The shape of a rainbow is an ARC, which is a three-letter word. A rainbow is a meteorological phenomenon that is created when sunlight and moisture interact in the atmosphere. It is seen as a multicolored ARC in the sky, with red on the outer edge and violet on the inner edge. Rainbows can be seen after rainstorms or when the sun is shining through mist or fog. They are often seen as a symbol of hope and beauty.