Rainbow

A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew. Rainbows can be full circles. However, the observer typically sees only an arc formed by illuminated droplets above the ground, and centred on a line from the Sun to the observer's eye. In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it. In a double rainbow, a second arc is seen about 10° outside the primary arc. Its colours are percieved to be in reverse order, with red on the lower side of the arc. The number of reflections have nothing to do with this; the centre of the second arc is the sun, not the anti-solar point, and the arc is seen about 130° from it. So it wraps around the top of the sky, and is seen upside-down. Red is still on the "outside," but that is the "down" side.