You Can't Make a Rainbow Disappear
Not only can you create rainbows, you can make them disappear too! And it doesn't involve something like chanting, "Rain, rain, go away. Come again another day!" All you have to do is grab a pair of polarized sunglasses, hold them in front of you vertically, and – poof – no more rainbow. What's going on?
Rainbows are highly polarized objects, which basically means their light waves are vibrating in one plane – in this case, a vertical one. Sunglasses are also polarized vertically. This is because they're created to block reflections and glare, which typically come off of pools of water and other flat surfaces, and are polarized horizontally. Horizontal light waves can't get through vertical polarizers. Thus, if you're wearing vertically polarized sunglasses, you can see vertically polarized rainbows. But turn those sunglasses on their side, effectively creating a horizontally polarized set of shades, and the rainbow's light waves will be blocked, causing the rainbow to mysteriously disappear [sources: Plait, Polarization].