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A Walk Through the Electromagnetic Spectrum
K. Morgan, Student Member, IEEE
Paper for ENGR302 Professor Paulo Ribeiro
May 15, 2002
Abstract – This paper explores the fundamental
mathematical principles behind electromagnetic waves and
also the various natures of light. This theory is presented to
give a background for a discussion of the different sections of
the electromagnetic spectrum, including its applications,
properties, and sources.
interact.
These equations are fundamental for an
understanding of electromagnetic waves, so they will be
expanded on below.
I. INTRODUCTION
Light today plays an important role in a vast array of
technologies. But light is not limited to what we can see
with the eye. In fact, the light that we see is a very small
portion of the wide range of light in nature. All light has
one thing in common: it is all electromagnetic waves.
When charges move, their movement is communicated to
the rest of the universe via an electromagnetic wave. So all
light, from radio, to visible, to x rays, to gamma rays, are all
variants of the same thing. This paper aims to explore a
little of the fundamental theory behind the electromagnetic
waves that make up the spectrum. Then it will take a walk
through the spectrum starting with the low-frequency radio
waves and ending up at the very high-frequency gamma
rays. At each stop along the walk, this paper will explore
the various ways mankind has found applications for that
part of the spectrum and also how that section has
manifested itself in nature. It is important to realize that the
divisions of the electromagnetic spectrum are artificial and
are only for organizational purpose. In reality, the spectrum
that has also been called “Maxwell’s rainbow” is continuous
and has properties that span over 25 orders of magnitude.
II. THE ELECTROMAGNETIC WAVE
The effects of electricity and magnetism were observed
as far back as the ancient Greeks. They knew that if amber
was rubbed with fur, it would attract bits of straw. They
also saw that pieces of a magnetic mineral called lodestones
repelled and attracted one another. It was not until 1820,
however, that the Danish physicist Hans Christian Oersted
realized that electricity and magnetism were related when
he found that an electric current in a wire could deflect a
compass needle. Until then, the two sciences had been
developing separately. Around the 1870s, they became
completely intertwined when James Clerk (pronounced
“clark”) Maxwell (Fig. 1) presented a mathematical
electromagnetic theory. Maxwell’s genius shines through
the elegance of his four equations, and they have held
through every test to which they’ve been subjected. They
completely describe the way electricity and magnetism
Fig. 1 – James Clerk Maxwell
A. Gauss’ Law for Electricity
Maxwell’s equations can be written either in integral or
differential form. Both forms are equivalent and provide
useful insights. Boldface symbols represent vectors that
have components in all three directions.
 D  dS    dv
S
v
(1)
and in differential form,
D  
(2)
In (1) and (2), D is electric flux, S is a vector normal to
the surface S, v is some volume, and  represents spacecharge density. In words, the equation states that the only
source that produces a nonzero flux through a closed surface
is free electric charge. The form of (1) makes it clear that
the amount of electric flux passing through a closed surface
S is equal to the total charge in the volume enclosed by S.
The form of (2) says that the divergence of the electric flux
at a point is equal to the charge density there.
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B. Gauss’ Law for Magnetism
 B  dS  0
S
(3)
and in differential form,
B  0
(4)
In (3) and (4), B is magnetic flux, and S is a vector
normal to the surface S. In words, the equation states that
no analog of free electric charges exists for a magnetic field
(there are no magnetic monopoles). The form of (3) makes
it clear that the net magnetic flux through any closed surface
is zero. The form of (4) says that magnetic flux does not
diverge from any single point.
C. Faraday’s Law
B
 dS
S t
 E  dl  
C
(5)
and in differential form,
E  
B
t
(6)
In (5) and (6), E is the electric field, l is a vector on the
contour C, B is the magnetic flux, and S is a vector normal
to the surface S. In words, the equation states that a timevarying magnetic field is a source of an (evidently timevarying) electric field. The form of (5) makes it clear that a
changing flux through a surface S gives rise to an electric
field on the contour of that surface. The form of (6) makes
it clear that a changing magnetic field at a point produces a
nonzero curl of E (an E that could produce a current in a
small loop). Faraday’s law is of profound importance since
electric generators are based on the principle of
electromagnetic induction.
