LAST AUGUST I DESCRIBED SOME of the dazzling and instructive visual effects that can be achieved with the pure, intense light of lasers. I said I would later set forth some ideas on how an amateur might prepare a laser light display in the home or the classroom. I shall now do so, giving in addition some of the many suggestions that came from readers as a result of my earlier piece.

The musical group Genesis has been accompanied by laser for a long time, and it was at one of the group's concerts that I saw a particularly beautiful display. During one of the numbers stage-hands blew onto the stage a thick fog of dry-ice vapor. A laser beam was directed upward through the fog at a small angle to the vertical. The beam was visible because small droplets of the fog scattered the light. A mirror mounted on a motor caused the beam to rotate about the vertical at a speed that matched the tempo of the music. The beam originated in a pulsed gas laser, so that the cone formed by it was chopped into discrete lines. I think the gas must have been a mixture of krypton and argon, which in a laser can generate more than one color, because what I saw was a glittering, rotating cone of light made up of four different colors. The effect was magical.

David Yoel, one of my students at Cleveland State University, devised a means of creating a more modest cone of light that my class would see end on rather than from the side. We had neither a multicolor laser nor a machine for making dry-ice fog, and so we worked with our 15-milliwatt helium-neon laser and smoke generated by a magician's stage explosive. The laser light was reflected by a mirror Yoel had mounted on the end of a rotating shaft by means of a hinge and a spring.

When the shaft was stationary, the mirror was perpendicular to it and reflected the laser beam direct to the middle of a screen. When the shaft was rotated, centrifugal force caused the mirror to tilt. The amount of tilt increased with the speed of the shaft, thereby increasing the deflection of the beam.

The rotation swept the beam around in a cone centered on the middle of the screen. Yoel controlled the size of the cone by adjusting the speed of the shaft. The stiffness of the spring is important in this apparatus; keep trying springs until you find a suitable one.

If you have a multicolor laser, you can split the cone into different colors by putting into the beam a dispersing device

such as a prism or a diffraction grating. Rig a motor that will make the device oscillate back and forth through the laser beam. The colors emitted by the laser emerge at different angles because of the dispersion. As the prism or grating oscillates, the emerging colors are swept across a slit the light must pass through to reach the rotating mirror. The color that goes into the air then varies as the prism or grating moves. Plans for making a laser that emits several colors appear in Light and Its Uses, which is cited in the bibliography for this issue.

The explosion of gunpowder serves the purpose of putting into the air a number of particles that will scatter the laser light. The particles augment the visibility of the cone. If the air were perfectly clean, neither the beam nor the cone would be visible except by reflection from the screen. We rig four flashbulbs that can be fired when a single button is pushed. They ignite lengths of magician's flash paper, which set off gunpowder sprinkled on the paper.

The particles are in the size range where they scatter light by both diffraction and reflection. As a result the light is strongly scattered forward and backward but weakly scattered to the sides. For demonstrations to students I put the laser on a stage and aimed it toward optical systems I had set up near the back of the room. The rotating mirror reflected the laser light back to a screen at the front of the room. (If you attempt a similar demonstration, make sure the beams are high enough to be well above the head of a tall person standing up in the room. Fix the laser and the optical systems rigidly in place to prevent an accidental misalignment from directing the beam into the audience.) When the laser was switched on just after the explosions, the air above the audience was laced with shimmering threads of red light.

Our laser's output of 15 milliwatts was more than enough for any one display, and so Yoel positioned small glass slides at an angle to the beam in order to achieve a variety of optical effects by splitting off part of the light. A slide transmits most of the light reaching it but reflects some. In order to control the direction of the reflected light he set up several solenoids, each with a small mirror mounted on its shaft. When a shaft was extended from a solenoid, the mirror directed the beam to a particular array of optical devices. When the shaft was retracted, the beam passed the mirror and went to another optical system on the other side of the solenoid. Yoel could shift the beam from one system to the other by means of a switch that controlled the current to the solenoid.

In my August piece I offered to send readers samples of a particularly useful Ronchi filter. Readers who obtained the filter can create sharper images in an interference pattern by peeling off the protective layer of Mylar on one side of the grating. I still have filters; if you would like one, send $1 and a mailing label to me at the Physics Department, Cleveland State University, Cleveland, Ohio 44115. The display generated by the filter is best when the laser develops at least a few milliwatts of power. It may be disappointing with weaker lasers.

