By carefully observing the wave motion of the water surface caused by a stone thrown into a pond, it is possible to ascertain that vibrating water particles have equal periods, because vertical movements of each oscillating particle are proportional to those of particles which are successively submitted to the same disturbance. In other words, oscillatory motion is isochronous as it is in pendulum clocks. Two particles that simultaneously reach the top of a wave crest are in phase; the distance between two crests is called wavelength, while the distance between the top of a crest and the underlying and supposedly stationary water surface is called amplitude of the oscillation. The time required by a particle to perform a complete oscillation is called period, while the number of oscillations in a second is called frequency.

As a consequence of this, and as anybody can although roughly observe on a water puddle, the longer is wavelength, the lower is frequency, and vice versa, high frequency always corresponds to short wavelength.

Sometime after the stone has fallen into the pond, wave motion gradually decreases from the centre to the periphery, producing damped waves. But if we throw a stone into the same point where the previous one had fallen, and then another one and so on, throwing objects of the same size, same weight (and applying the same strength) in a perfectly rhythmic and regular way, wave motion continues and produces continuous waves: we have thus created an oscillator.

Now let’s throw two stones in the pond in two different points and in different times, and we will see another phenomenon: the oscillatory and concentric motion created by the fall of one of the objects will meet the motion created by the other, water ripples will become more complex and will interfere with one another, producing interference. “Echo” interference can be observed at the edges of a tub when we throw even only one pebble: waves that reach the edge produce other waves which, moving into the opposite direction, interfere with the previous ones.

Similar things, even though invisible, occur each time we produce a sound. Our vocal cords vibrate and produce oscillations of the surrounding air particles: these are sound waves that propagate in all directions, thus reaching those particular “resonators” represented by our ears. Once again, as it happened in the water pond, we are dealing with vibrations of material particles, made of air. But if we were surrounded by vacuum, no sound waves could be possible for lack of an oscillatory medium. However, oscillatory movements can also be generated in vacuum, not by material particles but by disturbances within an electric field, magnetic field and also gravitational field.

In physics the word field, which reminds us of a piece of cultivated land, refers to an area of space changed by the presence of an electric particle or a magnet or a material mass, which respectively create an electric, magnetic or gravitational field.

(Giancarlo Masini)

 

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