Almost everyone has witnessed a majestic and beautiful rainbow. There are few people who won't take the time to appreciate them, but there are quite a few people who never stop to consider what causes them, or to consider the many facts we know about them.
One fact is that no two people see exactly the same rainbow. This is more easily explained by explaining the cause of the rainbow.
When sunlight streams into water droplets that are suspended in the atmosphere, at an angle of 42 degrees, the light passes through the near side of the water droplet where it is refracted, or bent. When the light then hits the other side of the droplet, it is reflected back while again being refracted. It leaves the droplet at the same angle it entered, or 42 degrees, and in the process the light is broken into its spectrum as it would be if it passed through a prism; red, orange, yellow, green, blue, indigo, and violet. The combination of this process on millions of tiny droplets is what causes a rainbow, with red on top, and violet at the bottom. This last is because of the property of the wavelengths of colors. Red has a longer wavelength, so it is found on top, while violet has a short wavelength and is found on the bottom.
The reason then that no two people see the same rainbow is because even standing side by side, they aren't looking at the exact same water droplets from the exact same angle.
The clarity and brilliance of the rainbow is a function of the droplet size. Very small droplets soften the effect, blurring the borders of the colors, while very large droplets don't offer as many prisms, so the effect is dulled. Medium sized water droplets create the most crisp looking rainbows.
We also usually perceive rainbows as arcs or parts of arcs. This is an illusion. Rainbows are actually circular, but we can't usually see the entire rainbow because of shadows or the horizon of the earth. Sometimes the entire circle is seen, though, as happens at times from an aircraft flying over clouds, or when there is a sun or moon "halo".
Rainbows can appear to be immense, and this is because of the angle. A 42-degree angle, viewed head on, can appear to span nearly the entire horizon. This is also the reason that as we walk toward a rainbow, it appears to recede from us at exactly the same speed. It really isn't moving at all, but rather the angle is being maintained.
Rainbows fascinate us, fill us with awe, and often give us a special appreciation. However, this is one weather phenomenon that is more thoroughly explained than almost any other. Even armed with this knowledge, though, it is very difficult to see a rainbow and to still not be taken in by its magnificent beauty.