Why are rainbows curved, knowing that light travels in straight lines?
Because raindrops are spherical. If raindrops were small cylinders, with their axis vertical, then we would see two vertical straight rainbows (with the base of these rainbows just about where our conventional rainbow feet would be) If the raindrop cylinders were at an angle, you’d see two parallel straight rainbows at a similar angle, the same distance apart as the vertical ones were. Rotate the cylinder raindrops and their rainbows and you can get an idea why spherical raindrops make circular rainbows. They would be completely circular if the ground didn’t get in the way. You don’t usually see the bottom part of the rainbow for two reasons: there is much less rain between you and the ground, so the rainbow is fainter, and the rainbow is harder to see against the ground.
