Download The Electromagnetic Spectrum: A History

14 Spectroscopy 21(3)
w w w. s p e c t r o s c o p y o n l i n e . c o m
March 2007
The Baseline
The Electromagnetic Spectrum:
A History
In all the time I’ve been doing this column, I’ve never discussed how the complete electromagnetic spectrum was originally mapped out. So, I’ll do it now. Spectroscopists
should understand that all regions of the electromagnetic spectrum are used in some
form of spectroscopy.
David W. Ball
W
hen a person says the word “light,” a listener —
even a spectroscopist — usually interprets that
as meaning “visible light.” That’s not unexpected, because throughout most of recorded history the
only light that we recognized was light that we could see.
However, that changed in the 1800s when it was finally realized that light was a more general phenomenon. Although
currently the term “light” refers to a lot more than just visible light, it’s more common to use the phrase “electromagnetic radiation” when referring to any form of light. The
collective range of possible lights is called the electromagnetic spectrum.
Properties of Light — A Quick Review
Light has both particle and wave properties, as has been discussed in a recent column (1). Because a photon of light has
a certain energy E and momentum p, it acts as a particle:
E = h
p = h/
Because light’s properties can be described by values of
wavelength , frequency , and wavenumber (see Figure
1), it acts as a wave. As with any wave, the velocity of light is
equal to the product of its frequency and wavelength:
c = Unlike that of other waves, the speed of light is a constant for a given medium. In a vacuum, the speed of light is
approximately 3.00 108 m/s — about seven and a half
times around the Earth every second. (See Figure 2 for an
example of the speed of light.)
Because the speed of light is a constant, there is a simple
relationship between the wavelength of a light and its frequency. For visible light, wavelengths range from 400 to 700
nm, which correspond to frequencies of 7.5 1014 to 4.3 1014 waves per second. The energy and momentum of each
photon of visible light can be calculated from the earlier
equations.
Other Than Visible Light
In 1800, British astronomer William Herschel was measuring the effect of various colors of light on a thermometer,
using a prism to disperse light from the sun. Upon putting
the thermometer past the red light, he noted an even larger
increase in temperature than when the thermometer was
bathed in visible light. It was obvious that there was “light”
beyond the color red; this light was eventually termed “infrared” light — literally, “below red” light.
Experiments with light showed that some colors could
darken certain silver salts — indeed, this is the basis of photographic film. In 1801, German scientist Johann Ritter
noted that the region of the spectrum just beyond the violet
edge of visible light was more effective at turning silver
halides dark. He reasoned that there was an invisible form
of light beyond violet, which he named “deoxidizing rays,”
later known as “chemical rays.” Once the nature of light was
better established in the late 19th century, the term “ultraviolet” — beyond violet — light was adopted.
16 Spectroscopy 22(3)
w w w. s p e c t r o s c o p y o n l i n e . c o m
March 2007
Direction of
propagation
tors. For additional information on
these operators, consult a calculus
text.) The constants 0 and 0 are permeability of free space and the permittivity of free space, respectively, both of
which are universal constants. In these
forms, E and B are related to each other
because two of the equations contain
them both; they are referred to as coupled differential equations. They can be
decoupled by applying the curl operator a second time to the third and
fourth equation (see reference 2, chapter 8). The results are
1 cm
2
~
∇ E = μ 0ε 0
2
∂ E
∂t
2
2
Figure 1: The wave nature of light.
Maxwell’s Laws
The mention of magnets in ancient literature goes back to the 4th century
B.C., while constructive use of electricity dates to 1800 with the invention of
the voltaic pile, the first crude battery.
