Electromagnetic spectrum
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Class
Freq
uency
300 EHz
γ
Energy
1 pm 1.24 MeV
Gamma rays
30 EHz
HX
Wave
length
10 pm
124 keV
Hard X-rays
3 EHz 100 pm 12.4 keV
SX
Soft X-rays
300 PHz
30 PHz
EUV
NUV
1 nm 1.24 keV
10 nm
124 eV
3 PHz 100 nm
12.4 eV
Extreme
ultraviolet
Near
ultraviolet
Visible
300 THz
NIR
1.24 eV
Near Infrared
30 THz
MIR
1 μm
10 μm 124 meV
Mid infrared
3 THz 100 μm 12.4 meV
FIR
Far infrared
300 GHz
EHF
Extremely high
frequency
30 GHz
SHF
VHF
1 cm
124 μeV
Super high
frequency
Radio
waves
UHF
1 mm 1.24 meV
3 GHz
1 dm 12.4 μeV
300 MHz
1 m 1.24 μeV
Ultra high
frequency
Very high
frequency
30 MHz
10 m
124 neV
HF
MF
LF
High
frequency
3 MHz
100 m 12.4 neV
300 kHz
1 km 1.24 neV
Medium
frequency
Low
frequency
30 kHz
VLF
10 km
124 peV
Very low
frequency
3 kHz 100 km 12.4 peV
VF / ULF
Voice
frequency
300 Hz
SLF
1 Mm 1.24 peV
Super low
frequency
30 Hz 10 Mm
ELF
124 feV
Extremely low
frequency
3 Hz 100 Mm 12.4 feV
Sources: File:Light spectrum.svg [1] [2] [3]
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic
radiation.[4] The "electromagnetic spectrum" of an object has a different meaning, and is instead the
characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.
The electromagnetic spectrum extends from below the low frequencies used for
modern radio communication togamma radiation at the short-wavelength (high-frequency) end,
thereby covering wavelengths from thousands ofkilometers down to a fraction of the size of an atom.
The limit for long wavelengths is the size of the universe itself, while it is thought that the short
wavelength limit is in the vicinity of the Planck length.[5] Until the middle of last century it was believed
by most physicists that this spectrum was infinite and continuous.
Most parts of the electromagnetic spectrum are used in science for spectroscopic and other probing
interactions, as ways to study and characterize matter.[6] In addition, radiation from various parts of
the spectrum has found many other uses for communications and manufacturing
(see electromagnetic radiation for more applications).
Contents
[hide]







1 History of electromagnetic spectrum discovery
2 Range of the spectrum
3 Rationale for spectrum regional names
4 Types of radiation
o 4.1 Boundaries
o 4.2 Regions of the spectrum
o 4.3 Radio frequency
o 4.4 Microwaves
o 4.5 Terahertz radiation
o 4.6 Infrared radiation
o 4.7 Visible radiation (light)
o 4.8 Ultraviolet radiation
o 4.9 X-rays
o 4.10 Gamma rays
5 See also
6 Notes and references
7 External links
History of electromagnetic spectrum discovery[edit]
For most of history, visible light was the only known part of the electromagnetic spectrum.
The ancient Greeksrecognized that light traveled in straight lines and studied some of its properties,
including reflection and refraction. Over the years the study of light continued and during the 16th
and 17th centuries there were conflicting theories which regarded light as either a wave or a
particle.[citation needed]
The first discovery of electromagnetic radiation other than visible light came in 1800, when William
Herscheldiscovered infrared radiation.[7] He was studying the temperature of different colors by
moving a thermometer through light split by a prism. He noticed that the highest temperature was
beyond red. He theorized that this temperature change was due to "calorific rays" which would be in
fact a type of light ray that could not be seen. The next year, Johann Ritter worked at the other end
of the spectrum and noticed what he called "chemical rays" (invisible light rays that induced certain
chemical reactions) that behaved similar to visible violet light rays, but were beyond them in the
spectrum.[8] They were later renamed ultraviolet radiation.
