6

LIGHT

As with electricity and magnetism, the study of light developed independently until 1865 when James Clerk Maxwell (1831-1879), by a purely theoretical argument based upon a missing symmetry in the known laws of electricity and magnetism, added a term to the equations and predicted a form of electromagnetic wave that travels with a speed exactly equal to the then-known speed of light (3 × 108 meters/second). In one swoop he consolidated light, electricity and magnetism, as well as other forms of electromagnetic waves, many of which were yet unknown. Although light is a form of wave, in many instances, its behavior can only be understood by resorting to a particle description, not unlike the description that prevailed from the time of the ancient Greeks up through the time of Isaac Newton (1642-1727). The modern theory is that light has a dual nature that requires a quantum mechanical description. Light is a particularly suitable choice for demonstrating electromagnetic waves because complicated instrumentation is unnecessary since our bodies come equipped with a pair of remarkably sensitive and versatile optical detectors.

Laser Safety Considerations

Lasers are especially useful sources of light for demonstrations because they are monochromatic, coherent, collimated, and intense. However, the increasing use of lasers in optics demonstrations poses special hazards particularly to an unwary audience[1-4]. Since most such demonstrations involve the use of relatively low power lasers operating in the visible range, the discussion will be restricted to them. High power lasers and lasers operating outside the visible range pose additional hazards and should only be used by a laser expert. The greatest hazard from visible lasers is damage to the retina of the eye. When light is focused by the eye, the intensity on the retina is concentrated by a factor 104 to 106 over that falling on the pupil of the eye. The dangers include the production of visible lesions; permanent, partial "bleaching" of the pigment for one particular light color; and complete blinding if the optical nerve is hit. Special glasses or goggles can be used to protect the eyes, but they are usually impractical in a demonstration before a large audience.

To estimate the danger, one must know the power or energy output, P, of the laser in watts or joules; the beam diameter, D, at the aperture of the laser in centimeters; and the beam divergence, Ø, in radians. The intensity of radiation at a distance d (in centimeters) away from the laser is then calculated from I = 4P/(pi)(D+dØ)2, in watts per square centimeter if P is the laser power, or joules per square centimeter if P is the laser energy. This value can be compared with the damage threshold values in Table 6.1. It can be seen that even small demonstration lasers can cause eye damage if the beam strikes the eye directly. The laser light can usually be safely viewed after diffuse reflection (such as from a piece of paper), but specular (mirror-like) reflections must be avoided. Note also that an individual laser may exceed the specifications listed by the manufacturer.

In order to enhance safety, one should never allow the laser to operate unattended. The illumination in the room should be kept as bright as possible to constrict the pupils of the observers. One should set up the laser so that the beam is not at normal eye level. A diffuse, absorbing, fire-resistant target should be used. One should remove all watches and rings and other shiny objects in the vicinity of the laser. Lenses can be used to defocus the beam, and shields can be placed to prevent the beam from going beyond the area needed for the demonstration. In general, the laser should be treated as an ordnance piece with similar security and safety measures. With these precautions, the laser permits many, very impressive, visual demonstrations.

REFERENCES

1. E. A. Lacy, Handbook of Electronic Safety Procedures, Prentice-Hall: Englewood Cliffs, New Jersey (1977).

2. D. Sliney and M. L. Wolbarsht, Safety with Lasers and Other OpticalSources, A Comprehensive Handbook, Plenum Press: New York (1980).

3. D. C. Winburn, Practical Laser Safety, Marcel Dekker, Inc.: New York (1985).

4. L. L. Davey, Editor, Code of Federal Regulations, Food and Drugs,Title 21, Office of the Federal Register, National Archives and Records Administration, U. S. Government Printing Office: Washington (1985).

