Physics and Astronomy Demonstration Resources

          Demonstration Index                          

36. GEOMETRICAL OPTICS

36a. Blackboard Optics (1). Plane mirrors and plastic semicircles are attached to the steel-based blackboards in the lecture halls and the reflection and refraction of rays of light from sources also attached to the boards is shown.

36b. The optical disk. The lenses attach magnetically to a disk, which has a light source on a movable arm attached at its perimeter. The movable source is very helpful in showing the laws of reflection and refraction as the reflecting (or refracting) device can remain stationary. The perimeter of the disk is marked out in degrees, thus making simple any measurements. For large class use with TV projector.

36c. Continuous spectrum. The collimated light from a carbon arc light is directed via a prism onto a screen. The light will be seen to be broken up into a rainbow. A slit and projection lens may be used to produce a "cleaner" spectrum.

36d. Blackboard optics (2). A semi-circular block of plexiglass is attached to the blackboard and a single ray of light passes through it. With the curved face of the block facing the source, an angle will be found where the light is totally internally reflected.

36e. A light pipe (1) is made from crofon plastic. Laser light is injected into one end and the light is seen to emerge from the other, some 3 meters away. When viewed in a darkened room, the whole fiber is seen to glow due to surfaced defects.

36f. A light pipe (2). A bent piece of plexiglass light pipe for laser light. The first few internal reflections can be seen.

36g. Light pipes (3) forming a bundle of 77000 coherently arranged fibers is available to show image transmission. An intense image of (say) a slide is focused onto one end of the fiber, the emergent image being projected onto a screen. The pipe is bent twice.

36h. Brewster's Angle. Light from an arc passes through a polaroid and then is reflected from a glass plate onto a screen. As the angle of the plate with respect to the incident ray is changed, the degree of polarization will be seen to alter. At the Brewster angle, the reflected light will be seen to disappear as can be seen by rotating the polaroid.

36i. A glass ray tank is half filled with slightly soapy water and laser beams are reflected from the surface, the incident ray coming from either above or below the surface. The ray is visible in the water by scattering off the soap, above the water by scattering of smoke particles from a smoke gun. Reflection, refraction and total internal reflection may be shown.

36j. Incidence and refraction. The Leybold ray tank shows at one time the refraction of rays emerging from water to air, and also the critical angle and total internal reflection. Two light sources are available, one producing about 10 rays, of which 7 refract and 3 totally internally reflect. The other source produces a single ray whose angle is variable. The transition from refraction to total internal reflection may be shown.

36k. Ray tracing can be shown for lenses and mirrors using blackboard optics or the optical disk. A multi-ray source produces rays, the deviation of these rays being shown. The ray sources can be arranged so that its rays emerge parallel, divergent or convergent. For lenses the use of 2 light sources in place of the multi-ray source allow one to ray trace, show the workings of a magnifying glass, telescope, microscope, etc. The rays may be drawn in with chalk and the simple lens formula applied. Practice is required to get good representations of optical instruments.

36l.Ripple tank (1) can show geometrical optics using plane waves reflecting and refracting from submerged objects because "shallow water waves travel slower" for depths of a cm or so. Excellent film strips available for this experiment; local ripple tank still in preparation.

36m. Microwave reflection. A microwave generator produces (roughly) planar wave fronts. If a metal reflector is placed in the beam of radiation and a detector moved around in the vicinity of the transmitter, but behind it, a position of maximum reception will be found when the angles of incidence and reflection are equal. Refraction may be demonstrated with water filled or wax prisms. The wax prism is equilateral, the water prisms right angled. The receiver is again moved until the position of maximum reception is found. With the right angle prisms, total internal reflection may be shown. It is possible to demonstrate the critical angle, but the apparatus as it now stands tends to make the effect somewhat unclear.