D. Ampere’s Law

C
currents and a time-varying electric field. The form of (7)
makes it clear that current and displacement-current (timevarying electric flux) passing through a surface S give rise
to a magnetic field on the contour of that surface. The form
of (8) says that current and displacement-current at a point
produce magnetic field loops at that point. Ampere’s law is
also of profound importance since electric motors rely on
the magnetic fields produced by currents in a wire.
E. The Uniform Plane Wave
The essence of any electromagnetic radiation (light) is
the traveling electromagnetic wave. In this section, the
equations describing a light wave are derived from
Maxwell’s four equations. In a uniform plane wave, all
electric and magnetic field lines are uniform throughout any
plane where z is constant. A strict uniform plane wave
doesn’t actually exist since all waves exhibit some
curvature. However, at points far from a source of light
(observing a star from earth), the wave is nearly uniform.
First, two assumptions are made about wave motion in
free space. One, there are no charges in the region of the
traveling wave, and two, since there are no charges, there is
no current density (there is displacement current, however).
With these assumptions, Maxwell’s equations can be
written as,
 D  0
(9)
B  0
(10)
B
t
D
H 
t
E  
(11)
(12)
Notice that  and J are both zero since there is no charge.
Now, let us assume that an electric field is changing
sinusoidally and has only an x component.
E  E x  E ( x, y, z ) cos(t  )
(13)
Using Euler’s identity and the standard phasor notion of
electrical analysis, we can write (13) as,
D 

H  dl    J 
  dS
S
t 

(7)
and in differential form,
H  J 
D
t
E s  E xs a x
(14)
Using the notation of (14), we can rewrite (9), (10), (11),
and (12) as,
(8)
In (7) and (8), H is the magnetic field, l is a vector on the
contour C, D is the electric flux, J is the current density, and
S is a vector normal to the surface S. In words, the equation
states that the sources of a magnetic field are electric
  Es  0
(15)
  Hs  0
(16)
  E s   j 0 H s
(17)
  H s  j 0 E s
(18)
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The wave equation can be produced with just a little
vector analysis,
    E s  (  E s )   2 E s   j 0   H s
 2  0  0 E s   2 E s
c
1
00
and the wavelength of the electromagnetic wave is
2 c

k0
f

Therefore,
 2 E s  k 02 E s
(19)
where k0 is the free space wavenumber, defined as
With a solution for Ex, a formulation of the magnetic
field can be found from (17).
H y ( z, t )  E x 0
k 0   μ0 ε 0
A solution of (19) is given by
E x ( z, t )  E x 0 cos(t  k 0 z )
(20)
 3  10 8 m/s
0
cos(t  k 0 z )
0
(21)
Fig. 3 is a visual representation of (21). It can be seen from
(21) that the oscillations of the magnetic field are in phase
with the electric field oscillations but are rotated 90 in
space.
Fig. 2 is a visual representation of (20). The traveling
wave nature of light can be deduced from this equation.
Since  is the radian time frequency, it can be seen that at a
particular point in space, the wave repeats itself over time.
That is, if one were to watch a particular point on the wave,
the electric and magnetic components would oscillate at the
frequency   2f . It can also be seen from (20) that the
wave repeats itself in space. That is, if one were to take a
“snapshot” of the wave at a particular instant, the electric
and magnetic components would oscillate through space at
a frequency k0.
Fig. 3
Obviously, energy is transferred with a traveling
electromagnetic wave. Similar to the definition of power in
electrical analysis is the power of an electromagnetic wave,
known as the Poynting vector,
P 
Fig. 2
Other important information can also be gleaned from
(20). First, it can be seen that the velocity of light in free
space is
1
EB
0
(22)
Note that the Poynting vector points in the direction of
power transfer as well as the direction of the traveling wave.
In (22), B can be put in terms of E and the power can be
averaged to provide a formula for the intensity of an
electromagnetic wave,
I
1
2
E rms
c 0
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F. A More Qualitative Approach
While the rigorous mathematical derivation of the wave
equation contains useful insights and shows the validity of
Maxwell’s equations, it doesn’t give a very intuitive idea of
electromagnetic wave propagation. It is instructive for
visualizing a traveling wave to look at a more qualitative
approach at how Maxwell’s equations imply wave
propagation.
