Magic Windows

"Hiding Images in Plain Sight: The Physics Of Magic Windows"

I recently made a physical object that defies all intuition. It's a square of acrylic, smooth on both sides, totally transparent. A tiny window. But it has the magic property that if you shine a flashlight on it, it forms an image. And if you take it out in the sun, it produces this 3D hologram. This post describes the math that went into making the object, and how you can create your own...