Electromagnetic radiation had been first linked to electromagnetism in 1845, when Michael
Faraday noticed that the polarization of light traveling through a transparent material responded to
a magnetic field (see Faraday effect). During the 1860s James Maxwell developed four partial
differential equations for the electromagnetic field. Two of these equations predicted the possibility
of, and behavior of, waves in the field. Analyzing the speed of these theoretical waves, Maxwell
realized that they must travel at a speed that was about the known speed of light. This startling
coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic
wave.
Maxwell's equations predicted an infinite number of frequencies of electromagnetic waves, all
traveling at the speed of light. This was the first indication of the existence of the entire
electromagnetic spectrum.
Maxwell's predicted waves included waves at very low frequencies compared to infrared, which in
theory might be created by oscillating charges in an ordinary electrical circuit of a certain type.
Attempting to prove Maxwell's equations and detect such low frequency electromagnetic radiation, in
1886 the physicist Heinrich Hertz built an apparatus to generate and detect what is now called radio
waves. Hertz found the waves and was able to infer (by measuring their wavelength and multiplying
it by their frequency) that they traveled at the speed of light. Hertz also demonstrated that the new
radiation could be both reflected and refracted by various dielectric media, in the same manner as
light. For example, Hertz was able to focus the waves using a lens made of tree resin. In a later
experiment, Hertz similarly produced and measured the properties of microwaves. These new types
of waves paved the way for inventions such as thewireless telegraph and the radio.
In 1895 Wilhelm Röntgen noticed a new type of radiation emitted during an experiment with an
evacuated tube subjected to a high voltage. He called these radiations x-rays and found that they
were able to travel through parts of the human body but were reflected or stopped by denser matter
such as bones. Before long, many uses were found for them in the field of medicine.
The last portion of the electromagnetic spectrum was filled in with the discovery of gamma rays. In
1900 Paul Villard was studying the radioactive emissions of radium when he identified a new type of
radiation that he first thought consisted of particles similar to known alpha and beta particles, but
with the power of being far more penetrating than either. However, in 1910, British physicist William
Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in
1914, Ernest Rutherford (who had named them gamma rays in 1903 when he realized that they
were fundamentally different from charged alpha and beta rays) and Edward Andrade measured
their wavelengths, and found that gamma rays were similar to X-rays, but with shorter wavelengths
and higher frequencies.
Range of the spectrum[edit]
Electromagnetic waves are typically described by any of the following three physical properties:
the frequency f, wavelength λ, or photon energy E. Frequencies observed in astronomy range
from 2.4×1023 Hz (1 GeV gamma rays) down to the local plasma frequency of the ionized interstellar
medium (~1 kHz). Wavelength is inversely proportional to the wave frequency,[6] so gamma rays
have very short wavelengths that are fractions of the size of atoms, whereas wavelengths on the
opposite end of the spectrum can be as long as the universe. Photon energy is directly proportional
to the wave frequency, so gamma ray photons have the highest energy (around a billion electron
volts), while radio wave photons have very low energy (around a femtoelectronvolt). These relations
are illustrated by the following equations:
where:


c = 299,792,458 m/s is the speed of light in vacuum and
h = 6.62606896(33)×10−34 J s = 4.13566733(10)×10−15 eV s is Planck's constant.[9]
Whenever electromagnetic waves exist in a medium with matter, their wavelength is decreased.
Wavelengths of electromagnetic radiation, no matter what medium they are traveling through,
are usually quoted in terms of the vacuum wavelength, although this is not always explicitly
stated.
Generally, electromagnetic radiation is classified by wavelength into radio
wave, microwave, terahertz (or sub-millimeter) radiation, infrared, the visible region is perceived
as light,ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its
wavelength. When EM radiation interacts with single atoms and molecules, its behavior also
depends on the amount of energy per quantum (photon) it carries.