Table 6.1

Approximate Damage Threshold Values for Retinal Tissue for Various CW and Pulsed Lasers in the Visible (0.4-0.7 µm) Range (minimum reported)[2]

          Pulse Length        Damage Threshold Intensity
            
              CWa                  10 x 10^-3 W/cm2
            100 nsec             (100 x 10^-6 J/cm2)b
             30 nsec               70 x 10^-6 J/cm2
             20 nsec              130 x 10^-6 J/cm2
              1 nsec             (120 x 10^-6 J/cm2)b
             30 psec               18 x 10^-6 J/cm2
aAssumes a one second maximum exposure.
bEstimates.

6.1

Spiral Light Guide

A long, solid, plastic, spiral rod is illuminated with a low power laser to illustrate the property of total internal reflection in a light guide.

MATERIALS

*Available from Central Scientific Company and Sargent-Welch Scientific Company

PROCEDURE

The rod is heated and bent into a spiral or other shape. The laser illuminates one end of the rod which is polished. The entire rod lights up along its length due to scattering off imperfections in the rod. A bright spot appears at the opposite end of the spiral. The effect is best when viewed in subdued background light. Such a configuration illustrates the principle of the light guide* (also called a "light pipe").

One can show other examples of light guides, especially the flexible kind made of many fine strands of glass (fiber optics). With modern, low loss fibers, light can be sent for many miles with very little attenuation[1-5]. Attenuation in modern fibers are less than 1 dB/km. Such technology could eventually revolutionize communications because the high frequency light provides a transmission medium with extraordinarily large bandwidth. Rates as high as 27 gigabits per second have been demonstrated. This is equivalent to 400,000 telephone conversations. By maintaining the relative position of the many strands within the bundle (a coherent fiber bundle), it is possible to pipe an image from place to place. Such devices are useful for seeing in places that are inaccessible to the eye, such as inside someone's stomach by use of an endoscope.

DISCUSSION

The light guide illustrates the principle of total internal reflection. According to Snell's law,** light traveling in a medium with index of refraction n1 and incident on a material with index of refraction n2 at an angle (theta)1 with respect to the normal at the interface, is reflected at an angle (theta)2 with respect to the normal. If n1 is greater than n2, then there is a critical angle (theta)1 = (theta)c such that (theta)2 = 90°. Light incident on an interface with (theta)1>(theta)c will experience total internal reflection. In this case, n1 = 1.51 for plexiglas, and n2 = 1.0003 for air. Thus the critical angle is (theta)c = sin-1(n2/n1) = 41.6°. The condition for total internal reflection is easily met provided the light guide is long and skinny and not bent at too sharp of an angle.

**The experimental discovery of this law is usually credited to Willebrord Snell (1591-1627), but it was also deduced from the particle theory of light by René Descartes (1596-1650) and is known as Descartes' law in France.

HAZARDS

This demonstration is safe provided one exercises normal caution to prevent the laser and its specular reflections from shining in anyone's eye.

REFERENCES

1. W. S. Boyle, Scientific American 237, 40 (Aug 1977).

2. S. J. Buchsbaum, Physics Today 29, 23 (May 1976).

3. E. A. Lacy, Fiber Optics, Prentice Hall: Englewood Cliffs, New Jersey (1982).

4. E. E. Basch, ed. Optical-Fiber Transmission, Howard W. Sams & Company: Indianapolis (1987).

5. C. D. Chaffee, The Rewiring of America: The Fiber Optics Revolution, Academic Press: San Diego (1987).

6.2

Water Light Guide

A stream of water illuminated with a laser or high intensity white light acts as a light guide.

MATERIALS

*Available from Sargent-Welch Scientific Company

PROCEDURE

The tank is filled with water which comes out a hole in the side in a stream[1,2]. The light is provided by a carefully aimed laser that shines through a window opposite the hole through which the water streams. The window can consist of a piece of glass bonded with putty or bathtub cement over a hole in the tank. Alternately, a light bulb can be lowered into the water. In the latter case, the bulb should be as bright as possible (a few hundred watts), and the inside of the tank should be white or shiny so as to reflect as much of the light as possible. The water can be caught in a trough on the floor. The tank should be tightly sealed to prevent escape of the light, but it must allow air to enter as the water leaves. The effect is best viewed in a darkened room. Fluorescin or other fluorescent substance may be added to the water for increased visibility. The tank should contain enough water to provide a nearly constant flow for about a minute (a few gallons), and the height of the water column should be a good fraction of a meter to provide sufficient pressure at the aperture as required for a steady stream of water. One can hold one's hand in the water stream to illuminate it and to show that the stream actually consists of water. As the level of water in the tank decreases, the stream bends at a sharper angle until suddenly the condition for total reflection is violated, and a bright spot is seen on the opposite wall.

