A rainbow occurs when sunlight shines on water droplets, causing the light to bend. The light bends again when it bounces off the back of the droplet and exits. The bending of the light, called refraction, allows a person to view the different wavelengths of light.
The shortest wavelength, violet, bends the most, while the longest, red, bends the least. Visible light is made up of all the colors of the rainbow and comes from the sun. For a rainbow to be visible, water droplets must be floating in the air and the sun must be behind the observer. Any clouds must be out of the way of the path of light, and the contrast must be sufficient for the observer to view it.
A rainbow is not an object and does not exist in a particular place. A rainbow naturally occurs as a full circle, a phenomenon that can be seen on occasion from an airplane, but from the ground only an arc is visible. Occasionally, a second rainbow, called a double rainbow, is visible above the first rainbow. This second rainbow is caused by a second reflection inside the water droplet. The colors of the second rainbow are reversed from the first, with red on bottom.