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3 An Introduction to Surface Waves
2. The disturbance is propagated through the material without any substantial overall motion of the material itself. A floating cork merely bobs
up and down on the water surface ripples, but experiences very little
overall movement in the direction of travel of the ripples. Closer observation shows, however, that the cork describes an almost circular path in
a vertical plane parallel with the direction of wave motion.
Thus, as wave motion is characterized by the propagation of form while the
material itself is not transported, the question arises as to what quantity is
transported by waves? This quantity is 'energy'. In particular, waves on a water surface transport mechanical energy, electromagnetic waves transport electromagnetic energy and sound waves cause vibration of air particles. Therefore,
we can generally define wave motion as a process whereby energy is transported
across or through a material without any significant overall transport of the
material itself. A wave is not a substance, such as water, air or rock; it is a
pattern of behaviour.
3.3 Types of Waves
Waves which travel through materials are called 'body waves'. Seismic waves
and sound waves are examples of body waves. However, in this book our main
concern is with surface waves. The most familiar surface waves are those
which occur at the interface between atmosphere and ocean, but surface waves
can occur at the interface between any two bodies of fluid. For example, waves
occur at the interface between two layers of ocean water of different densities.
Because the interface is a surface, such waves are, strictly speaking, surface
waves. However, in oceanography they are known as internal waves. They
will be considered in more detail in Chap. 6.
All surface waves can be regarded as either progressive waves, where energy
moves across the surface, or as standing waves which can be considered as
the sum of two progressive waves of equal dimensions, but travelling in opposite
directions. The motion of an elastic rope, horizontally oriented and fixed at
one end but moving vertically up and down at the other, describes a standing
wave.
Waves can also be classified depending on the nature of the restoring forces
which maintain their propagation. There are two main kinds of restoring forces
for surface waves, namely gravitational forces exerted by the Earth, Sun or
Moon, and surface tension. The attraction of sea water by the Sun and Moon
causes waves which have periods coinciding with the periodic motion of the
Earth and Moon. Because of the special, global character of these waves commonly called tides, they are usually treated separately from other surface waves
(see Chap. 5).
Most surface waves are generated by non-periodic forces. Winds blowing
on the sea surface displace water particles from their equilibrium position.
These waves are called wind-induced waves or simply wind waves (see
3 An Introduction to Surface Waves
2. The disturbance is propagated through the material without any substantial overall motion of the material itself. A floating cork merely bobs
up and down on the water surface ripples, but experiences very little
overall movement in the direction of travel of the ripples. Closer observation shows, however, that the cork describes an almost circular path in
a vertical plane parallel with the direction of wave motion.
Thus, as wave motion is characterized by the propagation of form while the
material itself is not transported, the question arises as to what quantity is
transported by waves? This quantity is 'energy'. In particular, waves on a water surface transport mechanical energy, electromagnetic waves transport electromagnetic energy and sound waves cause vibration of air particles. Therefore,
we can generally define wave motion as a process whereby energy is transported
across or through a material without any significant overall transport of the
material itself. A wave is not a substance, such as water, air or rock; it is a
pattern of behaviour.
3.3 Types of Waves
Waves which travel through materials are called 'body waves'. Seismic waves
and sound waves are examples of body waves. However, in this book our main
concern is with surface waves. The most familiar surface waves are those
which occur at the interface between atmosphere and ocean, but surface waves
can occur at the interface between any two bodies of fluid. For example, waves
occur at the interface between two layers of ocean water of different densities.
Because the interface is a surface, such waves are, strictly speaking, surface
waves. However, in oceanography they are known as internal waves. They
will be considered in more detail in Chap. 6.
All surface waves can be regarded as either progressive waves, where energy
moves across the surface, or as standing waves which can be considered as
the sum of two progressive waves of equal dimensions, but travelling in opposite
directions. The motion of an elastic rope, horizontally oriented and fixed at
one end but moving vertically up and down at the other, describes a standing
wave.
Waves can also be classified depending on the nature of the restoring forces
which maintain their propagation. There are two main kinds of restoring forces
for surface waves, namely gravitational forces exerted by the Earth, Sun or
Moon, and surface tension. The attraction of sea water by the Sun and Moon
causes waves which have periods coinciding with the periodic motion of the
Earth and Moon. Because of the special, global character of these waves commonly called tides, they are usually treated separately from other surface waves
(see Chap. 5).
Most surface waves are generated by non-periodic forces. Winds blowing
on the sea surface displace water particles from their equilibrium position.
These waves are called wind-induced waves or simply wind waves (see