TOYS, TRICKS, & TEASERS

By Donald Simanek

Reflections on an Illusion

An illusion that never ceases to fascinate is the mirror-produced real image. It can appear so real that you’ll reach out to grasp it, but your fingers close only on thin air.

»

Nineteenth-century books of science recreations often included the “phantom bouquet” (Figure A), which produced an upright real image of a bouquet of flowers hidden in a box below. The flowers had to be upside down because the concave mirror inverts the image.

You can duplicate this pretty effect with the concave side of a large shaving or cosmetic mirror, as shown in the old engraving in Figure A.

A somewhat more stable version uses a wooden box or stand enclosing the bouquet on all sides except the one facing the mirror. Paint the inside of the box matte black, or, better yet, line it with black plush or velvet cloth. Illuminate the bouquet, perhaps with a small desk lamp. The bouquet, and its image, will both be a distance R from the mirror, where R is the mirror’s radius of curvature.

A nice variation is the “phantom light bulb.” A light socket is fastened to the top of the box, and a second one is fastened upside down inside the box below it. Only the latter is powered. When the lower bulb is lit, its image appears in the socket above, if everything has been well aligned. If you use a clear bulb, get two of them. Put one bulb in each socket. Turn on the power, and the bulb above seems to be glowing. You can then unscrew the bulb and remove it, yet its phantom remains.

If you use a small-diameter mirror, the object should be relatively small. Use an old Christmas lamp and socket, or a decorative candelabra lamp. Frosted bulbs work best. Electrical and electronics supply stores have sockets that can be mounted on a flat surface.

One problem remains. You need a large concave mirror for the most effective presentation, and shaving or cosmetic mirrors are a bit small. Larger mirrors can be purchased at scientific supply houses, but are a bit pricey.

164 Make: Volume 18

An Educational Toy and

A Work of Art

For less than $50 you can buy such mirrors, packaged as a device that produces another neat version of this illusion. The Mirage is made by Opti-Gone ( optigone.com) in two versions: with 9" or 22" mirror diameter.

Assembled, it looks something like the conventional flying saucer, complete with a hole in the top from which you’d expect tiny aliens to emerge (Figure B). It’s sold by science stores, museum shops, and science-supply sources.

Most people put this device on a table and display the illusion as a work of art (Figure C). But I’m sure MAKE readers will want to take it apart and see what else can be done with its parts.

The Mirage consists of two parabolic mirrors, the upper one having a large hole in its center. The mirrors fit together in a clamshell arrangement, separated exactly two focal lengths apart. When light from a distant object falls on a spherical mirror, the rays converge to a point called the focal point that is about R/2 from the mirror, where again R is the mirror’s radius of curvature. The convergence isn’t perfect, and it’s much better when the mirrors are parabolic. For mirrors this size, you don’t notice the slight difference between spherical and parabolic.

The mirrors of the Mirage can be used separately. In a darkened room, let the light from an open window fall onto one mirror. An image of the window will appear near the mirror’s focal point. You can only see it if the reflected rays from the mirror surface enter your eyes, so you have to look back toward the mirror surface. This is a bit tricky because your head gets in the way of some of the incoming light. Or, you can place a small sheet of paper at the image location, and see a reduced-size image of the window projected onto the paper.