Cold Spot?

What can you do with a giant 12-foot spherical mirror? Plenty. Especially if you find yourself in a crowd of Exploratorium staff members, all bent on blowing each other's minds.

You can use it to see distant objects with frightening acuity. You can use to make 3-D mirages appear before your eyes. You can also put it to its most popular use—as the most astonishing funhouse mirror this side of Wonderland—and squeeze your own upside-down head in your hands.

Built by NASA, the mirror in question was originally intended for use in a flight simulator. Now, courtesy of the Smithsonian's Air and Space Museum, it is on indefinite loan to the Exploratorium.

Recently, Exploratorium staff gathered around the shiny new arrival and engaged in a mutual show-and-tell. In the process of putting the mirror through any and all paces, an interesting physics question arose: Can you focus cold?

It's well known that you can focus heat. A classic Exploratorium exhibit called Hot Spot uses a 5-foot parabolic mirror to focus infrared radiation from a space heater. Place your hand at the mid-air spot, and you can feel the focused heat—infrared light—as an eerily localized hot spot.

Pete Stevens upped the ante with a hastily assembled companion exhibit to Hot Spot: Cold Spot. He placed a head-sized chunk of dry ice (temperature: -109° F, or -78° C) in front of the mirror and challenged others to place their hands at the location of the ice chunk's image. Some felt the cold spot. Others didn't. Naturally, the discussion was, er, heated.

What do you think? Should a mirror that can focus heat also be able to focus cold?

Try this yourself—make your own Hot Spot exhibit.