James Watson of Ball State University called my attention to a Ronchi filter that is sold by the Holex Corporation (P.O. Box 27056, Philadelphia, Pa. 19118). The filter gives rise to a rectangular array of diffraction spots. Vic Rice of Campbell, Calif., sent me a sample of a halftone screen he employs to create the same optical effect. The screen can be either positive or negative.

In August I also described how interference fringes can be created by directing laser light through a sheet of clear plastic

that has been coated with airplane glue or Duco cement. I tried adding small pieces of plastic and metal to the glue. The best results came from plastic strips, none more than a millimeter wide, that are sold under the name Diamond Dust by the Permafrost Division of Potter Industries, Inc. (377 Route 17, Hasbrouck Heights, N.J. 07604). The material is also sold in hobby shops. With this system I got not only bright and dark fringes but also bright spots resulting from the refraction of light through the pieces of plastic. When I rotated the sheet through the beam, which was expanding because I had put a convex lens in its path, the bright spots slid around on the screen.

Several of the people who sent for Ronchi filters also told me about their own experiments with laser light. Christopher Heilman has devised a way to project a laser image that resembles a photograph of the sun made in one color: the color of an emission line of hydrogen. He employs a lens to expand the beam from his five-milliwatt helium neon laser and then directs the beam through a small glass sphere of uneven density. The result on his screen is an illusory sphere.

Roger Warden works with a textured plastic disk that he mounts on a slowly turning motor. When the laser beam intercepts one of the deep scratches he has made on the plastic, a starlike pattern appears on the screen. Warden also suggests trying small drops of water on the plastic. Frank B. Fadich uses cut lead glass crystals and plastic figures to create patterns with his five-milliwatt helium-neon laser. He also creates interesting figures when he inserts in the beam a small transparent ball with many flat faces. Such balls are available in hobby shops. Gertrude Reagan generates interference patterns by directing a laser beam through the neck of a gin bottle and out through the bottom. The patterns vary as she moves the bottom of the bottle around in the beam.

Gary F. Benedict of Chandler, Ariz., chops a continuous beam, such as one from a helium-neon laser, by inserting in it an opaque disk with teeth on its perimeter. As the disk is rotated the teeth periodically block the beam. This effect may be useful in obtaining a cone of many rays of light like the one T saw at the rock concert. Instead of totally blocking the beam you could work with a disk that has reflecting teeth. The light is reflected to other optical systems whenever the teeth are in the beam. Benedict also suggests using a rotating cylindrical glass rod with its long axis perpendicular to the laser beam. When the rod is inserted in the beam, it acts as a convex lens and redirects the beam. If the rod is rotated rapidly, the persistence of vision holds the pattern that is swept out.

Many people wrote to me about ways of reflecting a beam to form patterns on a screen. Tom Glanzman of Duke University reflects a laser beam from an encapsulated pool of mercury resting against a loudspeaker. The vibrations from the speaker generate ripples on the surface of the mercury. The reflection of the laser beam varies accordingly.

Robert J. Kearney of the University of Idaho mounts a small mirror on a rubber diaphragm that he attaches to an inexpensive speaker. The response of the mirror, he says, is relatively insensitive to the thickness of the rubber. The system can be adjusted by adding small weights to the rubber in order to control its patterns of vibration. (Any mirror used with a laser should be front-coated and of good quality, otherwise the beam will be excessively dispersed.)

One of the most interesting techniques came from Arthur Eisenkraft of Ossining, N.Y. He mounts a lightweight mirror on a speaker by means of a small loop of tape. The mirror is available from Metrologic Instruments, Inc. (P.O. Box 307, 143 Harding Avenue, Bellmawr, N.J. 08031). The mirror can move freely because of the looseness of the mounting. Eisenkraft tinkers with the position of the mirror until it reflects a laser beam both vertically and horizontally with equal strength. Without the adjustment the mirror would reflect the beam along an axis on the screen, thereby giving elongated patterns.

Eisenkraft was interested in the patterns that result when records of various kinds are played over the speaker With "Sgt. Pepper's Lonely Hearts Club Band" by the Beatles he got a mixture of pure Lissajous figures and erratic designs. The pure patterns were probably generated by the electronic music the Beatles meshed into the song.

