Around the Rainbow

Short answer: A rainbow isn't an object in a fixed place. It's a pattern of light that appears when sunlight enters falling drops, bends (refracts) going in, reflects once off the back of each drop, and bends again on the way out.

Different colors leave at slightly different angles, so the drops that send light to your eye all sit about 40 to 42° from the antisolar point. That's the point directly opposite the sun, which is always where the shadow of your head falls. Every point on a circle at that angle around your shadow sends rainbow light to you, so the primary rainbow is a ring, red on the outside and violet on the inside. From the ground, the antisolar point is below the horizon whenever the sun is up. You see only the part of the circle above the ground, and only if there are raindrops there. When the sun is right on the horizon you get a half circle, and the higher the sun, the smaller the arc. Once the sun is higher than about 42°, no primary bow appears at ground level. From an airplane, a mountain or a tall building, drops can lie below you too, so you can sometimes see the whole ring. A glory is a different effect: smaller, tighter colored rings around the shadow of an aircraft on cloud. The footage shows a bright rainbow arcing down past the horizon toward the water below, filmed from high above a waterfront.

In collections:Everyday Physics