First, we will assume that a wave already exists as shown
in Fig. 4.
slightly larger than B on the left so that the induced electric
field opposes the change in E.
Fig. 6
Fig. 4
Next, let us focus at the point on the wave in the gray
shaded circle (Fig. 4). At the instant in time and the
particular point in space shown, the magnetic field B is
decreasing (if the wave were to move slightly in the x
direction, B would be slightly smaller). Now let us draw a
very small rectangle around this point as shown in Fig. 5.
Fig. 5
Due to Faraday’s law of induction [(5)], a nonzero curl will
be induced. That is, at the point where B is decreasing, a
small electric field loop is induced. Moreover, the electric
field on the right side of the small rectangle is slightly
bigger than the field on the left side. This is because the
induced electric field is in the direction so as to produce a
magnetic field (out of the page) that opposes the changing
B.
The same argument can be made with the roles of the
electric and magnetic fields reversed. Suppose E is
decreasing at a point as shown in Fig. 6. According to
Ampere’s law [(7)], the changing E field induces a
magnetic field. B on the right side of the small rectangle is
Thus, it can be seen that the traveling electromagnetic
wave “rides” on itself. A changing electric field induces a
changing magnetic field. That induced magnetic field then
induces a changing electric field. That induced electric field
then induces another magnetic field, and so on. It sustains
itself and can, in theory, travel forever. An electromagnetic
wave is different from all other kinds of waves in that it
doesn’t require a medium to travel through. We are able to
see stars because electromagnetic waves travel many light
years through the vacuum of space.
It is important to note that the wave as seen in Fig. 4 does
not exist like that as a single “ray” of light. According to
Maxwell’s equations, such a ray by itself is impossible
because at each point on the ray, electric and magnetic field
lines have a nonzero divergence (they are coming out of the
line). This is contrary to both (9) and (10), which state that
the divergence of E and B is zero. Thus, one should not
think of pictures of an electromagnetic wave as in Fig. 4 as
a single ray of light. Rather the picture in Fig. 4 is meant to
represent the way E and B vary on a single line in an entire
field of varying electric and magnetic field lines. A picture
of this could be seen by taking the wave in Fig. 4 and
rotating it through every point in space. The key concept to
realize is that the electric and magnetic field lines are part of
closed loops of force that extend throughout space.
G. The Nature of Light
Maxwell’s equations describe mathematically what is
actually a very complex process that is difficult to
understand.
In fact, physicists today don’t have a
completely clear picture of the nature of light. Experiments
have shown that light behaves according to three separate
paradigms. The first is that light is an electromagnetic
wave, the second is that light is a stream of photons, and the
third is that light is a probability wave. Only one paradigm
doesn’t explain all the observed phenomena associated with
light. Thus, physicists say that light is all three paradigms
simultaneously.
The above derivations using Maxwell’s equations show
how light can be though of as an electromagnetic wave. It
was Thomas Young, however, who first proved that light is
a wave with his famous double-slit experiment in 1801. As
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shown in Fig. 7, incident light that can be considered a
uniform plane wave strikes the first plate with a single slit.
The single slit can then be thought of as a point source of
light. The light from this source travels through the two
slits on the other plate producing two more waves that
interfere with each other. The result is an interference
pattern (variations of intensity) on the screen. This result
can be explained only if the light travels as a wave. The
wave theory of light seems to best describe lower frequency
light.
Finally, light can be described as a probability wave.
This aspect of light is probably the strangest and the farthest
away from everyday experience. If a photon detector were
to be placed on the screen in Fig. 7, it would be seen that
photons strike the detector at random intervals. Thus, the
intensity of the light at a specific point can be though of as a
probability that a photon will strike that point. An even
stranger effect of light is that if instead of a wave striking
the double-slit plate in Fig. 7, a single photon were fired
from the point source, an interference pattern like that
shown in Fig. 9 would still build up on the screen.
Although the single photon can only travel through a single
slit, it still seems to be aware of the presence of the other
slit, so it interferes with itself.