The best pieces for creating laser tracings on the screen were recordings of electronic music by Keith Emerson. In particular Eisenkraft suggests the Karelia Suite by Sibelius, which is part of the album "Five-Bridge Suite" by Emerson and the Nice.

I have had good luck with much of the electronic rock music recorded by European groups. The Tangerine Dream group works almost exclusively with synthesizers. When its music is played on the two-mirror speaker I described in August, hypnotic Lissajous figures appear on the screen. The single-speaker system I also described in August responds only to electronic music in the lower frequencies.

In addition to turning electronic music into patterns on a screen you can convert the light back into sound. Eisenkraft demonstrated this capability by inserting a photocell in the beam. The signal from the photocell went successively to a preamplifier, an amplifier and a speaker, thus recovering the electronic signals encoded in the original music.

Teachers whose classroom equipment includes a ripple tank can mount mirrors on the paddle that generates the ripples. Several people also suggested mounting a mirror on the shaft of a motor with the plane of the mirror at a slight angle to the shaft. When the motor is engaged, the mirror turns, forming the beam into a cone of light. If the beam is chopped before it reaches the mirror, the effect is similar to the cone I saw at the Genesis concert. On the screen you see not a continuous trace of a pattern but dots that appear to chase one another along a curved path.

Gary E. Tomlinson of Grand Rapids, Mich., told me about a system of three such rotating mirrors. Each mirror is glued to the end of a shaft at an angle. By adjusting the angle of tilt of each mirror and the arrangement of the motors Tomlinson creates circles, ellipses, stars, squares and other figures on his screen. A patented system of this design is available from Lasertrace (26 Station Road, Westgate-upon-Sea, Kent, England CT8 8RT).

Gregory Yob of Palo Alto, Calif., has built a system of five motors with tilted mirrors mounted on their axes. He arranges for the mirror with the smallest tilt to be the first one reflecting the laser beam, which then goes to each of the other mirrors in succession. The power supply for the motors is shown in the illustration in Figure 5. Yob says the circuit is designed to make the motors operate synchronously.

Before the beam reaches the mirrors it is chopped by a wheel with 16 triangular teeth. The wheel is mounted on a motor that is attached to a lead screw. The motor is connected to a slide potentiometer so that Yob can control the speed of the wheel. The lead screw is driven by a gear-reduced motor. With this rig Yob can vary the extent to which the teeth intersect the laser beam. The more they cut into the beam, the less space there is for its transmission. Dots appear on the screen. When only the tips of the teeth intersect the beam, dashes appear.

Yob has devised a simple apparatus for modulating the reflection of a laser beam with his hand or his voice. His laser is mounted vertically in a stand and set on a tripod. The stand has a top about six inches square. The laser beam passes through a central hole in the top This arrangement also protects the operator from the vertical beam.

To intersect the beam Yob fastened a lightweight mirror to a piece of rubber stretched over a fruit-juice can. The rubber, which was made for balloons, and the mirror, which is coated with aluminum on its front surface, came from the Edmund Scientific Company (101 East Gloucester Pike, Barrington, N.J 08007). The other end of the can is left open. To direct the reflected beam Yob either moves the can in the laser beam or speaks into the open end of the can to make the mirror vibrate. He can also put textured plastic on the top of the laser housing. Moving the plastic through the beam creates the images I discussed in my August article. Yob says good displays can be obtained from a stained glass known as clear German crackle.

Abid Tanovic of Venice, Calif., wrote to me about his use of a dove prism to redirect the laser beam and form interesting patterns on a screen. A beam of light entering a slanted face of a dove prism parallel to the long side refracts toward that side, reflects from it and leaves the prism through the other slanted face, traveling in its original direction. At the long side the reflection is total, which means that because of the angle at which the beam hits that face and because of the index of refraction of the glass no light can refract out of the prism there.

If the prism is rotated about the axis of the incident light, the image is rotated twice as much. For example, if you look through the prism parallel to its long side while rotating it 180 degrees around your line of sight, the scene you see through the prism rotates 360 degrees. Tanovic incorporates this effect in his laser display by mounting a dove prism inside a tube he has fixed in a bearing block. The tube is rotated rapidly by means of a belt connected to a motor. He points out that the prism could be mounted directly in the center of the tube and that the motor should be variable in speed from 800 revolutions per minute down to zero in controlled steps.

