rainbow

A rainbow is an optical and meteorological phenomenon that appears as a multicolored arc in the sky when sunlight is refracted, reflected, and dispersed by water droplets in the Earth's atmosphere. The spectrum typically shows red on the outer edge and violet on the inner edge.

For a primary rainbow, sunlight enters a raindrop, is refracted (bent), reflected off the back of the drop, and refracted again as it exits. This process disperses the light into a spectrum. The bright arc observed from a given location emerges from many droplets at about 42 degrees relative to the direction opposite the sun.

A secondary rainbow occurs when light is reflected twice inside the droplets, producing a fainter arc with reversed colors at about 50–53 degrees. There can also be tertiary or quaternary rainbows in special conditions, but these are rare and faint.

Rainbows can form from mist or spray, and are often seen after rain or near waterfalls. A full circular rainbow can be visible from high vantage points when the sun is low. Other related phenomena include moonbows (lunar rainbows) produced by moonlight and fog bows, which are faint, colorless bows made by very fine droplets.

The word rainbow comes from Old English regnboga, from regn “rain” and boga “bow.”