"medium" in this illustration is the wheat and the "disturbance" is the wind
moving the stalks of wheat.
1-8. Wave motion can be defined as a recurring disturbance advancing
through space with or without the use of a physical medium. Wave motion,
therefore, is a means of moving or transferring energy from one point to
another point. For example, when sound waves strike a microphone, sound
energy is converted into electrical energy. When light waves strike a
phototransistor or radio waves strike an antenna, they are likewise converted
into electrical energy. Therefore, sound, light, and radio waves are all forms
of energy that are moved by wave motion. We discuss sound waves, light
waves, and radio waves later in this chapter.
1-9. A type of wave motion familiar to almost everyone is the movement of
waves in water. We explain these waves first to help you understand wave
motion and the terms used to describe it.
1-10. Basic wave motion can be shown by dropping a stone into a pool of
water (see figure 1-1). As the stone enters the water, a surface disturbance is
created, resulting in an expanding series of circular waves.
Figure 1-1. Formation of Waves in Water
1-11. Figure 1-2 is a diagram of this action. View A shows the falling stone
just an instant before it strikes the water. View B shows the action taking
place at the instant the stone strikes the surface, pushing the water that is
around it upward and outward. In view C, the stone has sunk deeper into the
water, which has closed violently over it causing some spray, while the
leading wave has moved outward. An instant later, the stone has sunk out of
sight, leaving the water disturbed as shown in view D. Here the leading wave
has continued to move outward and is followed by a series of waves gradually
diminishing in amplitude. Meanwhile, the disturbance at the original point of
contact has gradually subsided.
1-12. In this example, the water is not actually being moved outward by the
motion of the waves, but up and down as the waves move outward. The up
and down motion is transverse, or at right angles, to the outward motion of
the waves. This type of wave motion is called transverse wave motion.