36n. Microwave refraction. The focusing of microwave radiation with a wax lens is shown. The lens is set up about 1m from the transmitter, and the receiver moved along the "optical" axis until the position of the maximum is found. The focal length of the lens is about 30cm. As with all microwave demonstrations, care must be taken to minimize reflections of other projects in the vicinity that can produce odd effects due to interference. If the audio detector is used (see 35a), warbling intensity fluctuations will occasionally be heard as the demonstrator moves.

36o. Phantom light bulb: An inverted light bulb is located inside a box, on the outside of which is placed an empty socket, directly above the bulb. A concave mirror is set up at exactly 2 focal lengths from the light bulb and the image of the bulb is aligned with the empty socket. A person looking into the mirror will see a "phantom light bulb" in the socket, but at any wide viewing angle will see nought but an empty socket. Incidentally, the socket on top IS live, so a light bulb may be inserted if desired. Best for after lecture or with TV camera and projector.

36p. Radiant energy transfer: A heater is arranged at the focus of a parabolic mirror, and a book of matches at the focus of a second mirror. When the two are placed facing one another, and power applied to the heater, the matches will start to smoke after about 1 min, and burst into flames in less than two. If the match heads are blackened with a pencil, the times are greatly reduced.

36q. Focus of sound waves: A sound source (say a metronome) is placed at the back of the lecture hall and a microphone connected to an amp & speaker is placed at the focus of a parabolic mirror. When all the angles are correct, a very loud signal will be heard from the speaker. The microphone may be moved along the axis of the mirror to show the focussing effect.

36r. Subjective color may be shown by having a disk with a slot cut in it rotate in front of a bright red light. After images of green will be seen. Other demonstrations of this effect may be made. One very effective one is made from a disk of cardboard, one half of which is painted black. Arcs are drawn on the white half, each taking up 60 degrees in three groups of differing radii. If the disk is rotated at about 5 rps, pastel shades will be seen at each of the arcs, the color depending on direction and speed of rotation.

36s. Color mixing: The light from 3 projectors is arranged to overlap, and red, green & blue filters inserted, one into each projector. The intensities of the projectors are varied with three variacs. Just about any color can be produced with this system, and a discussion of color co-ordinates can be given. The concepts of Hue, saturation and intensity can also be presented.

36t. Water or ice lens: A plano-convex lens, made by freezing water in a watch glass, is used to focus light from a carbon arc source onto a match head. After a short while, the match will burst into flames. The demonstration must be performed as rapidly as possible, and the match head must be blackened to speed things up. Can use in vertical orientation by putting water into a horizontal watch glass. Can show that watch glass has no focusing effect by itself.

36u. Phase change at boundaries: The torsional wave machine (Shive type) is used with two of its sections, which have different wave velocities, coupled together. A wave is sent down the slower section, and is seen to speed up after the interface. An alternative is to have a spring connected to a piece of string, the whole thing being stretched across the lecture hall. A pulse sent down the string will be seen to slow down on hitting the denser medium. There will, of course, be reflections at the boundaries.

37. WAVE INTERFERENCE

37a. Laser Young's Experiment: Light from a laser placed at the back of the lecture hall is allowed to pass through a pair of slits placed side by side. The interference pattern on the projection screen will be about 5 feet wide. Many widths and separations of slits are available.

37b. Arc light Young's Experiment as in 37a except light source is a carbon-arc. Light passes through a color filter and a single slit, then through double slit. Pattern is quite weak compared with 37a, but you can see the effect of changing the color of the light. Using color TV should make it work, though this has not been checked out.

37c. Microwave Young's Experiment. Cover center portion of microwave horn to form two slits; explore intensity (using sound of modulated microwave source) of waves. Find central maximum, nodes on each side, then max again, then possibly a second minimum. Can estimate wavelength using first order minimum for which

r₂-r₁= λ/2.

37d. Sound Young's Experiment - (see experiment 19q).

37e. Water Wave Young's Experiment. The ripple tank using two point sources makes a sensational interference pattern. Can also use two slits with a plane wave passing through. Right now the film loops are far superior to our local ripple tanks.