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of
400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to
2500 nm. Detailed information about the physical properties of objects, gases, or even stars can
be obtained from this type of device. Spectroscopes are widely used inastrophysics. For
example, many hydrogen atoms emit a radio wave photon that has a wavelength of 21.12 cm.
Also, frequencies of 30 Hz and below can be produced by and are important in the study of
certain stellar nebulae[10] and frequencies as high as 2.9×1027 Hz have been detected from
astrophysical sources.[11]
Rationale for spectrum regional names[edit]
Electromagnetic radiation interacts with matter in different ways across the spectrum. These
types of interaction are so different that historically different names have been applied to
different parts of the spectrum, as though these were different types of radiation. Thus, although
these "different kinds" of electromagnetic radiation form a quantitatively continuous spectrum of
frequencies and wavelengths, the spectrum remains divided for practical reasons related to
these qualitative interaction differences.
Region of the
spectrum
Main interactions with matter
Radio
Collective oscillation of charge carriers in bulk material (plasma oscillation). An
example would be the oscillatory travels of the electrons in anantenna.
Microwave through
farinfrared
Plasma oscillation, molecular rotation
Near infrared
Molecular vibration, plasma oscillation (in metals only)
Visible
Molecular electron excitation (including pigment molecules found in the human
retina), plasma oscillations (in metals only)
Ultraviolet
Excitation of molecular and atomic valence electrons, including ejection of the
electrons (photoelectric effect)
X-rays
Excitation and ejection of core atomic electrons, Compton scattering (for low
atomic numbers)
Gamma rays
Energetic ejection of core electrons in heavy elements, Compton scattering (for
all atomic numbers), excitation of atomic nuclei, including dissociation of nuclei
High-energy gamma
rays
Creation of particle-antiparticle pairs. At very high energies a single photon can
create a shower of high-energy particles and antiparticles upon interaction with
matter.
Types of radiation[edit]
The electromagnetic spectrum
Boundaries[edit]
A discussion of the regions (or bands or types) of the electromagnetic spectrum is given below.
Note that there are no precisely defined boundaries between the bands of the electromagnetic
spectrum; rather they fade into each other like the bands in a rainbow (which is the subspectrum of visible light). Radiation of each frequency and wavelength (or in each band) will
have a mixture of properties of two regions of the spectrum that bound it. For example, red light
resembles infrared radiation in that it can excite and add energy to some chemical bonds and
indeed must do so to power the chemical mechanisms responsible for photosynthesis and the
working of the visual system.
Regions of the spectrum[edit]
The types of electromagnetic radiation are broadly classified into the following classes:[6]
1.
2.
3.
4.
5.
6.
7.
8.
Gamma radiation
X-ray radiation
Ultraviolet radiation
Visible radiation
Infrared radiation
Terahertz radiation
Microwave radiation
Radio waves
This classification goes in the increasing order of wavelength, which is characteristic of the type
of radiation.[6] While, in general, the classification scheme is accurate, in reality there is often
some overlap between neighboring types of electromagnetic energy. For example, SLF radio
waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as
electric power, although the latter is, in the strict sense, not electromagnetic radiation at all
(see near and far field).
The distinction between X-rays and gamma rays is partly based on sources: the photons
generated from nuclear decay or other nuclear and subnuclear/particle process, are always
termed gamma rays, whereas X-rays are generated by electronic transitions involving highly
energetic inner atomic electrons.[12][13][14] In general, nuclear transitions are much more energetic
than electronic transitions, so gamma-rays are more energetic than X-rays, but exceptions exist.