Fig. 7
Fig. 9
As the frequency of light increases to that of ultraviolet
and x rays, the photon theory of light seems to make more
sense than the wave theory does. At these frequencies, light
seems not like a wave, but like tiny packets of energy called
photons. In the photoelectric experiment as shown in Fig. 8,
it can be seen that light of a certain minimum frequency is
not able to knock electrons out of the target T no matter how
intense the incident light is. This is due to the fact that the
electrons need a certain minimum energy to be knocked out
of their orbitals. The energy of a photon doesn’t depend at
all on the intensity of the light it travels in; its energy is
given by E  hf where h is Planck’s constant and f is the
frequency. Increasing the intensity of light only increases
the number of photons present.
III. THE ELECTROMAGNETIC SPECTRUM
Fig. 8
The previous section of this paper explored the concept
of the electromagnetic wave. Maxwell’s equations allow
for waves with extremely long and extremely short
wavelengths (and all lengths in between). Collectively, this
continuous range of wavelengths is known as the
electromagnetic spectrum.
Our modern-day electromagnetic spectrum as seen in
Appendix A is a result of centuries of discoveries. In 1665,
Francesco Grimaldi claimed that light was a wave.
Gradually, forms of radiation near visible light (infrared and
ultraviolet) were discovered. After the introduction of
Maxwell’s electromagnetic theory, physicists realized the
possibility of much longer and much shorter waves.
Heinrich Hertz soon discovered longer waves in the 1880s,
and then Wilhelm Roentgen and others discovered shorter
waves in the 1890s.
This section aims to explore a very small portion of the
information scientists know about our current
electromagnetic spectrum.
A. Radio
Fueled by Maxwell’s electromagnetic theory, Heinrich
Hertz (Fig. 10) discovered and transmitted radio waves in
1887. It was M. G. Marconi, however, who came up with
the idea of using radio waves for communication. Thus, the
ever-popular radio was born. Radio continues today to have
a wide array of applications. In fact, the radio section of the
electromagnetic spectrum extends farther than any other
section, and it is the most regulated. Because of its use in
communication, the radio spectrum has been tightly
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regulated as shown in Appendix B.
Fig. 12
Fig. 10 – Heinrich Hertz
The frequency of radio waves ranges from about 10 3 to
10 Hz, and the wavelength from about 104 to 10-3 m. Fig.
11 shows that these waves range from about the size of a
mountain to the size of a period on a page. The energy of
radio photons ranges from about 10-30 to 10-23 J. Thus, radio
waves are very low-energy electromagnetic waves.
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.
A period
Mountains
Radar, which stands for radio detection and ranging, is
one of the most important applications of the radio
spectrum. It was developed by the British around the 1930s
for use in World War II. When the resonant-cavity
magnetron was invented, radar with very short wavelengths
(microwave) became possible. Instead of radar, it was
called LIDAR for light detection and ranging.
In a radar system, a transmitter produces a radio wave.
When the wave strikes distant objects, part of it is reflected
back to where it was transmitted where it can be detected.
Because the radio waves have a finite velocity, the time it
takes for the wave to return can be measured and used to
determine how far away the object is.
Humans are not the only source of radio waves. Many
objects in the universe emit electromagnetic radiation in the
radio spectrum. In fact, an organization called SETI
(Search for ExtraTerrestrial Intelligence) constantly
monitors the radio waves coming from space hoping they
can “listen in” on radio transmissions from other forms of
life in the universe. Radio astronomers use very large
telescope arrays to observe radio from space. Fig. 13 is an
image of the Crab Nebula; it is caused by the radio emission
from neutral hydrogen atoms. These arrays can be groundbased because there is a very large atmospheric window for
radio waves as shown in Appendix C.
Fig. 11
All manmade radio waves can be generated with variants
of the system pictured in Fig. 12. As shown, an energy
source excites an LC oscillator, which induces an AC signal
that travels the transmission line to the antenna. At the
antenna, charge oscillates back and forth giving rise to
changing electric and magnetic fields. These changing
fields travel outward from the antenna as an electromagnetic
wave. The frequency of the wave is   1 / LC .
