Maybe this should have been obvious, but it took me totally by surprise: rainbows are made entirely of polarized light! (I’m guessing this is because of how the light bounces off the insides of the raindrops on its way back to you.) So if you put on polarized lenses (like some sunglasses) and tilt your head sideways, you can make them disappear— or make them look twice as bright against the non-polarized sky!