DISCUSSION

The index of refraction of water is 1.33, and thus the critical angle for a water/air interface is sin-1(1/1.33) = 49°. So long as the water stream does not bend at too sharp an angle, light traveling along the length of the stream strikes the water/air interface at an angle greater than 49° with respect to the normal to the interface and is thus totally reflected.

HAZARDS

If a laser is used, care should be taken to ensure that it does not shine into someone's eye. If a light bulb is immersed into the water, it is best to use a low voltage bulb, such as an old 12-volt automobile headlamp to minimize the danger of electrocution. The lamp should be immersed in the water before it is turned on to prevent it from being broken by the thermal shock. The water stream should be aimed so that it will not damage other equipment, cause an electrical hazard or make the floor slippery.

REFERENCES

1. H. A. Robinson, ed., Lecture Demonstrations in Physics, American Institute of Physics: New York (1963).

2. A. Kshatriya, Am. Journ. Phys. 44, 604 (1976).

6.3

Tubeless Television

A visual image is made to appear in midair by waving a light-colored stick near the focal plane of a slide projector containing a slide of some appropriate subject, illustrating the persistence of vision and the scanning process in television.

MATERIALS

PROCEDURE

The slide projector is aimed at a dark-colored, irregularly shaped background some distance behind the lecturer so that the image of the slide on the back wall is very dim and out of focus. The stick is waved so rapidly that one's persistence of vision produces a continuous image. The effect is best when viewed in subdued light. It is necessary to focus the projector at a distance such that the image can be clearly seen from the back of the room and to wave the stick at the right place to obtain a well-focused image. Some practice is required to get the optimal distance and light level. Alternately, the slide projector can be placed on the floor behind the lecture table and aimed upward. A mirror at the back edge of the lecture table then deflects the light horizontally so that the projector is not visible to the audience. In the place of the stick, one can also use a vibrating string or rope. A picture of some well-known figure such as Albert Einstein is appropriate.

DISCUSSION

The principles involved are rather rudimentary and involve geometrical optics and the physiological process of persistence of vision. One can wave the stick at various rates to illustrate the extent of the visual persistence. The effect is analogous to the role of the persistence of vision in television and motion pictures and to the scanning process in the transmission of a television picture.

HAZARDS

There are no significant hazards with this demonstration other than inadvertently striking something with the stick or stumbling in the dark.

6.4

Optical Illusions

Transparencies containing optical illusions are projected on the wall or a screen to illustrate the role of subjectivity in scientific experiments.

MATERIALS

PROCEDURE

Although optical illusions are often discussed in the context of psychology and art, they can also be used to illustrate principles of physics. Books and articles[1-10] provide numerous examples of optical illusions.Many of these illusions can be photocopied onto transparencies and projected on a screen for viewing by a large audience. Impressive three-dimensional illusions can also be created but generally must be viewed from a particular angle and thus are less suitable for large groups.

As an example of how optical illusions can be used, one might show the figure of a clown that turns into a circus scene when rotated by 90° or the sketch of George and Martha Washington watching troops out a window that turns into a portrait of Washington when turned upside down. Such illusions are amusing and surprising, but can also be used to illustrate the role of subjectivity in physical observation.

DISCUSSION

Frequently, detailed observation of a phenomenon reveals information not apparent and sometimes even contradictory to that gleaned from a casual observation. The result of a physical measurement often depends upon the frame of reference of the observer as in the theory of relativity, and the act of making an observation can change the thing observed as in the Heisenberg uncertainty principle. As human beings, and as scientists in particular, we must observe carefully and objectively and guard against the tendency to see in nature only what we expect to see.