Antennas need to be approximately the same dimensions as
the wave they are intended to produce, so some antennas are
many kilometers long and some are small enough to fit in an
integrated circuit. One reason radio is used so heavily in
communications is that radio waves reflect off the
ionosphere, the part of the earth’s atmosphere composed of
charged particles. Because of the reflective properties of
this layer, radio signals can be broadcast to points far away
from the source.
Fig. 13
B. Microwave
The usefulness of microwaves was discovered by
accident in 1945 by the American engineer Percy Le Baron
Spencer at Raytheon. While working with radar (which can
also be in the microwave range), he found that food in his
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pocket would heat up when he was struck with radar
radiation. The concept was quickly applied commercially,
and the microwave oven was born.
The optimal frequency for heating food turns out to be
around 2.45 GHz, which is a wavelength of about 12.2 cm.
The inside dimensions of a microwave oven are even
multiples of 12.2 cm so that the waves constructively
interfere to deliver more power to the food being heated.
Microwaves heat food because they are the right
frequency to be able to break bonds between water
molecules. Molecules in water are effectively tiny dipoles.
As a result, they tend to “stick” to one another. When these
bonds are broken, energy is released and contributes to the
temperature of the water. Microwaves cause the tiny
dipoles to rotate (Fig. 14), thus breaking bonds with other
water molecules. The temperature of the water increases
and heats the food. Microwave ovens are able to heat food
faster than a convection oven because microwaves can
penetrate into the food.
C. Infrared
In 1800, William Herschel (Fig. 16) wanted to measure
the temperature of the different colors of the visible
spectrum. He found that the hottest part of the spectrum
was actually where there was no color at all. The region he
was measuring was what is now known as infrared. It is
called infrared because it is “below” red visible light on the
spectrum.
Fig. 16 – William Herschel
The frequency of infrared waves ranges from about 1010
to 1014 Hz, and the wavelength from about 10-3 to 10-6 m.
Fig. 17 shows that these waves are about the size of cells,
which is one reason why skin is able to detect infrared
radiation as the sensation of heat. The energy of infrared
photons ranges from about 10-23 to 10-19 J.
Fig. 14
Microwave radiation is also observed coming from
space. In 1965, scientists working on a radio telescope at
Bell Laboratories noticed a strange microwave signal
causing background noise in their telescope. It was later
determined that this signal was radiation remnants of the
Big Bang, and it was termed CMB for Cosmic Microwave
Background. A picture of this background radiation is
shown in Fig. 15.
Fig. 15
Cells
Fig. 17
Infrared radiation is produced by thermally excited atoms
and molecules. Thus, anything that is above absolute zero
will emit some infrared energy.
Infrared has found applications in remote controlling and
wireless data communication. Some light emitting diodes
will produce light in the infrared region. This is useful
because an infrared signal can be sent just as a signal would
using visible light, but the infrared signal is invisible to
humans. Infrared has also been used to image the clouds in
the earth’s atmosphere. Meteorologists are able to get better
cloud information using infrared and visible rather than just
visible light. Infrared light is also used in determining types
of chemical bonds.
Parts of the universe are above absolute zero, so space is
of course a source of infrared waves. Fig. 18 is an infrared
image of the Crab Nebula. These images must be taken
from a satellite because most of the infrared radiation from
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space is absorbed by the earth’s atmosphere (see Appendix
C).
Fig. 18
Fig. 19
D. Visible
The frequency of visible light is of the order of 10 14 Hz,
and the wavelength about 10-6 m.
Thus, visible
electromagnetic waves are near the sizes of cells as shown
in Fig. 17. While visible waves make up a very small
portion of the entire spectrum, they are the most important
to humans because of the amazing detection instrument
called the eye. The energy of visible photons is about 10 -19
J.
