A meteorological phenomenon, the rainbow is caused by the reflection, refraction, and dispersion of light resulting in a spectrum of light appearing in the sky. It takes the form of an arc. Rainbow is also formed in full circles. But, on an average, an observer sees only a semicircle.
In theory, every rainbow is a circle, but only its upper half can be seen by the ground observers. Rainbow always appears in that section of the sky which is opposite to that of the sun. Therefore, as the sun approaches the horizon, more of the colorful circle comes into our view. That is, the largest section of the circle (50%) is normally seen during sunrise or sunset.
There is a primary rainbow and a secondary rainbow. In a primary rainbow, the arc shows red on the outer part and violet on the inner side. Primary 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. The order of colors is reversed in the secondary rainbow, with red on the inner side of the arc. Double reflection of sunlight inside the raindrops causes the secondary rainbow. When a double rainbow occurs, the second arc is seen outside the primary arc.
Heard about the twinned rainbow? Unlike a double rainbow that consists of two separate and concentric rainbow arcs, the very rare twinned rainbow appears as two rainbow arcs that split from a single base.The colors in the second bow, appear in the same order as the primary rainbow. In twinned rainbows, colors do not reverse as in a secondary rainbow.
Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude angle. Because of this, rainbows are usually seen in the western sky during the morning and in the eastern sky during the early evening.