HAZARDS

There are no significant hazards in the observation of these illusions.

REFERENCES

1. J. Frederick, Classical Illusions, McPherson & Co.: New Paltz, New York (1985).

2. E. Lanners, ed. (Translated and adapted by H. Norden), Illusions, Holt, Reinhart and Winston: New York (1977).

3. M. Gardner, Mathematical Magic Show, Random House: New York (1977).

4. S. Luckiesh, Visual Illusions; Their Causes, Characteristics andApplications, Dover Publications: New York (1965).

5. S. Simon, The Optical Illusion Book, Four Winds Press: New York (1976).

6. L. Kettelkamp, Tricks of Eye and Mind, The Story of OpticalIllusion, William Morrow and Company: New York (1974).

7. D. D. Hoffman, Scientific American 249, 154 (Dec 1983).

8. F. Attneave, Scientific American 225, 62 (Dec 1971).

9. G. H. Fisher, Perception and Psychophysics 4, 189 (1968).

10. M. L. Teuber, Scientific American 231, 90 (Jul 1974).

6.5

Talking Head

Reflections from a mirror mounted beneath a table give the illusion that a disembodied head is sitting on top of the table.

MATERIALS

PROCEDURE

The mirror is mounted underneath the table so as to cover the whole space from the under side of the table top to the floor[1]. A shag carpet or straw on the floor helps conceal the bottom edge of the mirror. Near the back of the table in the center and behind the mirror, a circular hole is cut through the table top of sufficient diameter to pass the head of the subject. The table is initially covered with a cloth that reaches to the floor on every side.

A volunteer is taken from the audience around to the back of the table by an assistant who helps the volunteer kneel or sit with the head protruding through the hole in the table while the demonstrator, who at this point emulates a magician, holds the tablecloth in such a way as to conceal from the audience what is happening to the volunteer. A little patter about the similarities and differences between science and magic fits in well here. A few grunts and groans from behind the cloth add to the drama. Finally, when the volunteer is in place, the cloth is removed, and the audience is presented with the illusion that a disembodied head is resting on the table. One can carry on a conversation with the head, culminating perhaps in a pun about restoring the body while the victim is "ahead." In a magic show, the volunteer would be removed from the table, and the audience would be left to wonder how it was done. However, in a science demonstration, the volunteer should be removed in view of the audience while the demonstrator explains the trick.

The illusion is most effective if the mirror is very clean and its edges are concealed. The table should be placed well away from other obstructions that would be eclipsed by the mirror. If a carpet is used, it must be aligned carefully so that there is no discontinuity at its edge where it goes behind the mirror. One should be careful not to stand directly in front of the table lest the reflection of one's legs be seen in the mirror. The table should not be placed too close to the audience to avoid reflections of the audience. The illusion works best if the audience is seated slightly above the level of the top of the table. The proximity of the audience to the head would seem to favor discovery of the trick, but, on the contrary, it is indispensable to its success.

DISCUSSION

Although this demonstration is more amusing than educational, it serves to introduce, motivate, and illustrate the idea that the angle of incidence is equal to the angle of reflection in geometrical optics. It also illustrates the danger of being deceived in making observations of nature and of the importance of considering all possible explanations of a phenomenon before reaching a conclusion.

HAZARDS

There are no significant hazards with this demonstration. Care should be taken when moving the mirror to avoid breaking it, and the volunteer should be cautioned not to lean or push against the mirror.

REFERENCE

1. D. H. Charney, Magic, Strawberry Hill Publishing Company: New York (1975).

6.6

Pepper's Ghost

The phenomenon of the partial reflection of light at the interface between two media with different indices of refraction is illustrated by an illusion in which a person or other object is made to disappear and reappear at will.