The concept of light reception and perception in the eye
is not completely understood, but scientists do have some
idea of how it works. Visible waves coming into the eye are
focused onto the retina (Fig. 19), which contains different
kinds of cells capable of converting the incoming light to a
chemical signal to be sent along the optic nerve. The outer
segment of the receptors in the eye contain photopigment
molecules. When a photon strikes one of these molecules,
the molecule undergoes a shape change called
isomerization. There are four types of receptors: 1. Rods,
which don’t detect color, but are used to detect lowintensity light. 2. L-receptors (for long wavelengths) are
most sensitive to red light. 3. M-receptors (for middle
wavelengths) are most sensitive to green light. 4. Sreceptors (for short wavelengths) are most sensitive to blue
light. The color receptors are also known as cones. The
atmospheric windows chart in Appendix C shows why
evolution decided that the eye should see visible light. As
can be seen, there is a small window that allows the sun’s
visible radiation to reach all the way to the earth’s surface.
An interesting question to ask is “why is the sun
yellow”? The primary law governing blackbody radiation is
the Plank Radiation Law [(23)] where  is the wavelength,
T is the absolute temperature, h is Planck’s constant, c is the
speed of light, and k is Boltzmann’s constant.
E ( , T ) 
2hc 2
1
5
hc / kT
 e
1
(23)
Since the surface temperature of the sun is about 5800 K, it
emits radiation at the intensities and wavelengths shown in
Fig. 20 according to (23). Notice that the sun produces
electromagnetic waves of many different wavelengths, but
the radiation is most intense at about 5000 Angstroms,
which is the wavelength of yellow light.
Fig. 20
By way of comparison, the Crab Nebula pictured in Fig.
13 and Fig. 18 is shown in visible light in Fig. 21.
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Fig. 21
E. Ultraviolet
The discovery of ultraviolet light came about in a story
similar to that of infrared. In 1801, Johann Ritter (Fig. 22)
was trying to find out which color of light caused the
quickest chemical reaction of silver chloride. To his
surprise, he found that light past the visible violet light
caused the most vigorous reactions. Thus, ultraviolet light
was discovered. It is called ultraviolet because it is
“beyond” the visible violet light in the electromagnetic
spectrum.
so there are not many manmade sources. There are,
however, many commercial products designed to block
ultraviolet (often called UV). Suntan lotion is designed to
block UV rays from the sun. Although most of the sun’s
UV radiation is blocked by the earth’s atmosphere, even the
small amount that reaches the surface can be detrimental to
human health. It is known that long-term exposure to UV
rays can cause cancer.
While there are not many manmade UV sources,
scientists still study it, and they generate very high intensity
ultraviolet radiation at facilities like the ALS (Advanced
Light Source) in San Francisco.
Astronomers also study the ultraviolet radiation coming
from space. Stars and other objects in the universe that are
much hotter than the sun are good sources of ultraviolet.
Satellite telescopes like the Hubble Space Telescope must
be used to observe ultraviolet because almost all ultraviolet
radiation is absorbed by the earth’s atmosphere as shown in
Appendix C. Fig. 24 is an ultraviolet image taken by the
Hubble.
Fig. 24
Fig. 22 – Johann Ritter
The frequency of ultraviolet waves ranges from about
1014 to 1016 Hz, and the wavelength from about 10-6 to 10-8
m. Fig. 23 shows that these waves are on the order of virus
and protein sizes. The energy of ultraviolet photons ranges
from about 10-19 to 10-17 J.
Viruses
F. X-Ray
X-rays, like many other parts of the electromagnetic
spectrum, were discovered by accident in 1895 by Wilhelm
Roentgen (Fig. 25). He was a German physicist working on
cathode ray tubes when he noticed a glow coming from a
chemical called barium platinocyanide. He found that the
rays coming from the tube could pass through many objects
including flesh. They had a much higher frequency than
visible light and were called “X-rays” because of their
unknown nature.
Proteins
Fig. 23
There are not many applications that use ultraviolet light,
Fig. 25 – Wilhelm Roentgen
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The frequency of x rays ranges from about 10 16 to 1019
Hz, and the wavelength from about 10-8 to 10-10 m. Fig. 26
shows that x rays are molecular and atomic scale
electromagnetic waves. The energy of x-ray photons ranges
from about 10-17 to 10-14 J.
Molecules
shown in Fig. 28 emit x-rays because they are at extremely
high temperatures.
