A rainbow is an optical and meteorological phenomenon that is caused by both reflection and refraction of light in water droplets in Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. All rainbows are full circles, however, the average observer sees only approximately the upper half of the arc, the illuminated droplets above the horizon from the observer's line of sight.