MATERIALS

PROCEDURE

The simplest demonstration consists of a candle and a beaker of water[1]. The beaker of water is placed behind the glass, and the candle is placed an equal distance in front of the glass so that the audience sees the beaker through the glass and sees the reflection of the candle in the glass. It looks as though the candle is burning under the water. A black shield between the candle and the audience prevents the audience from seeing the candle directly.

A more elaborate demonstration[2] requires a much larger piece of glass (at least five feet square). The glass is placed at a 45° angle to the audience so that the audience sees a combination of light passing through from behind the glass and light reflecting off the glass at a 90° angle from the line of sight. Spotlights on dimmer controls can alternately illuminate the area behind the glass or the area off to the side. One light should be turned up while the other is turned down in such a way that the total light intensity is nearly constant. This can be done automatically with a single control, or with practice by manipulating a pair of controls, one in each hand. In this way, people and other objects can be made to appear and disappear at will. It helps if the background behind both objects is dark and if a partition is provided to prevent the audience from directly seeing the object off to the side. The room lights should be lowered to prevent the members of the audience from seeing their own reflection. By proper manipulation of the lights, a volunteer can be turned into a ghost or skeleton and back again. It is important that the person and ghost or skeleton be placed in exactly the right position and that a volunteer be chosen who is nearly the same height as the ghost or skeleton (see figure). With proper placement, the figure will change continuously from one to the other independent of the viewer's position in the audience. In practice, a viewing angle of about 60° can be accommodated, depending upon the width of the glass. This arrangement also makes an effective and dramatic entrance and exit for the demonstrator. Glitter sprinkled from above in place of the skeleton aids in the appearance and disappearance.

DISCUSSION

A light beam incident on the interface between two transparent media of different indices of refraction (such as air and glass) is partially transmitted and partially reflected. Thus the light emitted from such an interface consists of the superposition of light traveling through the glass from behind and light reflected off the glass from a direction such that the angle of incidence is equal to the angle of reflection. By controlling the relative intensities of the two sources of light, one scene can be made to transform into another.

HAZARDS

There are no hazards with this demonstration other than the danger of breaking the glass.

REFERENCE

1. D. H. Charney, Magic, Strawberry Hill Publishing Company: New York (1975).

2. J. P. VanCleave, Teaching the Fun of Physics, Prentice Hall Press: New York (1985).

6.7

Prism Rainbow

A rainbow is produced by passing a collimated beam of white light through a glass prism illustrating that white light is made up of many different colors.

MATERIALS

PROCEDURE

White light from a slide projector or other bright source is allowed to pass through a glass prism and projected on a screen or light colored wall. A bowl of water with a mirror immersed in the water can be used instead if a prism is not available. The effect is best if the light is highly collimated and the room darkened. This can be accomplished by placing a slide with a transparent slit about a centimeter wide in the projector. Alternately, if the sun is in the proper position, light passing through a window and a hole in a shade can be used. A laser can be used to demonstrate that the effect occurs only for white light. To illustrate better the formation of rainbows by raindrops, a spherical flask of water can be used. Mirrors can be used to rotate the spectrum into the usual position (arched in the center with red on the top). With a variac the intensity of the lamp can be controlled to illustrate that at low temperature the light is mostly red but that the shorter wavelength colors successively appear as the temperature of the lamp is increased. Point out that there is radiation below the red (infrared) and above the violet (ultraviolet) but that our eyes are not sensitive to these wavelengths. The band of wavelengths to which the eye is sensitive is the same as the wavelengths for which we receive the maximum radiation from the sun. This apparent coincidence is probably a result of biological evolution.

DISCUSSION

The dispersion of white light into its constituent colors occurs because the index of refraction of water and most glasses varies with the wavelength of the light. Typically, the index of refraction is given by n = A + B/(lambda)2, called the Cauchy formula, where A and B are experimental constants that depend upon the type of glass and (lambda) is the wavelength. Substances with a large value of B have the most dispersion. For most glasses, the index of refraction is about a percent greater at the short wavelength (violet) end than at the long wavelength (red) end of the visible spectrum. The result is to refract the violet light more at the interface between the air and the glass. The triangular shape of the prism causes the refraction at the entrance and exit interfaces to add rather than cancel as they would for a plate of glass.

