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ELECTROMAGNETIC
SPECTRUM
It may seem that astronomers and astrophysicists have
a particularly difficult task.  They
have to study stars, including our Sun, from a great
distance. However, we don't have to touch an object
to learn about it. Humans are quite experienced at "remote
sensing". We receive most of our information about
the world from sensors that receive information from
a distance; our eyes, ears and nose.The most important
tools for astronomers and astrophysicists are those
that improve their sight. The invention of the telescope
in the early 1600s was one critical step. However,
the most important advances have come in our understanding
of light itself.
The publication in 1704 of "Opticks" by
Sir Isaac Newton was important to our understanding
of light . In the process of explaining his theories
of light, Newton revealed the results of his experiments
with a prism that clearly showed that visible light
from the Sun is a mixture of a continuous spectrum
of colors.
 It
is convenient to describe this color spectrum as Red,
Orange, Yellow, Green, Blue, Indigo, and Violet (Roy
G. Biv). However, it is important to realize that there
is a continuous blending of colors from red to orange
to yellow and so on. For more on Color.
The important discovery that visible
light was made of a continuous spectrum of colors of
light led to two other important discoveries in the
1800s. In 1800 the astronomer, Sir
William Herschel, performed experiments to understand
why different filters on his telescopes seemed to heat
up differently. Herschel separated sunlight into its
spectrum with a prism. He used three thermometers with
their bulbs blackened with soot. He put one bulb in
each color of the spectrum and used the other two bulbs
as controls. He found that each color of light produced
a temperature higher than the controls. He also observed
that the temperature increased as he progressed from
the blue to the red. When he measured the temperature
just beyond the red, he found this region with no visible
light produced the highest temperatures.
 Further
experiments by Herschel showed that he could reflect,
refract, absorb and transmit whatever was in this region.
Herschel correctly concluded that there was a portion
of the light beyond the red (infrared). You can perform The
Herschel Experiment at home or in the classroom.
>When Johann
Wilhelm Ritter heard of Herschel's work, Ritter
reasoned that there might be invisible radiation
on the other end of the spectrum - beyond violet.
Ritter was an accomplished chemist, and he used his
knowledge of chemistry to test his theory. He knew
that blue light decomposed silver chloride to silver
more efficiently than red light. He reasoned that
non-visible light beyond blue might be even more
efficient. He was right! When he exposed paper covered
with silver chloride to the complete spectrum of
sunlight that had passed through a prism, the silver
chloride in the region beyond violet (where there
was no visible light!) decomposed the fastest. From
this experiment Ritter knew that there must be light
beyond violet. This radiation came to be known as
ultraviolet light.
The new light spectrum included invisible
radiation at both ends of the visible spectrum.
 The
new radiation could be reflected, refracted, absorbed
and transmitted just like visible light.
It is useful, at this point, to consider
an idea about light presented by Christiaan
Huygens in 1690. Huygens explained light as a wave.
In the early 1800s Thomas
Young supported the wave theory of light with a
number of important experiments. (In 1905 Einstein
showed that light also has particle properties. We
now understand that light has both wave
and particle properties.) The theory that light
has wave-like properties allows us to consider light
in terms of its wavelength. The wavelength can be explained
in this simple diagram.
 Understanding
the crest or trough of a water wave is simple. Understanding
the crest and trough of a light wave is not! For a
dynamic exploration of wavelengths of light and the
nature of light, the Electromagnetic
Waves tutorial of Physics 2000© is an excellent
resource. Also Imagine the Universe Level
I and Level
II and Star Light, Star
Bright may help.
The wavelength of visible light is measured
in Ångstroms (10-8cm) and ranges between 4000Å (violet)
and 7000Å (red). Scientists also use nanometers
(nm) that are 10-9 meters. In nanometers the range
is 400nm to 700nm.
It is sometimes convenient to measure
the frequency of waves. The frequency is
the number of complete cycles -crests or troughs -
that pass a point in a period of time. The number of
cycles per second is called a hertz (HZ).
The 1800s were rich with experiments
and theoretical advances that added greatly to the
work of Herschel, Ritter and Young. Hans
Christian Oersted found that an electrical
current in a wire caused a magnetic
field around the wire. Reacting to Oersted's findings, Michael
Faraday was able to generate an electric current
with a changing magnetic field. James Clerk Maxwell. Maxwell showed
that a few relatively simple mathematical equations
could express the behavior of electric and magnetic
fields and their interrelated nature; that is, an oscillating
electric charge produces an electromagnetic field.
Maxwell also calculated that the speed at which an
electromagnetic field moved away from its source is
approximately that of the speed of light. Furthermore,
his equations showed that the electromagnetic phenomena
had wave properties. He proposed that the phenomenon
of light is therefore an electromagnetic phenomenon.
Because charges can oscillate with any frequency, Maxwell
concluded that visible light forms only a small part
of the entire spectrum of possible electromagnetic
radiation.
Maxwell published his work in A Treatise
on Electricity and Magnetism in 1873. In 1886 Heinrich
Hertz constructed very simple circuits that proved
Maxwell's theory. Hertz was able to transmit radio
waves (or frequencies) through air to a receiver. Hertz
viewed his demonstration as simply verification of
Maxwell's math. However, Guglielmo Marconi immediately
saw an application - wireless telegraphy. After experimenting
for several years he was able to demonstrate trans-Atlantic
communication. This was the beginning of the wireless
radio and radar. 
In the same year that Marconi began experimenting
with his wireless transmissions, William Carl Roentgen
was discovering another type of electromagnetic emission
- the x-ray.
Roentgen experimented with the penetrating
power of x-rays. He "took pictures" of his
wife's hand, various metal objects (including his gun),
wood and glass. The use of x-rays as a medical tool
was obvious to Roentgen and others. Roentgen's experimentation
with his gun and other metal objects predicted the
use of x-rays in airport security and to detect metal
fatigue.
The common use of the word 'light' to
mean only visible light causes some confusion. Scientists
often refer to forms of light as electromagnetic radiation
and the entire spectrum as the Electromagnetic Spectrum.
 As
another way of looking at the different types of electromagnetic
emissions, this table provides information on the wavelength
and frequency for different "types of light".
These many discoveries and the invention
of instruments to allow us to "see" more
than the visible wavelengths of light have provided
astronomers with important tools to explore our universe
from afar. Find out How
Astronomers Use the Electromagnetic Spectrum to
look at the universe in all of its light.
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Astronomers use the electromagnetic Spectrum
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