If we look away from the sun, the shadow of our head on the ground marks the point called the antisolar point, where the sun is directly opposite of us. If the sun is in the sky, the antisolar point is below the horizon. If the sun has set, the antisolar point is above the horizon.

What does this "antisolar point" stuff mean? All it is telling us is the direction of the sun and where we can expect the rainbow to form.

What we have, then, is a block of raindrops being front-illuminated by the sun, refracting the light into its various colors, and reflecting those colors back. The important thing to notice is that the red light gets refracted back at 42° from our antisolar line, and the blue light gets refracted back at about 40° from our antisolar line.

That is, every raindrop within 42° of the antisolar line is reflecting red light back to our eyes. If we look at every raindrop that is 42° from this line, what shape are we seeing?

If we look at every raindrop within about 40° of the antisolar line, we would see a circle of raindrops centered on the line. Unfortunately, the horizon gets in the way of most of the rain, so we only get to see an arc or a bow; however some people have reported seeing full rainbows from airplanes and mountain-tops.

So we find that the raindrops that contribute to your rainbow all lie on a cone with its apex at your eye. In scientific jargon, we might say:

Fortunately, we don't need to be that scientific to understand the nature of a rainbow...

If you are admiring a rainbow with a friend, you are both seeing different rainbows.

Each rainbow is your own.

"...I once saw two rainbows in the sky. How did that happen?"

Why is it brighter inside the rainbow?