The Mirascope

I saw my first mirascope upon my initial visit to the Exploratorium when I was eight. I can still recall my astonishment. The exhibit entices you to pick up an illuminated spring, about four centimeters high and two centimeters in diameter— yet when you reached out to touch it, the spring is simply just not there! This was some serious physics magic! Sometimes mistaken for a hologram, this image is not produced using a LASER with the physics of interference and diffraction, but instead produced only by mirrors and reflection.

Here is a demonstration of a mirascope:

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The physics of the mirascope is fairly simple and yet the resulting 3D ghostly image is astonishing in its appreance. The mirascope consists of two parabolic mirrors facing each other in a clamshell fashion. The key to the design is that the focal point of each parabolic mirror sits at the vertex of the other, and a hole is made in the top mirror's vertex where the image is produced. To understand how reflection of light can create such an image, consider the special geometry of a parabola. A ray diagram illustrates how parallel rays of light that reflect off a parabolic curve will all meet at its focus (figure 1). This is the operating principle of satellite dishes or any parabolic reflector. It of course works in reverse: a light source located at the focus will reflect off the curve and leave the dish as parallel rays, a phenomena used by microwave communication antennas and searchlight reflectors.
 

figure 1: Parallel rays reflect and converge at the focal point F
(Wikipedia)

Placing two parabolic mirrors into the mirascope configuration (as seen in figure 2) puts the object to be viewed at one focus. The light leaving the object at this focal point reflects off the top mirror into parallel rays directed down. These rays then hit the bottom mirror which reflects them a second time to converge at the top focal point, creating the image at the top.





figure 2: Diagram included on the packaging
of the Opti-Gone Mirage

Amazingly, the mirascope was discovered by accident. Here’s a brief summary of the account (as described in these student conference proceedings by Adhya and Noé, page 367).  Sometime around 1969 a custodial worker at UC Santa Barbara was cleaning out a storage closet in the physics department. The closet contained a collection of carefully stacked WWII surplus searchlight reflectors— parabolic mirrors, each with a hole in its center for an arc lamp to protrude through. Serendipitously, these reflectors were stacked and stored in a clamshell fashion. The worker, Caliste Landry, found that there was dust “floating” in air at the top hole of one of the reflectors that “could not be cleaned”. He reported what he found to one of the young physics faculty members, Virgil Elings, who figured out the physics of the situation. Elings and Landry were awarded a patent two years later for their “Optical Display Device”. The rights of the patent were acquired by Opti-Gone International in 1977, and for decades Opti-Gone was the main seller of mirascopes which they marketed under the name “Mirage”. Elings went on to found Digital Instruments Inc. in the 1990s, where he attained many patents on scanning probe microscopy—  however, the mirascope patent, a physics toy, was his first!