[[Image:Rainbow.jpg|thumb|400px|right|A rainbow, seen from the island of Curaçao. Note: the bright spot is a clearing in the clouds; the sun is ''behind'' the photographer.]]
A rainbow is a semi- circular colour spectrum appearing in the sky due to the result of refraction and multiple reflections of sunlight in droplets of water . Rainbows appear when water droplets are lit by direct sunlight, a condition that rarely occurs ''during'' a storm, because the clouds block the sun. Rainbows are frequently seen ''after'' a storm, when the falling rain has moved into the distance and the sun is roughly behind the observer when the observer is facing the patch of rain.
Complete circular rainbows can be seen under the right conditions; in particular, when viewing the mists over Niagara Falls early or late in the day. Circular rainbows can sometimes be seen from an airplane when it flies over clouds, the rainbow actually encircling the airplane's shadow on the clouds. One way to think about the rainbow is that the mist or rain is like a projection screen, and the rainbow can only be seen where the projection screen is. When the projection screen is large enough, you see the entire circular rainbow.