Prism spectrometers are useful for determining the composition, temperature, and distance of stars. In the laboratory, spectrometers are used to identify the composition of materials, since each chemical element has its own characteristic spectrum of emission lines.

The scattering of sunlight off moisture and density fluctuations in the atmosphere is what makes the sky blue and the sunset red[1]. When the sun is near the horizon, light from overhead is mainly a result of the large scattering of the blue end of the spectrum, while light coming from the direction of the sun is very little scattered and thus appears red.

HAZARDS

One should not look directly into a bright source of light but rather observe the spectrum as projected on a screen or wall.

REFERENCE

1. R. Greenler, Rainbows, Halos, and Glories, Cambridge University Press: Cambridge (1980).

6.8

Laser Gun

A sufficiently powerful laser is used to burst balloons from across the room.

MATERIALS

PROCEDURE

If a sufficiently powerful laser is available, spectacular demonstrations can be done if one is careful to avoid the hazards inherent with intense light sources (see the section on Laser Safety Considerations). A laser of at least a watt average power is desirable. Pulsed lasers with high peak power and lasers outside the visible range can also be used but are more dangerous and are not recommended. The beamline should be arranged to avoid the possibility of shining the laser into someone's eye, and the usual precautions of avoiding specular reflections should be strictly adhered to. The laser beam should terminate on a beam dump capable of dissipating all the power of the laser without significant reflection or heating. A long, corrugated metal pipe, blackened on the inside and gently bent into a circular arc makes a suitable beam dump. Reducing the background room light makes the laser beam more visible but also makes the eye more sensitive and susceptible to damage.

If the laser is of marginal power, one can focus the laser beam with a pair of lenses. A diverging lens with a long focal length is placed in the beam. The diverging beam is then brought to a focus with a converging lens with a short focal length. Not only does this make a very intense spot, but it defocuses the beam and makes it safer for the audience.

With a laser of this class, it is possible to burn holes and ignite pieces of paper and to burst balloons. A light-colored paper or balloon reflects most of the light and is more difficult to burn or burst than one with a dark color which efficiently absorbs the laser light. A particularly nice demonstration is to enclose a black balloon inside a clear one. The laser beam can be made to pass though the clear balloon and burst the black balloon. Balloons filled with flammable gas (methane, hydrogen, etc.) cannot be ignited directly because such gases are transparent to the laser beam, but with an appropriate piece of paper taped to the balloon, the paper can be set on fire, and the gas indirectly ignited.

DISCUSSION

Although a watt is a relatively small amount of power, the laser has the ability to concentrate this power in a spot of only a few millimeters diameter. If the surface on which the laser beam is incident absorbs the light efficiently enough, the heat can puncture or ignite the material. The color and to a lesser extent the texture of the target determines how much of the light is absorbed. Light not absorbed is either reflected or transmitted through the target. A magnifying glass outdoors on a sunny day can similarly be made to concentrate light to the point where it ignites a target. With a sufficiently bright incandescent or arc lamp and an appropriate lens, the same can be done in the lecture room to illustrate that there is nothing special about using a laser other than its natural collimation.

When performing this demonstration, it is difficult to avoid implications about the prospect of directed energy weapons[1,2]. Although it appears to illustrate the feasibility of such applications, it can equally well be used to illustrate the ease with which a target can be made such that practically none of the laser light is absorbed.

HAZARDS

This demonstration is potentially very dangerous. If the light from a laser of this class impinges on one's eye it can cause almost instant blindness. This demonstration should thus be done only by someone experienced with the use of high power lasers and only after taking the necessary precautions to ensure that the laser beam cannot be reflected into someone's eye. The laser is also capable of burning one's skin and setting fire to any flammable materials placed in its path.

