TERMS USED IN WAVE MOTION
There are a number of special terms concerning waves that you should know. Many of the terms, such as CYCLE, WAVELENGTH, AMPLITUDE, and FREQUENCY were introduced in previous NEETS modules. We will now discuss these terms in detail as they pertain to wave propagation. Before we begin our discussion, however, note that in the figure, wave 1 and wave 2 have equal frequency and wavelength but different amplitudes. The REFERENCE LINE (also known as REST POSITION or POINT OF ZERO DISPLACEMENT) is the position that a particle of matter would have if it were not disturbed by wave motion. For example, in the case of the water wave, the reference line is the level of the water when no wave motion is present. With this in mind, let's go on to our discussion of the four terms, as shown in figure 1-7.
Figure 1-7. - Comparison of waves with different amplitudes.
Refer to wave 1 in figure 1-7. Notice how similar it is to the sine wave you have already studied. All transverse waves appear as sine waves when viewed from the side. In figure 1-7, wave 1 has four complete cycles. Points ABCDE comprise one complete cycle having a maximum value above and a maximum value below the reference line. The portion above the reference line (between points A and C) is called a POSITIVE ALTERNATION and the portion below the reference line (between points C and E) is known as a NEGATIVE ALTERNATION. The combination of one complete positive and one complete negative alternation represents one cycle of the wave. At point E, the wave begins to repeat itself with a second cycle completed at point I, a third at point M, etc. The peak of the positive alternation (maximum value above the line) is sometimes referred to as the TOP or CREST, and the peak of the negative alternation (maximum value below the line) is sometimes called the BOTTOM or TROUGH, as depicted in the figure. Therefore, one cycle has one crest and one trough.
A WAVELENGTH is the distance in space occupied by one cycle of a radio wave at any given instant. If the wave could be frozen in place and measured, the wavelength would be the distance from the leading edge of one cycle to the corresponding point on the next cycle. Wavelengths vary from a few hundredths of an inch at extremely high frequencies to many miles at extremely low frequencies; however, common practice is to express wavelengths in meters. Therefore, in figure 1-7 (wave 1), the distance between A and E, or B and F, etc., is one wavelength. The Greek letter lambda is used to signify wavelength. Why lambda and not "l"or "L"? This is because "L" is used conventionally as the symbol for inductance, and "l" is used for dimensional length; therefore, l is used to indicate the length of waves.
Two waves may have the same wavelength, but the crest of one may rise higher above the reference line than the crest of the other. Compare wave 1 and wave 2 of figure 1-7 again. The height of a wave crest above the reference line is called the AMPLITUDE of the wave. The amplitude of a wave gives a relative indication of the amount of energy the wave transmits. A continuous series of waves, such as A through Q, having the same amplitude and wavelength, is called a train of waves or WAVE TRAIN.
Time is an important factor in wave studies. When a wave train passes through a medium, a certain number of individual waves pass a given point in a specific unit of time. For example, if a cork on a water wave rises and falls once every second, the wave makes one complete up-and-down vibration every second. The number of vibrations, or cycles, of a wave train in a unit of time is called the FREQUENCY of the wave train and is measured in HERTZ. If 5 waves pass a point in one second, the frequency of the wave train is 5 cycles per second. In figure 1-7, the frequency of both wave 1 and wave 2 is four cycles per second (cycles per second is abbreviated as cps).
In 1967, in honor of the German physicist Heinrich Hertz, the term HERTZ was designated for use in lieu of the term "cycle per second " when referring to the frequency of radio waves. It may seem confusing that in one place the term "cycle "is used to designate the positive and negative alternations of a wave, but in another instance the term "hertz" is used to designate what appears to be the same thing. The key is the time factor. The term cycle refers to any sequence of events, such as the positive and negative alternations, comprising one cycle of electrical current. The term hertz refers to the number of occurrences that take place in one second.