The term electromagnetic wave describes the way electromagnetic
radiation (EMR) moves through space. Different forms of EMR are
distinguished by their wavelengths, which vary from many yards (meters)
to a distance smaller than the diameter of an atomic
nucleus. The full range, in decreasing order of
wavelength, goes from
radio waves
through microwaves, visible light, ultraviolet and X-rays to gamma rays
and is known as the electromagnetic spectrum. Electromagnetic waves
have many applications, both in science and in everyday life.
Light Waves
In many respects, an electromagnetic wave behaves similarly to
ripples on water, or to sound travelling through a medium such as air.
For example, if a light is shone onto a screen through a barrier with
two narrow slits, a pattern of light and dark stripes is seen. This is
called an interference pattern: where the crests of the waves from one
slit meet those from the other, they reinforce one another, forming a
bright stripe, but where a crest meets a trough, they cancel out,
leaving a dark stripe. Light can also bend around an obstacle, like
ocean breakers around a harbor wall: this is known as diffraction. These
phenomena provide evidence of the wave-like nature of light.
It was long assumed that, like sound, light must travel through some
kind of medium. This was given the name “ether,” sometimes spelt
“aether,” and was thought to be an invisible material that filled space,
but through which solid objects could pass unhindered. Experiments
designed to detect the ether by its effect on the speed of light in
different directions all failed to find any evidence for it, and the
idea was finally rejected. It was apparent that light, and other forms
of EMR, did not require any medium and could travel through empty space.
Wavelength and Frequency
Just like an ocean wave, an electromagnetic wave has peaks and
troughs. The wavelength is the distance between two identical points of
the wave from cycle to cycle, for instance, the distance between one
peak, or crest, and the next. EMR can also be defined in terms of its
frequency, which is the number of crests that pass by in a given time
interval. All forms of EMR travel at the same speed: the speed of light.
Therefore, the frequency depends entirely on the wavelength: the
shorter the wavelength, the higher the frequency.
Energy
Shorter wavelength, or higher frequency, EMR carries more energy than
longer wavelengths or lower frequencies. The energy carried by an
electromagnetic wave determines how it affects matter. Low frequency
radio waves mildly perturb atoms and molecules, while microwaves cause
them to move about more vigorously: the material heats up. X-rays and
gamma rays pack much more of a punch: they can break chemical bonds and
knock electrons from atoms, forming ions. For this reason, they are
described as ionizing radiation.
The Origin of Electromagnetic Waves
The relationship between light and electromagnetism was established
by the work of the physicist James Clerk Maxwell in the 19th century.
This led to the study of electrodynamics, in which electromagnetic
waves, such as light, are regarded as disturbances, or “ripples,” in an
electromagnetic field, created by the movement of electrically charged
particles. Unlike the non-existent ether, the electromagnetic field is
simply the sphere of influence of a charged particle, and not a
tangible, material thing.
Later work, in the early 20th century, showed that EMR also had
particle-like properties. The particles that make up electromagnetic
radiation are called
photons. Although it seems contradictory,
EMR can behave as waves or as particles, depending on the type of
experiment that is carried out. This is known as the wave-particle
duality. It also applies to subatomic particles, whole atoms and even
quite large molecules, all of which can sometimes behave as waves.
The wave-particle duality emerged as quantum theory was being
developed. According to this theory, the “wave” represents the
probability of finding a particle, such as a photon, at a given
location. The wave-like nature of particles and the particle-like nature
of waves have given rise to a great deal of scientific debate and some
mind-boggling ideas, but no overall consensus about what it actually
means.
In quantum theory, electromagnetic radiation is produced when subatomic particles release energy. For example, an
electron
in an atom can absorb energy, but it must eventually drop to a lower
energy level and release the energy as EMR. Depending on how it is
observed, this radiation can appear as a particle or an electromagnetic
wave.
Uses
A great deal of modern technology depends upon electromagnetic waves.
Radio, television, mobile phones and the Internet rely on the
transmission of radio frequency,
(Radio frequency refers to an alternating electrical current with certain
properties that allow it to be broadcast from an antenna.)
EMR through air, space or fiber optic cables. The lasers used to record
and play DVDs and audio CDs use light waves to write to and read from
the discs. X-ray machines are an essential tool in medicine and airport
security. In science, our knowledge of the universe comes largely from
analysis of light, radio waves and X-rays from distant stars and
galaxies.
Hazards
It is not thought that low energy electromagnetic waves, such as
radio waves, are harmful. At higher energies, however, EMR poses risks.
Ionizing radiation, such as X-rays and gamma rays can kill or damage
living cells. They can also alter DNA, which can lead to cancer. The
risk to patients from medical X-rays is considered negligible, but
radiographers, who are exposed to them regularly, wear lead aprons —
which X-rays cannot penetrate — to protect themselves. Ultraviolet light, present in sunlight, can cause sunburn and can also cause skin cancer if exposure is excessive.
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