WunderPhotos®

When sunlight hits drops of rain they act like a prism, splitting the light into different colors, each bent at a slightly different angle. Because the sun, is a circle, and light travels in a straight line, the rainbow is a circle as well. In order for the sun's rays to refract so we can see them, the sun has to be within 42 deg of the horizon either rising or setting. That's pretty low, which means we can never see the full circle of a rainbow - we just see the top of the arc!(quote)