Wave Motion and Interference
by throwing simultaneously two stones into still water, and a number of interesting variations may be obtained by varying the size of the stones and their distance apart.
The experiment may be arranged for projection by using a surface of mercury instead of one of water, and agitating it by means of a tuning-fork, to the ends of whose prongs are attached light pieces of iron wire which dip slightly into the mercury.
The arrangement of the apparatus is shown in Fig. 11. The light of an electric lamp is concentrated on a small mirror, by which it is reflected through a lens to the tuning-fork, whose ends dip into a surface of mercury. It is reflected by
the mercury surface back through the lens and passes to another mirror, by which it is reflected to form an image on a distant screen. Fig. 12 shows the resulting disturbance of the surface. The circular ripples which diverge from the points of contact of the forks are represented by the circles. These move too rapidly to be seen in the actual experiment, but may be readily recognized in an instantaneous photograph. The heavy lines are the lines of maximum disturbance, where the two systems of waves meet, always in the same phase; while the lighter parts between represent the quiescent portions of the surface, where the crests of one system meet the troughs of the other, forming stationary waves. Fig. 13 is a photograph of the actual appearance.
Light Waves and Theib Uses
Another striking instance of interference is furnished by two tuning-forks of nearly the same pitch. Take, first, two similar forks mounted on resonators. When these are sounded by a cello bow, the resultant tone may or may not be louder than the component tones,but it is constant—or, at least, dies away very slowly. If, now, one of the forks be loaded by
fastening a small weight to the prong, the sound sinks and swells at regular intervals, producing the well-known phenomenon of “beats.” The maximum occurs when the two vibrations are in the same phase. Gradually the loaded fork loses 011 the other until it is half a vibration behind; then there is a brief silence. This may be shown graphically by allowing each fork to trace its own record along a piece of smoked glass, and by adding the two sine curves, as shown in Fig. 14.
The matter of the interference of light waves requires special treatment on account of the enormous rapidity of the vibrations. This statement, however, inverts the actual chronology, for this rapidity is inferred from the interference experiments themselves.