37f. Ultrasonic Young's Experiment. Two ultrasonic transducers are connected to an oscillator and mounted side by side. A third transducer is used as a detector, an oscilloscope serving as an indicator. The interference pattern can be easily shown, as can the effects of changing source separation. The frequency is fixed.

37g. Fresnel Biprism: two small angle prisms are cemented together at their bases. Light from a somewhat expanded laser beam is shined on the boundary of the two prisms. An interference pattern appears on the screen. An arc lamp through a narrow slit or a long straight-filament unfrosted light bulb should also work to give colored fringes.

37h. Lloyd's mirror. Light from an arc lamp passes through a single slit and falls on a screen. Part of the light is intercepted by a front- surface mirror placed almost touching the slit. The result is interference on the screen between direct light and reflected light. Use of TV may be necessary. (In preparation)

37i. Microwave Lloyd's mirror. The microwave analogy of the above experiment may be set up. Waves from a microwave generator fall onto a detector either directly of after reflection off a mirror (a sheet of metal). The interference pattern can be shown by moving either the detector or the mirror. Changing the position and angle of the reflector can be said to be analogous to radio fading.

37j. Fresnel's mirrors. Light from a laser is made to diverge with a long focal length lens. The light falls at grazing incidence on a pair of front surface mirrors which are slightly tilted relative to one another. The two reflected wave fronts act as though there were two virtual sources. Interference pattern results where both "sources" are illuminating the screen. (In preparation)

37k. Soap film interference: Light from a carbon arc is reflected off a soap film, an image of the film being projected onto a screen. As the thickness of the film changes due to evaporation, magnificent colored fringes will be seen to run across the film, the motion being presumably due to convection. As the light is very intense, the films rarely last more than 30 sec, even with heat filters. With luck, however, just before the film breaks a region of blackness will appear at the bottom of the image (the top of the film). This black region is due to destructive interference between the waves when the thickness is less than one quarter wavelength.

37l. Newtons rings may be shown to a large class by forming an image of the apparatus on a screen by light from an arc reflecting off it. Colored rings will be seen at the center of the image, the size and separation of which can be varied by changing the tension between the flat and curved pieces of glass. The sodium vapor lamp gives good black minima.

37m. Pohl's fringes: Light from a mercury lamp is reflected off a piece of mica suspended vertically. The interference fringes formed by the light reflecting off the front and back surfaces of the mica will cover much of the lecture hall's walls and ceiling. The fringes show some color due to the multi-frequency nature of the mercury emission. Screen students from the source using a black cloth.

37n. RF interference - (See experiment 35g).

37o. A small Michelson interferometer is available for use with a laser. When set up, the device will produce interference fringes (either circular or parallel) in a patterns of about 50 cm diameter, and of sufficient brilliance for a large class to see. The effects of moving one mirror may be shown. The apparatus is so sensitive that the vibrations of the building cause a continuous jiggling of the fringes.

37p. A microwave Michelson interferometer can be constructed and the effects of moving mirrors, inserting different dielectrics into one of the paths and so on shown. By virtue of the greater wave length, the apparatus is far bigger than that of 37o, thus allowing the lecturer to point out all the pieces easily.

37q. Ruler interference: Light from a laser strikes a machinists rule (metal, engraved scale) at grazing incidence, the reflected light falling onto a screen some 4 or 5 meters away. The interference pattern is clearly visible, and, knowing the separation of the rulings on the scale, along with the distance of the screen from the scale, a calculation of the wave length of the laser light can be made.

37r. Double slit simulation: A double slit pattern can be modeled on the overhead projector using two sheets of plastic onto which have been drawn a number of concentric circles. The moire fringes formed when the two are placed on top of each other and projected are used to represent the maxima, and lie on hyperbolae. The separation between the two "sources" can be changed.

