Light Refraction
When light passes through the air into a raindrop, the light refracts (changes direction) inside the raindrop. The light bows inward toward the back of the raindrop, where it is reflected, bows back towards the front of the raindrop and then escapes. Inside the raindrop, the light is split into a spectrum of different colors.
Reflection and Magnification
The rainbow is a reflection of the refraction occurring inside the raindrops. The raindrops act as a mirror to the refraction happening inside. The raindrops also magnify the reflection of the refraction inside.
Angle of the Sun
Sunlight must hit a raindrop at just the right angle to create a rainbow. The sun must be at a 42-degree angle above the horizon to cause a rainbow to form. The sun must be behind the viewer for the rainbow to be seen. Two people looking at the same point in the sky may not both see a rainbow if one is facing the sun and the other has his back to the sun.
Other Water Droplets Can Form Rainbows
A rainbow can form in any water droplet so long as that water droplet is airborne. This includes spray from a rapid river, a fountain, a waterfall or water droplets from a garden sprinkler. Suspension of the water droplets puts the droplets at the right angle to refract incoming sunlight. Rainbows that appear on sunny days are passing through rain at high altitudes or as a result of showers occurring at distances further than the naked eye can see.
Multiple Rainbows
Two or more rainbows appearing at once are caused by light reflecting twice inside the same raindrop. One rainbow, called the primary rainbow, is usually more prominent than the other rainbow, called the secondary rainbow. The secondary rainbow is usually seen above and outside the primary rainbow and has the order of its colors reversed. This is because the light has already been reflected once through the primary rainbow.