A rainbow is formed when a raindrop refracts light, but why then does the whole sky not become a huge rainbow when it rains? Would the light not be dispersed into ordinary white light? What causes it to look as if each end is nearly touching the ground?
[Physics] What makes a rainbow happen
opticsreflectionrefraction
Related Solutions
You are seeing only one color from each droplet. Even though each droplet reflects the whole spectrum, only one color gets to your eye. The rest of the light from a single droplet is sent somewhere else than your eye (maybe into the eye of someone standing near you). So if you see a thick blue band in a rainbow, the blue band is formed by many reflections from many droplets. You cannot see many thin rainbows (each one from a single droplet), because each droplet contributes to your complete rainbow image as one "pixel" only, in a sense.
Each individual droplet reflects a whole cone of light - a complete circular rainbow (the image below has only 2 colors for clarity, but you get the idea - see this video):
All droplets in a rain reflect a huge amount of these cones (circular rainbows), but you can see only a small fraction of every cone from each droplet. All these fractions sum into the rainbow you see. So - how is the vertical portion of a rainbow formed? Simply - it is formed by the rays from the vertical portions of the light cones reflected by the droplets that are located in the direction where you see the vertical portion of the rainbow.
Again - the rays that you do not see are sent somewhere else. You can see them if you move around, but then you cannot see the rays that you were seeing in your previous location.
That means that a single cone from a single droplet can contribute to different parts of different rainbows, seen by different people: it can contribute to the upper horizontal part of a rainbow seen by you (that would be a color ray from the lower section of that individual cone), but at the same time it can contribute to the vertical portion of a rainbow seen by someone else who is higher and to your left (that would be a color ray from the left vertical portion of that individual cone).
I suspect the rainbow is from retroreflector glass beads rather than salt. Tiny glass beads are placed on the surface of road markings, halfway embedded, making them reflect light from vehicles back to them with total internal reflection. But when they get loose they act as cloud droplets instead, making rainbows.
Best Answer
It depends on the position of the sun. A rainbow does not exist at a particular location in the sky. Its relative position depends on the position of the observer and the sun. All raindrops refract sunlight in the same way, but only the light from some raindrops reach the observer's eye. This light is what constitutes the rainbow for that observer. The bow is centred on the shadow of the observer's head, known as the antisolar point and forms a circle at an angle of of about 42° to the line between the observer's head and its shadow.