A rainbow is a meteorological phenomenon that is caused by reflection , refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It 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 full circles. However, the observer normally sees only an arc formed by illuminated droplets above the ground,  and centered 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 outside the primary arc, and has the order of its colours reversed, with red on the inner side of the arc. This is caused by the light being reflected twice on the inside of the droplet before leaving it.
How Do Rainbows Form?
When the Sun is shining and there is rain as well you may see a rainbow in the sky! As sunlight passes through the water droplets, it is bent and split into the colors of the rainbow. Sunlight is known as visible or white light and is actually a mixture of all visible colors. Rainbows appear in seven colors because water droplets break white sunlight into the seven colors of the spectrum red, orange, yellow, green, blue, indigo, violet. You can only see a rainbow if the Sun is behind you and the rain in front. The main rainbow becomes visible at an angle of around 40" from the horizon. You might be able to see a second rainbow above the main one in which the colors are in reverse order. You can even make your own rainbow using a garden hose or water sprinkler to form the water droplets in the air on a sunny day.
Low Sun and Water Droplets
Rainbows Near and Far Some scientists think rainbows also exist on Titan, one of the moons of the planet Saturn. Titan has a wet surface and humid clouds. The sun is also visible from Titan, so it has all the ingredients for rainbows. Also called earthshine. Also called a stacker rainbow. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Jeannie Evers, Emdash Editing. Caryl-Sue, National Geographic Society.
If you had no idea at all about what a rainbow is or what causes it, you might actually believe some of the legends that different ancient cultures have created to explain it. A rainbow requires water droplets to be floating in the air. The Sun must be behind you and the clouds cleared away from the Sun for the rainbow to appear. A full rainbow is actually a complete circle, but from the ground we see only part of it. From an airplane, in the right conditions, one can see an entire circular rainbow. The sunlight shines on a water droplet. As the light passes into the droplet, the light bends, or refracts, a little, because light travels slower in water than in air because water is denser. Then the light bounces off the back of the water droplet and goes back the way it came, bending again as it speeds up when it exits the water droplet. Light enters a water droplet, bending as it slows down a bit going from air to denser water. The light reflects off the inside of the droplet, separating into its component wavelengths—or colors.