Electromagnetic Radiation and Wave Behaviour 109
We will defer our discussion of the wave-particle duality of light to
the time when we discuss atomic physics, and turn our attention instead
to the wave nature of light. Let us first consider the nature of wave
behaviour in general by discussing a more familiar example, namely, the
waves that travel on the ocean. When one looks at the surface of the
ocean on a windy day, one sees alternate rows of crests and troughs. As
one observes the movement of the water, one observes that the crests and
troughs are moving toward the shore. One might erroneously conclude
that the water is moving towards the shore but, in fact, the water
composing the wave is actually moving up and down. It is oscillating in
the direction perpendicular to the direction in which the wave is moving.
This can be easily verified by watching a buoy bobbing up and down in
the water. At the shore, it is true that at certain moments water moves
toward the shore, but it is also true that, at other moments, an equal
amount of water moves away from the shore as the wave washes back
into the sea.
One should not confuse the two different types of motion one
encounters in wave behaviour. One motion is the motion of the wave or
really, the waveform, which is continuous and unidirectional. The other
motion is the actual movement of the medium, which is always an
oscillatory motion. In the example of waves upon the ocean, the
oscillatory motion of the medium is perpendicular to the motion of the
waveform. This type of wave is called a transverse wave and is
differentiated from a longitudinal wave in which the medium oscillates
back and forth in the same direction in which the wave moves.
Perhaps the best-known example of a longitudinal wave is a sound
wave. A sound wave requires the existence of transmitting medium.
Most of the sound waves with which we come in contact propagate
through the atmosphere although sound waves can also propagate
through solids and liquids. There are no sound waves on the surface of
the Moon however, because there is no atmosphere.
Let us consider the production and propagation of sound waves in our
atmosphere. A sound wave is produced as a result of the rapid vibration
of some object, like a violin string for example, which causes the
molecules of air surrounding it to move back and forth like the vibrating
string. This causes alternate condensations and rarefactions of the air
molecules. As the string moves to the right, it pushes the air molecules
together creating a condensation; as it moves back to the other direction,
it leaves a rarefaction. The motion of the air molecules back and forth is