A commercially made Mirage Maker or Mind Boggling Optic Mirage, or the smaller, less expensive 3-D Mirascope (see below), all available from various online vendors
To Do and Notice
Put an object in the bottom of the apparatus. A coin works well, but a small, colorful object that looks like a push-button, resting on a “PUSH” sign, is an amusing alternative.
Notice that the object or button appears to be in the hole in the top of the device. Try to grab the object or push the button. There’s nothing there!
What's Going On?
You are seeing an image formed by two concave mirrors facing one another. The object is placed at the center of the bottom mirror. The curvature of the mirrors is such that the object is at the focal point of the top mirror.
When light from a point on the object hits the top mirror, it reflects in parallel rays. These parallel rays hit the bottom mirror and reflect so that they reassemble to form a point located at one focal length from the bottom mirror. The mirrors are placed so that the focal point of the bottom mirror is located at the hole in the top of the device. The end result is that light from every point on the object is assembled into an image in the hole.
The ray diagram may help explain this effect (click to enlarge).
The image produced by this apparatus is known as a real image, because the light that forms it actually passes through the location of the image. However, if you place a piece of waxed paper or onionskin paper at the location of the real image, the image will not appear on the paper. The outside regions of the mirrors that do not reflect light to your eyes do reflect light to the paper. The edges of the mirrors have large aberrations and create an image so blurred that it cannot be seen. This image is also known as an aerial image because it appears in the air and not on a screen.