(Previous experiments involving electricity largely involved static electricity
or, as with Benjamin Franklin and the
kite, lightning.) In 1820, Danish scientist Hans Ørsted noticed that the needle on a nearby compass deflected
when he turned the current on and off
a voltaic cell. This suggested that there
was a connection between electricity
and magnetism, and inspired additional work by scientists such as Faraday, Ampère, Volta, and Ohm (all of
whom now have electricity-related
units named after them). Many of the
theoretical advances involving electricity and magnetism were summarized
by James Clerk Maxwell in 1864. The
four fundamental equations are called
∇ B = μ 0ε 0
2
Maxwell’s equations, and in a space
where there is no current or charge,
they are
·E = 0 (Gauss’ law)
·B = 0 (Gauss’ law for magnetism)
∇E = B
t
(Faraday’s law of induction)
∇B = 0ε0
E
t
(Ampère’s law)
Here, E is the electric field, B is the
magnetic flux density, · is the divergence operator, and is the curl operator. (Both the divergence and curl
operators are partial differential opera-
∂ B
∂t
2
The equation has the same form for
each quantity. This form is a secondorder differential equation whose solution is the classic wave equation: a sine
or cosine function (which are similarly
behaved functions that differ mainly in
phase) whose velocity is the reciprocal
square root of the constants multiplying the second derivative on the right
side; that is,
1
speed =
μ 0ε 0
The numerical value of this equation is about 3.00 108 m/s, which
was very close to the experimentally
measured speed of light, so Maxwell
made the following statement (3):
This velocity is so nearly that of
light, that it seems we have strong
reason to conclude that light itself
(including radiant heat, and other
Figure 2: It takes light one half of a nanosecond (0.0000000005 s) to go from one end of this line to the other. To see how far light travels in one
second, redraw this line two billion times end-to-end.
w w w. s p e c t r o s c o p y o n l i n e . c o m
March 2007
radiations if any) is an electromagnetic disturbance in the form of
waves propagated through the electromagnetic field according to electromagnetic laws.
Indeed, this is how light currently is
perceived, which is why light is synonymous with the term electromagnetic radiation. This also means that
the
1
velocity μ 0 ε 0
of
light c is given by
c=
Because 0 and 0 are universal
constants, it stands that c, the speed of
light, is a universal constant, an idea
that forms the basis of Einstein’s theory of relativity.
Notice that nothing in Maxwell’s
equations restricts the possible values
of the wavelength/frequency of electromagnetic radiation; they only restrict its velocity. Infrared light, visible
light, and ultraviolet light are all the
same phenomenon, but with different
values of wavelength and frequency
(and the same velocity). What
Maxwell’s equations imply is that
there exist other ranges of wavelength/frequency for electromagnetic
radiation. The entire range of possible
wavelengths and frequencies for electromagnetic radiation makes up the
electromagnetic spectrum.
770 nm
400 nm
Radio
waves
Microwaves
Infrared
Visible
18 Spectroscopy 22(3)
Ultraviolet
X
rays
Gamma
rays
(Region limits are approximate)
Figure 3: A representation of the complete electromagnetic spectrum.
magnetic radiation intentionally in a
different region of the spectrum.
Using a transmitter, Hertz generated
radio waves and detected them by
using a loop of wire that had a small
gap between the ends. A spark would
be generated upon receipt of radio
waves by the loop, which was acting as
an antenna. By setting a curved zinc
plate some distance away from the
generator to reflect the waves (a precursor to radar) and by varying the
distance between the generator and
the antenna, Hertz was able to deter-
mine the wavelength of the radiation
(about 4 m) and determine its other
properties, like velocity. In measuring
this last quantity, Hertz confirmed
that the radiation being given off was
a form of light, verifying the applicability of Maxwell’s equations. Hertz
also generated microwave radiation in
a similar fashion. Modern methods of
generating microwave radiation use
solid-state semiconductor devices or
vacuum tube-based instruments.
The infrared portion of the spectrum
commonly is divided into sections, al-
The Rest of the Spectrum
Figure 3 shows a diagram of the electromagnetic spectrum. The cutoffs for
each region are approximate and can
vary somewhat, depending upon the
reference. Some older references also
can show cosmic rays on the far end
next to gamma rays; though once
thought to be very high-frequency radiation, cosmic rays are now known to
be composed of particles, not electromagnetic radiation.
Maxwell died in 1879, at age 48. In
1887–1888, German physicist Heinrich Hertz (who himself died at age 37
in 1894) was the first to create electro-
Figure 4: An X-ray of Anna Röntgen’s hand, the first X-ray of a human body part, taken by
Wilhelm Röntgen in December 1895 as part of his initial studies of this new type of
electromagnetic radiation. The phalanges are clearly visible as darker regions of the radiograph,
as is the ring on her finger.
w w w. s p e c t r o s c o p y o n l i n e . c o m
March 2007
NMR
spectroscopy
Vibrational
spectroscopy
Gamma rays
X-rays
Infrared
Microwaves
Radio waves
Rotational
spectroscopy
Ultraviolet
X-ray photoelectron
spectroscopy
Electronic
spectroscopy
Visible
20 Spectroscopy 22(3)
-ray
spectroscopy
EPR
spectroscopy
Figure 5: The regions of the electromagnetic spectrum and the types of spectroscopy they
participate in. (Adapted from reference 5.)
though the number of sections and
their limits varies. One common set of
divisions lists the far infrared (wavelength range 0.75–5 m), the mid infrared (5–30 m), and the near infrared
(30–1000 m). The near-infrared region is adjacent to the visible portion of
the spectrum.
