Rainbows are what happens when sunlight is split into its various colors by the prismatic action of moisture in the air. You are most likely to see one after a rain shower, when the sunlight returns and shines on the storm that's headed off to the east, or when the air retains enough water to act as a prism. We call it a rainbow because it is an arc, or bow, in the sky. The thing is that we are only seeing a small portion of the rainbow as an arc. The full phenomenon is a circle, but we can't see it because it's so big. The earth gets in the way. You might even see a rainbow as an almost-straight line if it's big enough and is either obstructed or has gaps in the moisture.
Theoretical astrophysicist Ethan Siegel explains how sunlight becomes separated into its component colors in a way that you don't have to be an astrophysicist to understand. He explains why all rainbows are alike, except for their size and the way they may be obstructed. And he gives us two ways to see one as a full circle: the first one involves flying over a rainbow, which may be beyond our ability. The second one only requires a water hose, which I find myself doing every time I use a hose in the middle of a sunny day. Read all about rainbows at Big Think.
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