Arch Rainbow

Rays of a rainbow bend twice. As they enter the drops, the rays are bent, then they reflect off the back of the drops and bend again as they exit out the front of the drops.
The rainbow is circular because when a raindrop bends light, the light exits the raindrop at a 40-42 degree angle away from the angle it entered the raindrop.
The violets and blues bend at a 40 degree angle, and the oranges and reds bend at a 42 degree angle.
Light can also enter droplet, be reflected off the back of the droplet, only to be reflected of the front, and then off the back again before leaving. It is bent at each phase in this process, and a second rainbow can appear above the first one. In a double rainbow, the second rainbow will appear where it does because the light will be cumulatively bent some 50 to 53 degrees. This gives it its position "outside" the primary rainbow. Also, the colors will be inverted in this bow.
Rainbows don't have "ends" but are full circled, but we can't see this because the horizon of the earth is in the way.
Source(s):
WikiHow


A rainbow is curved due to angle requirements. Light from the sun that
bounces through a drop of water, comes into your eye, and appears red must
bounce at a specific angle. Join two sticks together at a set angle. Place
one end at a dot to represent the sun. Place the other toward your eye.
The place where the sticks join, where the water drop is, can be rotated
around in a circle. A rainbow is shaped like a circle.

You never see the entire rainbow because of the horizon. When you look at a
rainbow, the Earth gets in the way of most of the circle. The portion you
see is just the top portion. Draw a circle and cover a little more than
half of it. What is left is the shape of a rainbow.
Source(s):
http://wiki.answers.com/Q/Why_is_the_rainbow_curved



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When we see rainbows we see them as arcs when in truth they are formed as circles. There is a point called the antisolar point that the rainbow seems to form around. From the picture we see the antisolar point formed as a line from the observers head by the sun shining from behind.--We don't see the full circle of the rainbow because the horizon gets in the way. As the sun goes down we are able to see more of the rainbow, and the higher the sun is in the sky the smaller the arc seems.--Sometimes people at high elevations, like in planes, see rainbows as full circles because they do not have the horizon to block their view.


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There are always two rainbows; the top (secondary bow), and usually fainter one has the order of the colours inverted with respect to the primary bow (the one in the picture).