Let us turn to the other side of the
spectrum. The ultraviolet region of the
electromagnetic spectrum can be separated in several ways. Ultraviolet A radiation (abbreviated UVA), also called
long-wave UV light, has a wavelength
range of 400–315 nm. Ultraviolet B radiation (UVB, or medium-wave UV)
has a wavelength range of 315–280 nm,
while ultraviolet C (UVC, short-wave
UV, or germicidal UV) has a wavelength
of less than 280 nm. Scientists also
speak of near UV (380–200 nm, or
NUV), far or vacuum UV (200–10 nm,
FUV or VUV), and extreme UV (31–1
nm, EUV or XUV). For spectroscopists,
the term vacuum UV means wavelengths of UV light that air absorbs significantly, so that any spectroscopy
using light of this wavelength range typically must be performed in a vacuum.
Because the energy of a light photon is inversely proportional to its
wavelength, electromagnetic radiation
in the UV range or shorter wavelength
can have severe physiological consequences, as the energy of the photon is
similar to that of an average chemical
bond. Solar radiation contains signifi-
cant amounts of UV radiation, exposure to which causes skin tanning and,
if in excess, sunburn. Long-term exposure to solar UV radiation damages
skin and can lead to cancer.
X-rays were first studied systematically by Prussian (now part of Germany) scientist Wilhelm Röntgen in
1895. Using a partially evacuated discharge tube called a cathode ray tube,
Röntgen noticed that his detector, a fluorescent screen of barium platinocyanide, glowed when a discharge was
passed through the tube even though
the window of the tube was covered.
Convinced that he had discovered a
new form of radiation, he named it “Xrays” after the common algebraic variable for an unknown quantity. In his
initial experiments, Röntgen noted the
differential passage of X-rays through
matter of differing compositions; less
than a week later, he took the first X-ray
of the human body, that of his wife’s
hand (see Figure 4). Röntgen’s work was
first announced in January 1896, and
the medical applications of X-rays were
almost immediate. X-ray spectroscopy
came a bit later, developed most notably
by Karl Manne Siegbahn, a Swedish
physicist who won the 1924 Nobel Prize
in Physics for his work in developing Xray fluorescence spectroscopy.
X-rays are a type of ionizing radiation, so-called because X-ray photons
have enough energy to eject an electron from an atom, creating ions. (In-
deed, this can be used as a form of
spectroscopy, which was discussed
previously in this column [4].) The
production of ions in this fashion typically is not good, and people usually
have to take safety precautions when
working with X-rays or electromagnetic radiation of smaller wavelength.
Gamma rays were discovered by
French scientist Paul Villard in 1900
during some of the initial investigations into the properties of radioactivity. They were first thought to be
particles emitted during the course of
radioactivity; hence their name, following the identification of alpha particles and beta particles. William
Bragg showed that they ionized gas in
1910, and Ernest Rutherford measured their wavelengths by diffracting
them with crystals, demonstrating
that they were indeed electromagnetic
radiation. Gamma rays are the shortest wavelength, highest frequency, and
highest energy form of electromagnetic radiation.
All regions of the electromagnetic
spectrum are used in spectroscopy.
Figure 5 shows how each region contributes to a form of spectroscopy,
many of which have been addressed in
previous columns.
References
(1) D.W. Ball, Spectroscopy 21(6), 30 (2006).
(2) D.J. Griffiths, Introduction to Electrodynamics (Prentice-Hall, Englewood
Cliffs, New Jersey, 1981).
(3) J.C. Maxwell, Phil. Trans. 155, 459
(1865).
(4) D.W. Ball, Spectroscopy 18(11), 36
(2003).
(5) D.W. Ball, Field Guide to Spectroscopy
(SPIE Press, Bellingham, Washington,
2006).
David W. Ball is
a professor of chemistry at Cleveland State
University in Ohio.
Many of his “Baseline”
columns have been
reprinted in book form
by SPIE Press as The
Basics of Spectroscopy,
available through the
SPIE Web Bookstore at
www.spie.org. His most recent book, Field
Guide to Spectroscopy (published in May
2006), is available from SPIE Press. He can
be reached at [email protected]; his website is academic.csuohio.edu/ball.