Why are rainbows arc shaped?

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. read less

The amount light bends as it passes through the droplet depends on the light's wavelength, or color -- red light bends the most, orange and yellow slightly less, and so on, down to violet, which bends the least. Since each color is bent through a specific angle -- red light comes from the sun and is reflected back at an angle 42 degrees away from its original direction, while blue light bends only 40 degrees -- each color appears at a different place in the sky. Red, say, denotes all those locations in the sky 42 degrees away from an imaginary line connecting the sun to the back of your head. Together, these places trace out an arch. Since blue appears only 40 degrees from this imaginary line (which also connects your head to its shadow), the blue arch of a rainbow is always below the red. read less