Why are rainbows always curved? –David, San Francisco
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It takes something of an intuitive leap to see why this should be so, but let’s give it a crack. Water droplets reflect light at an angle of between 40 and 42 degrees, depending on the wavelength. (The difference in wavelengths is what separates rainbows into different colors. But that’s a story for another day.) Because of the sharp angle, you only see rainbows when the sun is (1) behind you and (2) low in the sky. When the sun is high, the light reflecting off the droplets passes over your head and you see nothing.
Now for a little creative visualization. The sun is low and behind you. All the sunbeams head in, strike the cloud of water droplets ahead of you, and bounce back at an angle of 40 degrees. Naturally the beams can bounce 40 degrees any which way–up, down, or sideways. But the only ones you see are the ones that lie on a cone with a side-to-axis angle of 40 degrees and your eyes at the tip.
