A rainbow is an optical and meteorological phenomenon that is caused by the reflection of light in water droplets in the Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc.©Matt Fricker Photography
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 while 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 outside the primary arc, and has the order of its colors reversed, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets. The image below was taken by Molly Glasoe on 26th July 2013, and clearly shows the reversed colors.
©Molly Glasoe / perthweatherlive.com
Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. Interestingly, the angle of the sun in relation to your eye when looking at a rainbow is always 42°. Also, the lower the sun is to the horizon, the higher in the sky the rainbow will appear.