38. WAVE DIFFRACTION

38a. Single slit diffraction-laser: A variable single slit is placed in the beam of light from a laser and the transmitted light allowed to fall on a screen about 15 meters away. The diffraction pattern as the slit is narrowed is seen to get wider, break into striations and finally to reduce to a single bright line across the screen. Be sure that the two jaws of the slit move symmetrically. A wire (or hair) is placed in the path of a beam of light from laser, the beam eventually falling on a screen some 15 meters away. The diffraction pattern is clearly visible.

38b. Single slit diffraction-arc. As in 37b, substituting a single slit for the double slit.

38c. Single slit diffraction-microwaves. As with experiment 37c, but single slit replaces double slit. Slit width should exceed 4 or 5 cm.

38d. The ripple tank is set up to produce plane waves, and a single slit placed in the path of the waves. The spreading of the waves after passing through the slit as a function of wave length and slit width can be shown. The motion can be "frozen" by stroboscopic illumination. The film loop of this experiment is far superior to our tank results at this time.

38e. White light diffraction: Light from a carbon arc is passed through a single slit and allowed to fall on a screen. The diffraction pattern at the edge of any object placed in the path can be seen. As with all the white light physical optics demonstrations, the image is very dim (compared to the laser equivalents) but is bright enough to show to a class with the TV system.

38f. Poisson's spot-white light: Light from a lensless carbon arc placed at the back of the lecture hall falls onto a coin suspended from its edge from a stand half way down the hall. The shadow of the coin falls on a screen. If the TV system is set up well and focussed onto the center of the shadow, a bright spot will be seen. (Focussing at the edge gives a very good show of the diffraction effects at an edge).

38g. Poisson's spot-microwaves: A beam of microwaves falls on a circular metal disc set about 1.5m from the horn. The area of the "shadow" is probed with a detector, and at the center of the shadow a region of reasonable high intensity will be found. The focus of this intensity region can be traced, and is found to lie along a line from the center of the disc.

38h. Mills Cross: A very fine cross is marked in the center of a piece of blackened photographic film and a laser beam is shown through it. The diffraction pattern produced this way consists of a rectangular array of maxima, with regions of very high intensity at the corners of the rectangles. The Mills cross is sometimes used in radio and radar antennas, especially in direction finders. Cal also show 2-D diffraction by shining laser through a fine wire mesh.

38i. A large number of diffraction gratings are available, all of which may be illuminated by either the laser or the light from a carbon arc. 100, 500 & 13,000 line/in gratings are available as well as a 13,000 line/in blazed grating, where one side of the spectrum is brighter than the other.

38j. Sound Diffraction grating: A sound diffraction grating may also be shown. High frequency sound waves are allowed to fall on a number of slots in a board, and the maxima and minima are plotted with a microphone. The effect of changing wave length can be shown.

38k. Multiple slits: The increase in intensity and the decrease in width of an interference pattern as the number of slits between the source and the screen is increased is demonstratable. The light from a laser falls on a slide which has 1,2,3....6 slits cut in it. As the laser is directed onto an increasing number, the pattern changes as mentioned above. A beam expander is needed for N=5 and N=6.

38l. Zone Plates: An optical and a Microwave zone plate are available. In each case, the plate is set up some distance from the source so that the source appears to be a point radiator. The focus of the optical plate is shown by moving a screen along the axis until the bright spot is found. In the case of the microwave system, the detector is moved until the region of maximum intensity is found. The individual zones of the microwave plate may be removed, both odd and even being available in the complete set.

38m. Resolving power: Two brightly illuminated pin holes are observed through a telescope with a TV camera acting as the "eye". Card sheets with holes of different diameters cut in them are placed, one after the other, in front of the 4" objective. With a large aperture, the pin holes are seen to be cleanly resolved. As the diameter of the hole is decreased, the diffraction pattern grows until the pin holes merge. Alterations must be made to the TV camera's sensitivity as the objective diameter is reduced.