REFERENCES

1. K. B. Payne, Laser Weapons in Space: Policy and Doctrine, Westview Press: Boulder, Colorado (1983).

2. N. Bloembergen and C. K. Patel, Rev. Mod. Phys. 59, 3, part II (1987).

6.9

Laser Beam

The beam from a low-power laser is used for a number of simple demonstrations.

MATERIALS

*Available from Metrologic Instruments Inc., 143 Harding Avenue, Bellmawr, NJ 08031 (609) 933-0100

PROCEDURE

A low-power (few milliwatt), continuous, visible laser (helium-neon or equivalent) is set up so as to project a beam across the front of the lecture hall. The room is darkened to render the beam visible. The beam can be made much brighter by clapping together two chalk-board erasers covered with chalk dust or by use of a commercial smoke generator.

The possible uses for such a laser beam are very numerous[1,2]. One can illustrate the monochromatic nature of the laser light by passing it through a glass prism and comparing the result with the result of passing a collimated beam of white light through the same prism. The beam can be scattered off various objects. A fine-mesh screen or diffraction grating produces an interesting interference pattern when illuminated with a laser. The beam can be passed over a knife edge, through various fluids and vapors or through lenses of various types. A laser beam can be reflected off a small mirror mounted on a loudspeaker connected to a source of sound. With two such speakers, one can construct a crude, low-frequency oscilloscope and display Lissajous patterns by connecting the speakers to sinusoidal sources with harmonically related frequencies.

For a sophisticated audience, the following puzzling demonstration is recommended. An equal amount of water is poured into each of two Styrofoam drinking cups. The laser is aimed at one of the cups for a few seconds. The cups are turned upside down. Water comes out of one cup, but little or none comes from the cup that was illuminated with the laser. The water was soaked up by a sponge surreptitiously placed in the bottom of the cup before the demonstration. This trick doesn't help one understand light but does give a good opportunity to discuss the importance of careful observation and controlling all the variables in an experiment.

DISCUSSION

A laser is characterized by being intense, collimated, monochromatic and coherent. A laser beam travelling through clear air is essentially invisible because there is very little scattering of the light off the air molecules. Chalk dust or smoke provide a multitude of tiny particles from which the laser beam can scatter. Since the laser beam is monochromatic, it is deflected through a constant angle by a prism and is not dispersed. Coherence means that the photons of the light are all vibrating in synchronism with the same phase rather than with random phases as in ordinary light. The monochromatic and coherent nature of the beam makes it ideal for interference and diffraction experiments.

HAZARDS

The only significant danger from a laser in this class is eye damage from looking directly into the beam. Care should be taken in setting up the demonstration to ensure that the laser beam cannot be directed or reflected into someone's eye.

REFERENCES

1. H. H. Gottlieb, ed., 101 Ways to Use a Laser, Metrologic Instruments Inc.: Bellmawr, New Jersey (1984).

2. E. Schmidt, Phys. Teach. 27, 30 (1989).

6.10

Fluorescence

Materials illuminated with ultraviolet light are made to emit visible light.

MATERIALS

*Available from Aldrich Chemical Company, 940 W. Saint Paul Avenue, Milwaukee, WI 53233

PROCEDURE

After a motivation consisting, for example, of viewing a rainbow spectrum of white light, one introduces the idea of invisible light, in particular, infrared and ultraviolet. The room lights are extinguished, and an ultraviolet lamp turned on. Ultraviolet lamps can be purchased from most vendors of scientific supplies or from florists or hardware stores. Various minerals and other fluorescent materials are placed under the lamp. Some articles of clothing fluoresce nicely. Common household products that flouresce are Murine eye drops and Pearl Drops toothpaste. The adventurous demonstrator can brush his or her teeth with such toothpaste and continue the lecture with teeth glowing bright yellow. The coating on the inside of the glass of a fluorescent lamp works well except for the fact that ultraviolet light does not easily penetrate the glass. Breaking such tubes is not recommended since the fluorescent material and the mercury in such tubes are highly toxic.

