A rainbow is a full circle of colors, but an observer will only see a small portion of that circle creating the appearance of an arch. The lower the sun is in the sky, the more of the full circle you will see. You may also notice the red color of a rainbow is on the outside of the curve while the blue color is on the interior of the curve. Because the sunlight is reflected back out from the raindrop, you should have your back to the sun if you want to see a rainbow.
Types of Rainbows
As sunlight enters a raindrop, it is refracted and hits the inside back end of the water droplet. The light is then reflected back to the front of the raindrop at a 42 degree angle. This image shows the path of sunlight through water. Since the colors of the visible spectrum are at different wavelengths (shorter=blue, longer=red), the colors are separated.
Other Sites Featuring Rainbows