Waves 3


	Let us take as an example water, in order to explain a few concepts which will help understand the experiment of electron interference. An instrument present in many physics laboratories to investigate water waves is the ripple tank. This device contains an engine which causes an arm to vibrate, the latter being connected to a terminal partly immersed in water. The vibration thus transmitted to water, generates waves.

Example of ripple tank., lab device for educational experiments on water waves. An engine transmits a regular motion of oscillation to one or more arms partly immersed in water. The act produces waves, propagating on the water surface. In the case illustrated (an excerpt from the 1976 movie “Electron Interference”) the vibration is applied to two spherically shaped terminals. As we shall see in a while, the waves coming from two sources, overlapping, originate the phenomenon of interference.


	The shape of waves produced by the ripple tank depends on that of the terminals used, for instance if that is a planar plate, the front wave is rectilinear. If the object is spherical or pointed, as in the case of the film above, circular waves are produced. A different way to produce a circular wave is to locate on the path of the plane wave an obstacle with a small opening in the middle (small in the sense of being smaller than the wavelength, as we shall see in the next page). Under these conditions, a circular wave propagates beyond the obstacle, as shown in the next film excerpt shown.

Production of waves in the ripple tank: the case of rectilinear waves and that of circular waves. The latter in this case are not produced through a spherical terminal, but in the alternative way, placing an obstacle with a little opening on the path on which plane waves travel (excerpt taken from the 1976 Italian TV show “Electronic Microscopy” – Elaboration carried out on material from the archives of RAI Rai-Teche)

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