Download Transversal Waves

Transversal Waves
The Question
This is a question about physics. I know the difference between transversal and
longitudinal waves. My question relates to transversal waves. I can comprehend why a
wave on a surface of water or along a string is transversal, and why sound waves are
longitudinal. But why is electromagnetic radiation transversal? In other words, why does,
for instance, visible light move up and down between two extremes while moving? Why
does it reverse direction at the high and low points of a wave, and why does it continually
change its direction?
The Answer
Electricity and magnetism are forces that are active even from a distance, as opposed to
forces which require direct contact such as friction and pressure. If one electrically charges
an object by rubbing it up, a force can be felt even at a distance, such as with a comb which
attracts snippets of paper. The same goes for magnets, which can attract nails without
direct contact. Because this force works from a distance, electromagnetism is described as
a force field. Objects will experience this force, if they are sensitive to it. Gravity is another
example of a force active at a distance, which can also be described as a force field.
If a vertical livewire, connected to a battery, is stuck through a horizontal cardboard with
some iron filings on top of it, the iron filings will form circles around the livewire at the
centre. The filings will indicate the direction of the magnetic field. The electric current,
flowing through the wire, is created by an electric field powered by the battery. The electric
field will move the electrons along the wire. As shown by the filings, a magnetic field will be
generated by the electric field, perpendicular to this electric field. However, the circular
magnetic field will not suddenly exist everywhere. Just like ripples of water stirred by a
stone thrown in a pool, the magnetic field will expand in all directions because the circles
become bigger and bigger. There is an important difference, however: the direction of the
magnetic field travels along the circles, whereas waves of water are perpendicular to the
force.
Should a livewire be connected to an alternating electric field, a magnetic field will be
created expanding in circles in continually changing directions. A pattern of consecutive
circles will be created along a magnetic field that is continually varying in size and
direction. The magnetic field will generate an electric field itself as well. This is exactly how
a dynamo works: a magnet spins around within a coil, generating an electric field. An
electric current is thus powered. The electric field will be perpendicular to the magnetic
field.
Because of the alternating electric field in the livewire, an alternating circular electric field
is created parallel to the live wire, and an alternating magnetic field perpendicular to the
live wire will be created at the same time. Both fields will expand in all directions in bigger
and bigger circles. This is called an electromagnetic wave. Because both the electric and
magnetic field are perpendicular to the direction of the propagation, the electromagnetic
wave is called transversal.
Should one picture this wave at one moment in time, points in space may be distinguished
where the electric field is pointing upwards, and pointing downwards a little further on.
Somewhere between these points, the strength of the electric field will hence be zero. The
same goes for the magnetic field. In order to picture the field strength, a line is often drawn
from the source – the livewire, in this case – perpendicular to the direction and strength of
the fields. This line will look like a wave, even though it is not! As shown, electromagnetic
waves do not behave like waves along a string. Therefore, light does not move on between
two extremes. Light just moves straight ahead.
Kind regards,
Drs. H. Jordens
Faculty of Mathematics and Natural Sciences
University of Groningen
Translated by Anniversary Project Group