Atoms
Fig. 26
Fig. 28
Scientists at the ALS in San Francisco produce extremely
intense x rays for experimental purposes. Perhaps the most
common application, however, is the x-ray machines used
in doctors’ offices. Fig. 27 is a simplified schematic of how
x rays are produced in these machines. A voltage is applied
to a filament F, which heats up to the point that electrons
boil off its surface. A high voltage V sets up an electric
field that accelerates these electrons toward the target T (this
is similar to the electron beam in a cathode ray tube). The
target (which is often made of Tungsten) emits photons in
the x-ray region when struck by electrons. These photons
then pass only through the window W, which is usually
made of quartz. X rays have such small wavelengths that
they pass through flesh, but are absorbed by bones, which
are denser than flesh. This is how images are formed on xray film. The film starts out white, and the areas exposed to
x rays darken. X rays are very high energy and can tear
through molecules like DNA.
This is why it is
recommended that people limit their exposure to x rays.
G. Gamma-Ray
In 1898, Ernest Rutherford (Fig. 29) discovered gamma
rays while performing experiments with nuclear emissions.
He named them gamma rays to follow the theme of alpha
and beta rays, whose existence he had discovered earlier.
Fig. 29 – Ernest Rutherford
The frequency of the most energetic waves called
gamma-rays ranges from about 1019 to 1024 Hz, and the
wavelength from about 10-10 to 10-14 m. Fig. 30 shows that
these waves are near the size of the fundamental particles in
physics. The energy of gamma-ray photons ranges from
about 10-14 to 10-10 J, which represents an extremely high
energy for a single photon. Gamma rays are the most
electron
energetic
of the electromagnetic spectrum.
neutron
Fig. 27
One of the main areas of astronomy is x-ray astronomy.
Satellite telescopes such as Harvard’s Chandra X-ray
Observatory can take striking pictures of the x-ray
emissions of cosmic objects. Objects like the Crab Nebula
proton
Atomic Nuclei
Fig. 30
11
Manmade gamma rays are produced only in complex
physics experiments involving atomic nuclei and in some
medical equipment. Doctors use these high-energy waves
to kill cancerous cells. Gamma-ray radiation from spent
nuclear fuel must be tightly controlled since this highenergy radiation can be very hazardous to human health,
even at very low doses.
A very interesting natural source of gamma-ray photons
is the proton-proton cycle as shown in Fig. 31. This process
is the energy source of the stars and is also known as
thermonuclear fusion. Two protons combine to form
deuteron, a process that also produces a positron and a
neutrino. This proton-proton event is actually extremely
rare (otherwise the sun would explode almost instantly), but
because of the huge number of protons in the sun, deuterium
is actually produced at the rate of 10 12 kg/s. Two gamma
rays are first produced when the positron annihilates a free
electron. Deuterium again combines with a proton to form
helium and a gamma-ray photon. Thus, in one cycle, six
gamma-ray photons are created.
Fig. 32
IV. CONCLUDING REMARKS
The intricacies of light continue to remain a mystery to
physicists. The exact nature of light cannot be explained.
Light seems to be a strange mixture of electromagnetic
waves, photons, and probability. The fact that we do not
fully understand light has not kept humans from taking full
advantage of the electromagnetic spectrum. New uses for
light are always being discovered and are pushing
technology forward. Knowledge of the electromagnetic
spectrum has also helped us learn more about the universe.
The four different images of the Crab Nebula in this paper
show that we can’t limit ourselves to things that can only be
seen by the eye.
V. REFERENCES
Fig. 31
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Space is a source of a wide range of electromagnetic
waves, and gamma rays are no exception. Probably the
most interesting phenomenon in gamma-ray astronomy was
discovered in the late 1960s. Detectors on the Vela satellite
recorded bursts of gamma rays (Fig. 32) coming from deep
space. Today, astronomers record these events about once
every day. During these bursts, the whole sky lights up with
gamma-ray radiation. While the source of these bursts is
unknown, it is suspected that they may be glimpses of
cataclysmic events during the early stages of the universe.
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12
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13
Appendix A. Electromagnetic Spectrum Chart
14
Appendix B. Radio Chart
15
Appendix C. Atmospheric Windows