A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.
In a so-called "primary rainbow" (the lowest, and also normally the brightest rainbow) the arc of a rainbow shows red on the outer (or upper) part of the arc, and violet on the inner section. This rainbow is caused by light being reflected once in droplets of water. In a double rainbow, a second arc may be seen above and outside the primary arc, and has the order of its colours reversed (red faces inward toward the other rainbow, in both rainbows). This second rainbow is caused by light reflecting twice inside water droplets. The region between a double rainbow is dark. The reason for this dark band is that, while light below the primary rainbow comes from droplet reflection, and light above the upper (secondary) rainbow also comes from droplet reflection, there is no mechanism for the region between a double rainbow to show any light reflected from water drops, at all.
Although legendary, triple rainbows (in the same style and angle as double rainbows) are impossible, since a third reflection of light inside water drops would put their rays close to the direction of the Sun, and they would thus be invisible. Some phenomena (such as "supernumerary arcs" very close to, and inside primary arcs) may be mistaken for "triple rainbows." It is also impossible for an observer to manoeuvre to see any rainbow from water droplets at any angle other than the customary one (which is 42 degrees from the direction opposite the Sun). Even if an observer sees another observer who seems "under" or "at the end" of a rainbow, the second observer will see a different rainbow further off-yet, at the same angle as seen by the first observer. Thus, a "rainbow" is not a physical object, and cannot be physically approached.
A rainbow spans a continuous spectrum of colors; the distinct bands (including the number of bands) are an artefact of human colour vision, and no banding of any type is seen in a black-and-white photo of a rainbow (only a smooth gradation of intensity to a maxima, then fading to a minima at the other side of the arc). For colours seen by a normal human eye, the most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv). However, colour-blind persons will see fewer colours.
Rainbows can be caused by many forms of airborn water. These include not only rain, but also mist, spray, and airborne dew.