110 The Poetry of Physics and The Physics of Poetry
in the same direction as the motion of the vibrating string. The vibrating
column of air adjacent to the string will, in turn, cause the column of air
adjacent to it to vibrate and then, this column of air will act on the
column adjacent to it and so on and so forth and in this way, the sound
wave will propagate through the air. Each column of air will vibrate back
and forth and hence, there will be no net displacement of the air as the
sound wave propagates from the vibrating string to the ears of a listener.
The air molecules that are contact with violin strings will not come in
contact with the listener’s ear. The vibration of these air molecules will
propagate through the air to the listener’s ear as a consequence of the
contact the air molecules make with each other through collisions. The
vibrations of the final column of air adjacent to the eardrum of listeners
will cause their eardrum to vibrate with the same frequency of the
original violin string. The vibrations of the eardrum are transmitted
through tiny bones to a cavity, the cochlea, containing a fluid where they
activate nerve cells, which transmit the information to the brain. The
human ear is capable of detecting frequencies in the range from sixteen
vibrations per second to twenty thousand. The greater the frequency of
the sound wave, the higher the pitch that we detect. The loudness of the
sound wave depends on the strength of the vibrations or on the distance
through which the string vibrates. The harder the string is struck, the
greater is the amplitude of its vibration and hence, the louder the sound.
The frequency of the string does not depend on the strength with which it
is struck but rather on the length of the string, its thickness and the
amount of tension with which it is strung.
We have now considered both transverse waves, (waves on the ocean)
and longitudinal waves (sound waves). In both cases, the wave is
transmitted as the result of oscillatory motion of a physical medium. In
the case of ocean waves, the water was oscillating up and down in the
direction transverse to the propagation of the wave. In the case of the
sound wave, the air molecules were oscillating back and forth in the
direction longitudinal to the wave motion. We encounter a somewhat
different situation when we consider electromagnetic radiation since no
medium is required to propagate the wave motion. Light can travel in a
vacuum. i.e. empty space. Instead of the oscillation of some physical
medium, electromagnetic waves involve the oscillation of the electric
and magnetic fields. The oscillation of these fields is transverse to the
direction of the wave propagation and hence, light is a transverse wave.
We are still left with the mystery of how a wave is able to propagate
through empty space. This mystery is related to the mystery of action at a