Double rainbow seen over plains. The outer rainbow is called a secondary rainbow. A rainbow is a spectrum of sunlight caused by the refraction and reflection of light in raindrops. The primary rainbow forms a circle 42 degrees from the shadow of the observer's head (the antisolar point). The secondary rainbow is formed by a double reflection in raindrops, and appears some 52 degrees from the antisolar point. The double reflection means it is fainter, and its colour order is reversed. Filmed in Colorado, USA, in summer.