A particularly appealing demonstration is to add about 0.01 g of fluorescein (Na2C20H10O5) to 500 ml of an aqueous solution of 0.8% polyethylene oxide (Polyox WSR-301). Polyethylene oxide is a viscoelastic liquid with many strange and delightful properties such as the ability to self-siphon[1]. With a pair of 1000 ml beakers, one can start pouring the liquid from one beaker to the other. When one tries to stop pouring it, the liquid continues to run up and over the rim of the upper beaker in seeming defiance of gravity. The stream of liquid can be cut with a pair of scissors, and the upper part of the stream will jump back into the upper beaker. The fluorescein renders it visible under ultraviolet light and provides quite a striking visual effect.

DISCUSSION

Fluorescence is the process whereby a substance absorbs light at a short wavelength and then re-emits it at a longer wavelength. All that is required is appropriately spaced energy levels in the atoms that constitute the material. The fluorescent lamp is the most common example. In such a lamp, electrons produced by a filament at one end of the tube travel down the tube and excite atoms of mercury vapor in the tube. The mercury emits ultraviolet light which strikes the fluorescent coating on the inside of the glass.

The atoms of a fluorescent material typically re-emit their radiation after about 10-8 seconds. By contrast, phosphorescent materials continue to emit after the incident radiation is removed, sometimes for hours. Such materials are used in paints and on the hands of watches and clocks. Other light-emitting processes are chemiluminescence in which the light is generated by a chemical reaction and triboluminescence in which the light is generated by a mechanical stress in a crystal.

Viscoelastic liquids consist of molecules containing long chains of atoms, often numbering into the millions. If such materials could be viewed under enormous magnification, they would look like a plate of spaghetti. With the molecules sufficiently entangled, they exhibit both high viscosity and elasticity. One needs to exercise some care in handling these materials since heat or vigorous stirring can break the molecules and degrade the effect. They should be prepared within a day of use and stored in a refrigerator.

HAZARDS

Intense ultraviolet light can damage the eyes. One should avoid prolonged staring at such lamps. In extreme cases, other parts of the body can receive a sunburn.

REFERENCE

1. A. A. Collyer, Physics Education 8, 111 (1973).

6.11

Twinkling Stars

A laser beam passed over the top of a Bunsen burner produces a spot on the wall that twinkles like a star.

MATERIALS

PROCEDURE

A laser is set up so that the beam passes over the top of a Bunsen burner and makes a spot on the wall some distance away. After pointing out the spot on the wall to the audience, the Bunsen burner is lit. The spot begins to dance around like a twinkling star. Then the Bunsen burner is extinguished, and the spot stops moving.

Alternately, in place of the laser, one can use a slide projector and a photographic slide of stars. Lacking a suitable slide, one can make one with aluminum foil with a number of pin holes perhaps arranged in the form of the Big Dipper or some other easily recognized constellation.

Don't miss the chance to play that famous composition by Mozart that everyone knows as "Twinkle, Twinkle, Little Star."

DISCUSSION

Light travels in a straight line only when traveling in a medium with constant index of refraction. The index of refraction of air depends slightly on temperature. Thus the hot air above the flame of the Bunsen burner deflects the light slightly, and in a time-varying way, due to the turbulence of the air above the flame. This is exactly what happens when one views the stars through the turbulent atmosphere of the earth. In that case the temperature gradients are smaller, but the path length is much longer.

The myth abounds that stars twinkle, but not planets. If this is so, it is only because on the average the planets appear larger, and thus the effect is less noticeable. The apparent position of the moon presumably dances around in a similar way, but it occupies such a large segment of the sky that we never notice it.

This phenomenon is what makes it ineffective to simply build ever larger telescopes to improve the resolution of objects in the sky. Satellite-based telescopes or telescopes positioned on the moon would benefit from larger size.

HAZARDS

The usual precautions concerning lasers should be observed. Set up the demonstration so that there is no chance of the laser beam being directed into someone's eye. The Bunsen burner is an obvious source of fire and burns, especially if the demonstration is done in subdued illumination.

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J. C. Sprott