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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.
Frequency and Time
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.
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CHARACTERISTICS OF WAVE MOTION
The two types of wave motion, transverse and longitudinal, have many of the same characteristics,
such as frequency, amplitude, and wavelength. Another important characteristic that these two types of
wave motion share is VELOCITY. Velocity of propagation is the rate at which the disturbance travels
through the medium, or the velocity with which the crest of the wave moves along. The velocity of the
wave depends both on the type of wave (light, sound, or radio) and type of medium (air, water, or metal).
If longitudinal waves are plotted as a graph, they appear as transverse waves. This fact is illustrated in