“Rainbows are one of the most spectacular light shows observed on earth,” observed author Donald Ahrens in his text Meteorology Today. Technically speaking, the traditional rainbow is sunlight spread out into its spectrum of colors and diverted to the eye of the observer by water droplets.
The “bow” part of the word describes the fact that the rainbow is a group of nearly circular arcs of color all having a common center. They appear when raindrops reflect sunlight, thus breaking white sunlight into colors.
When light enters a water droplet, the different wavelength colors bend at slightly separate angles. Some of the light reflects off the back of the droplet and is bent a second time as the droplet emerges from the light beam. Drops at different angles send distinctively different colors to the eye.
When light is hitting raindrops at a proper angle, a secondary, larger rainbow will appear outside of the main rainbow. This secondary rainbow is fainter in color than the main one because the light has been reflected twice by each raindrop. This double reflection also reverses the colors in the secondary rainbow. More than one rainbow can be viewed at once when light bounces around inside a water droplet, and is split into its constituent colors. Double rainbows are produced when this happens inside a droplet twice, triple when it happens three times, and (see where this is going) quadruple when it happens four times.
To see a rainbow, an observer must have her back to the sun and rain must be falling in some part of the sky. Since each raindrop is lit by the white light of the sun, a spectrum of colors is produced. No two observers will ever witness exactly the same rainbow because each will view a different set of drops at a slightly different angle. Also, each color seen is from different raindrops
Sunlight is made up of the whole range of colors that the eye can detect. The range of sunlight colors, when combined, looks white to the eye. This property of sunlight was first demonstrated by Sir Isaac Newton in 1666. Light of different colors is refracted by different amounts when it passes from one medium (air, for example) into another (water or glass, for example).
Rainbows that appear after dark are called moonbows. That makes sense, because the light rays being bent come from reflections off the moon instead of directly from the sun.