbows too. At sunset the arch is a half circle with its top about 42 degrees above the horizon, and if the sun is more than 42 degrees above the horizon, no rainbow can be seen from the ground.

Next time you see a rainbow, look at the sky around it. The sky inside a rainbow is brighter than the sky outside it, and a rainbow shows up best against a dark storm cloud. Big raindrops make bright, vivid rainbows, and small drops make paler ones. Behind the simple arch lies a chain of events: sunlight bent, bounced and bent again, sorted by color into bands, then caught by one pair of eyes at one spot.

Words to know

• indigo: deep blue-violet
• spectrum: band of colors
• wavelength: wave length
• diffracts: spreads out
• antisolar: opposite the sun