Physics and Astronomy Demonstration Resources

          Demonstration Index                          

39. ATOMIC STRUCTURE

39a. Photoelectric effect: A zinc plate is carefully polished with emery paper and placed on an electroscope and charged negatively. Upon illuminating the plate with light from a mercury or carbon arc, the electroscope is seen to discharge. If the initial charge is positive, no effect is seen. Placing a sheet of glass (which absorbs UV) in the beam of light illuminating a negatively charged plate will cause the plate not to discharge, as will rotating the plate so that a dirty face (hence high work function) faces the light source. There are many alternative ways of setting up this demonstration. A good alternative for humid weather uses the same electroscope and plate, but places a spiral electrode between the plate and the light. The plate is maintained either negative or positive with respect to the electrode and the deflections observed as before. A simple addition to the electroscope allows it to be used as a "ticking ammeter", the number of deflection ("ticks")/sec being a measure of photo-current.

39b. h/e Experiment: Light from a mercury arc is split with a grating into its spectral lines, the light from each line being allowed to fall in turn onto a vacuum photocell. A potential difference is maintained across the cell by a battery and potentiometer system such that the anode of the cell is at a negative potential with respect to the photocathode. For each spectral line, the counter potential is varied until the current through the cell drops to zero. It will be found that the required potentials for stopping the photoelectrons will depend on the frequency of the illumination. Currents may be only a few nano-amps. Data can be taken and a plot of frequency vs stopping potential is made. The slope is h/e. Not an easy experiment, needs practice.

39c. Spectra: A continuous spectrum from a carbon arc, an absorption spectrum of a sheet of Didymium glass (showing many band and line absorptions) and a line spectrum from a mercury arc can be projected onto the screens in the lecture halls. In each case the spectrum is about 6 feet wide. The mercury spectrum shows a yellow doublet, a green, a blue and a violet line. There are also a couple of lines in the near ultraviolet that can be observed when a fluorescent screen is placed in that region of the spectrum. (There is also a bright UV line in the carbon arc spectrum.) 39d. More spectra: Students are given (to keep) sections of grating material, and are asked to observe the spectra of a large number of sources placed at the front of the lecture hall. The He, Na, Cd, K, & T1 sources are very bright and easy to see. The N, CO₂, Ar, H, O, H₂, O₂, and many other sources are somewhat dim, but many can be seen throughout the halls.

39e. Film vibration modes: The apparatus of 37k is set up, but this time a small loud speaker is placed behind the soap film. With the speaker being driven with a variable frequency oscillator, the image of the soap film is observed and at certain frequencies is seen to have bright rings on it, but none at frequencies in between. The lines are the nodes of oscillation and may be seen, on close observation, to have a number of pips around them all equally spaced.

39f. A number of photo-voltaic cells are available, for demonstrating the conversion of light into electricity. One is capable of producing about 500ma & 6v in bright sunlight.

39g. The Franck-Hertz experiment may be performed wherein electron collisions with mercury atoms is observed. The kinetic energy of the electrons is varied and the current transmitted through the tube is measured. When the accelerating potential is equal to (or some multiple of) the resonance potential, inelastic collisions will occur, and the current through the tube drops. With a good set-up, about 5 peaks (4 valleys) will be observed.

39h. A set of holograms are available for display after lecture (or with TV), each being about 15cm square. They may be observed on the provided viewer (which uses filtered white light) or with a laser. If a laser is used, the image is bright enough to be viewed by the TV system. One notable thing about using the TV is that the lens must be wide open, and thus the depth of focus is shallow. It will be found necessary to refocus when viewing the more distant parts of the image after viewing the nearer. Perspective changes can be shown by moving the camera.

39i. Thermal radiation: That all bodies radiate can be shown by using a thermopile connected via an amplifier to a galvanometer. With sufficient gain, the radiation from a hand can be "seen" at about 1m. Match flames, soldering irons and the like can be seen from much greater distances.

