One of the more fascinating facts about rainbows is that no two people can see the exact same rainbow. You may think you're seeing the same thing; you might even describe the rainbow you're seeing to the friend next to you, who will agree that what she's seeing looks just like the one you described. But you're truly not seeing the same thing. Here's why:
When you're looking at a rainbow, you're looking at light that's reflected by raindrops sitting above the horizon. But your horizon is always different – albeit, sometimes only slightly different – from everyone else's, and vice versa. To put it a little differently, the center of the rainbow arc you're seeing sits on an imaginary line stretching from your eye to the sun. Since your eyes and those of someone else's, even someone next to you, can't be in the same place in space simultaneously, the two of you can never see the same rainbow. If that's not enough to ponder, consider this: Even our own two eyes see slightly different rainbows [sources: National Geographic, Rao, Science Kids].