If a laser beam is sent through the rotating prism, it forms a circle on the screen. The size of the circle depends partly on how far from the center of rotation the laser beam entered the prism: the closer to the center the beam is on entering, the smaller the circle is. It also depends on the angle at which the beam enters the prism. To create more interesting formations on the screen Tanovic places near the prism diffraction gratings, Fresnel lenses, textured glass, shaped plastic or any of the other things I have described. The resulting images are fascinating as they form, rotate, spread and contract. With a pulsed laser or a chopped beam the images on the screen lie in a circle, and each successive image is oriented differently.

Tanovic lights his displays with a krypton laser emitting blue, green, yellow and red. He directs the beam through a regular prism in order to separate the colors. They fall at different places on a mirror he: has mounted on a vibrating speaker. Then the light passes through his rotating dove prism and proceeds to the screen. The colors on the screen are in their spectral order from the center of the display to the perimeter. By rearranging the optical components he can make the central color either blue or red.

The techniques I have described are just a few of the possible ones for a laser-light show. To make laser beams visible oil dispersions blown into the air can be substituted for the dry-ice fog and the smoke from theatrical explosive. Beware of making a mess with the oil. I once ruined a screen with it. A laser beam will also give rise to interference patterns if it is directed through a viscous fluid that is being mixed by heating. As the light encounters varying refractive indexes in the fluid the beam interferes with itself and casts magnificent interference patterns on a screen.

Intriguing interference patterns develop when a laser beam is directed through a plastic material that is being slowly melted. Some of the commercial laser-light shows employ plastics that are melted by the laser beam itself, but this effect takes more power than an amateur's laser is likely to have. Finally, the 3 entire laser show can be controlled by a home computer programmed to actuate the optical devices in time to the music from a tape recorder.

In December, 1979, I described a set of experiments involving sauce béarnaise, a warm emulsified, mixture served over meats. The sauce consists primarily of dilute vinegar, wine, egg yolks and butter. In my experience it is a highly frustrating sauce to prepare because of its tendency to curdle, frequently just as I am serving it to a guest. In my discussion I raised three questions. What factors stabilize the sauce so that it remains smooth and appealing? What factors cause the droplets of butter in the sauce to flocculate, that is, to coalesce into unappetizing pools? If the sauce does fail in this way, what can the preparer do to salvage it? Colleen Kelly, Rachel Kleinman, Karen Mehlman and Craig Deutsche, who are students at the Westlake School for Girls in Los Angeles, wrote to me about experiments they did to answer these questions.

The answers I gave were not definitive because the present knowledge of the physics and chemistry of the sauce is sketchy. Two models have been formulated to describe the interaction of the butter droplets and the mechanism of a failure of the sauce. One model sees the droplets as being coated with negative electric charges. Surrounding the droplets is an "atmosphere" of positive charge. When two droplets of butter move close to each other, they repel each other because their atmospheres have charges of the same sign. With sufficient charge on the droplets and in their atmospheres they will not easily flocculate. The sauce is said to be stable.

In the other model the droplets are visualized as being coated with lecithin from the yolks of the eggs. Each molecule of lecithin is oriented with its lipophilic end pointing toward the butter droplet and its hydrophilic end pointing away from the droplet. The hydrophilic end binds water from the surrounding solution. As a result the butter droplets are covered with a protective layer of water molecules. Butter droplets that might flocculate are prevented from doing so by the water layer.

According to the first of the models, the sauce fails if the charge on the butter droplets is too low. When droplets collide, they coalesce, eventually forming a pool of butter. In the other model failure results if the droplets have insufficient lecithin. Then they do not have enough protective water to prevent coalescence.

As an egg ages, the lecithin disintegrates, reducing the ratio of lecithin to cholesterol. Whereas lecithin is an emulsifier for butter droplets in water, cholesterol is an emulsifier for water droplets in butter. An aged egg may not provide the lecithin necessary to stabilize the sauce but may still provide the cholesterol that can ruin the sauce.

