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A rainbow's position depends on the exact relationship between the observer, sunlight, and individual water droplets. The light entering one person's eyes therefore comes from a different collection of droplets than the light reaching someone standing nearby. Both people can point toward what appears to be the same rainbow, but physically they're receiving light from different droplets. Moving changes the geometry again. A rainbow isn't a fixed object sitting somewhere in the distance. In a sense, every observer gets a personal rainbow.
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