39j. Thermal emissivities: The apparatus of 39i is set up in front of a cube filled with hot water. The faces of the cube are covered with materials of differing emissivities. The changes in radiation intensities for the surfaces is shown.

39k. Continuous spectrum: The spectrum of a white light source (a filament) is observed with diffraction gratings given to all students. The temperature of the filament is varied with a variac, and the progressive diminution in the intensity of the blue end of the spectrum when compared to the red as the temperature falls is shown.

39l. Inelastic electron collisions: An electron beam is passed through a tube containing helium at low pressure. The velocity of the electron beam is varied and the number of SCATTERED electrons observed on a collector ring surrounding the electron beam. At certain potentials, there will be a great increase in the number of scattered electrons, those that have collided inelastically with the helium atoms in the tube. If the initial velocity of the electrons is changed continuously (say sinusoidally) and the x axis motion of an oscilloscope beam modulated in the same way, the current from the collector may be used to drive the y deflection and a continuous plot of the current made. The tube in use at the moment shows 3 separate peaks, corresponding to 3 energy levels, in the curve before ionization sets in.

40. MISCELLANEOUS MODERN PHYSICS

40a. Electron diffraction: Electrons are accelerated through a potential difference of between 5 & 10 Kv, and fall on a target. The diffracted electrons form an image on a fluorescent screen at the front of the tube. The target has 2 types of material deposited on it. In the 1st, 3rd and 4th quadrants is aluminum oxide powder. With the electron beam passing through any of these, a ring pattern is produced, the diameter of the rings being proportional to the electron wavelength (which may be varied as the pattern is observed). The 2nd quadrant contains hexagonal graphite. Steering the beam here will produce one of 3 patterns.

1.) Rings, as above, but of different diameters produced by tiny Carbon crystals.

2.) Rings that are obviously made up of a number of irregularly spaced bright spots (larger crystals).

3.) A hexagonal diffraction pattern from one crystal of carbon.

This last is quite difficult to get. Steering and focus are critical. All the patterns can be shown fairly well on the TV system, the central maximum being blocked out to prevent overloads on the camera.

40b. Probability demo: A couple of probability demonstrators are available. In each case, balls are allowed to roll down a ramp that has nails placed in a regular array on its surface. Collectors at the bottom allow for counting the number of balls deflected laterally through some distance.

40c. Crystal diffraction of microwaves: Parallel microwave radiation falls onto a styrofoam cube inside which is an array of small metal reflector balls. The scattered radiation is picked up by a microwave lens and focussed onto a detector. As the cube ("crystal") is rotated, angles will be found where the scattered radiation is quite intense. The angle of the receiver to the transmitter is variable. If the output of the detector is fed into an oscilloscope and the crystal rotated continuously in synchrony with the time base of the "scope", a "Bragg Pattern" can be shown. The crystal may be sat on the turntable so that the orientations of the reflectors inside may be made to correspond to the 100, 110 or 111 axes.

40d. Electron beam tubes: The old standby, the Maltese Cross tube, may be shown to demonstrate linear propagation of electrons. A cold cathode and a hot cathode versions are available.

40e. Gas discharge tube: A long glass tube with electrodes in either end is connected to an induction coil and the tube observed as the pressure inside slowly is reduced. Streamers, striations, Crooke's dark space and so on are all easily seen. What is not seen is the quantity of x-rays produced by the tube, so long operation is not advisable. Once pumped to near darkness, another gas (say CO₂) may be slowly admitted to the tube, and the change in color of the discharge seen.

40f. Frustrated Total Reflection: Microwave radiation is totally internally reflected in a water prism as seen by a detector placed to receive the reflected radiation. A second receiver placed in line with the transmitter sees nothing. If, however, a second 90 degree prism is placed so that the long faces of the two are parallel, and the second is moved slowly towards the first, progressively more energy will be transmitted and less reflected until, when they are much less than 3cm (1 wavelength) apart nearly all the energy is transmitted. A nice analogy to penetration of potential barriers in wave mechanics.