The students who told me about their work were concerned with the role of the egg yolks in the sauce. Mehlman made a successful sauce of vinegar, butter, lemon juice, cream and egg yolks. The egg yolks had been kept in the refrigerator, out of their shells, for four days. After preparing the sauce she added one and a half teaspoons of laboratory-grade cholesterol to a third of a cup of sauce. The mixture flocculated, but she could restore its smoothness by stirring it vigorously. The additional cholesterol apparently destroyed the emulsification of the sauce.

Kleinman made two batches of sauce from egg yolks, lemon juice and unsalted butter. In the first batch she used fresh eggs; the sauce was smooth and stable. For the second batch she aged the eggs for a week. the sauce flocculated. She then stirred in a tablespoon of liquid lecithin, which is available at healthfood stores. The sauce was restored.

Half of this batch was left as an experimental control. To the other half Kleinman added two teaspoons of cholesterol. The sauce flocculated. These results appear to support the hypothesis that lecithin is a stabilizing agent and cholesterol is a destabilizing one. Still, the sauce is unpredictable. When Kleinman made a third batch and added several teaspoons of cholesterol, the sauce thickened but would not flocculate.

Kelly prepared a sauce with egg yolks, red-wine vinegar and margarine. When she added several teaspoons of cholesterol, the sauce flocculated. It was restored to smoothness when she stirred in liquid lecithin. In another batch of sauce she used eggs she had stored for six days. The sauce was not stable until she stirred in several teaspoons of lecithin.

The students also tested their sauces for an effect of electric charge on the butter droplets. If the sauce is stabilized by charged atmospheres surrounding each droplet, the ions released by inorganic salts may deplete the charge of the atmospheres enough for the droplets to flocculate. In some cases salts releasing multivalent ions should promote flocculation more than salts releasing singly valent ions.

The students prepared samples of sauce to which they added variously sodium chloride, magnesium chloride, aluminum chloride, iron (III) chloride, sodium sulfate or sodium orthophosphate (NaCl, MgCl₂, AlCl₃, FeCl₃, NaSO₄ or Na₃PO₄). No consistent pattern emerged. The sauce showed no increased tendency to flocculate when multivalent ions were released by the salt. The students note that with different recipes different results may follow the addition of a salt. They conclude that the stabilization of a sauce béarnaise is more likely to be due to the lecithin than to charged atmospheres.

I also heard from Madeleine Kamman, a professional chef and cooking teacher who wrote The Making of a Cook, one of the few scientifically oriented cookbooks. She politely took me to task on my anatomy of a failed sauce. "The trouble is that you lovely scientists want to scientify everything. That sauce is fickle." I agree, but then again her own approach is hardly unscientific.

Kamman's secret for stabilizing a sauce béarnaise for as long as four hours, which is necessary in a restaurant, is to prepare an initial infusion that is more liquid than usual. The sauce is put into a pan that is put into another pan containing water at a temperature of 140 degrees Fahrenheit to keep the sauce warm. As liquid evaporates, the sauce thickens. The smart cook will know exactly when to remove it from the warming pan before the butter starts to separate into a layer. At that point a bit of salted water at a temperature of 120 degrees F. is whisked into the sauce. Its effect, Kamman says, is to reduce the acidity that builds up during evaporation. The addition of salted water is also partly for taste. Moreover, it and the whisking may be necessary to break up butter droplets that flocculate during the long period of sitting.

I think flocculation is less likely in the initial infusion because the extra water makes collisions of butter droplets less likely. As the water evaporates flocculation becomes more likely. The distribution of charge in the sauce may also change. Whisking and the addition of water are then needed to reverse flocculation and to make collisions of butter droplets less likely.

Bibliography

FUNDAMENTALS OF OPTICS. Francis A. Jenkins and Harvey E. White. McGraw-Hill Book Company, 1976.

A MATRIARCHAL SOCIETY: SAUCES. Madeleine Kamman in The Making of a Cook. Atheneum Press, 1978.

LASER ART & OPTICAE TRANSFORMS. T. Kallard. Optosonic Press, 1979.

LASERS. In Light and Its Uses: Making and Using Lasers, Holograms, Interferometers, and Instruments of Dispersion. With introductions by Jearl Walker. W. H. Freeman and Company, 1980.

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