By analogy to electronic transitions, muonic atom transitions are also said to produce X-rays,
even though their energy may exceed 6 megaelectronvolts (0.96 pJ),[15] whereas there are many
(77 known to be less than 10 keV (1.6 fJ)) low-energy nuclear transitions (e.g., the 7.6 eV
(1.22 aJ) nuclear transition of thorium-229), and, despite being one million-fold less energetic
than some muonic X-rays, the emitted photons are still called gamma rays due to their nuclear
origin.[16]
The convention that EM radiation that is known to come from the nucleus, is always called
"gamma ray" radiation is the only convention that is universally respected, however. Many
astronomical gamma ray sources (such as gamma ray bursts) are known to be too energetic (in
both intensity and wavelength) to be of nuclear origin. Quite often, in high energy physics and in
medical radiotherapy, very high energy EMR (in the >10 MeV region) which is of higher energy
than any nuclear gamma ray, is not referred to as either X-ray or gamma-ray, but instead by the
generic term of "high energy photons."
The region of the spectrum in which a particular observed electromagnetic radiation falls,
is reference frame-dependent (due to the Doppler shift for light), so EM radiation that one
observer would say is in one region of the spectrum could appear to an observer moving at a
substantial fraction of the speed of light with respect to the first to be in another part of the
spectrum. For example, consider the cosmic microwave background. It was produced, when
matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state.
These photons were from Lyman series transitions, putting them in the ultraviolet (UV) part of
the electromagnetic spectrum. Now this radiation has undergone enough cosmological red
shift to put it into the microwave region of the spectrum for observers moving slowly (compared
to the speed of light) with respect to the cosmos.
Radio frequency[edit]
Main articles: Radio frequency, Radio spectrum and Radio waves
Radio waves generally are utilized by antennas of appropriate size (according to the principle
of resonance), with wavelengths ranging from hundreds of meters to about one millimeter. They
are used for transmission of data, via modulation. Television, mobile phones, wireless
networking, and amateur radio all use radio waves. The use of the radio spectrum is regulated
by many governments through frequency allocation.
Radio waves can be made to carry information by varying a combination of the amplitude,
frequency, and phase of the wave within a frequency band. When EM radiation impinges upon
a conductor, it couples to the conductor, travels along it, and induces an electric current on the
surface of that conductor by exciting the electrons of the conducting material. This effect
(the skin effect) is used in antennas.
Microwaves[edit]
Main article: Microwaves
Plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation.
The super-high frequency (SHF) and extremely high frequency (EHF) of microwaves are on the
short side of radio waves. Microwaves are waves that are typically short enough (measured in
millimeters) to employ tubular metalwaveguides of reasonable diameter. Microwave energy is
produced with klystron and magnetron tubes, and with solid state diodes such
as Gunn and IMPATT devices. Microwaves are absorbed by molecules that have a dipole
momentin liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave
radiation is used in Wi-Fi, although this is at intensity levels unable to cause thermal heating.
Volumetric heating, as used by microwave ovens, transfers energy through the material
electromagnetically, not as a thermal heat flux. The benefit of this is a more uniform heating and
reduced heating time; microwaves can heat material in less than 1% of the time of conventional
heating methods.
When active, the average microwave oven is powerful enough to cause interference at close
range with poorly shielded electromagnetic fields such as those found in mobile medical devices
and poorly made consumer electronics.[citation needed]
Terahertz radiation[edit]
Main article: Terahertz radiation
Terahertz radiation is a region of the spectrum between far infrared and microwaves. Until
recently, the range was rarely studied and few sources existed for microwave energy at the high
end of the band (sub-millimeter waves or so-called terahertz waves), but applications such as
imaging and communications are now appearing. Scientists are also looking to apply terahertz
technology in the armed forces, where high-frequency waves might be directed at enemy troops
to incapacitate their electronic equipment.[17]
Infrared radiation[edit]
Main article: Infrared radiation
The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz to
400 THz (1 mm - 750 nm). It can be divided into three parts:[6]


Far-infrared, from 300 GHz to 30 THz (1 mm - 10 μm). The lower part of this range may
also be called microwaves. This radiation is typically absorbed by so-called rotational modes
in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The
water in Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere
in effect opaque. However, there are certain wavelength ranges ("windows") within the
opaque range that allow partial transmission, and can be used for astronomy. The
wavelength range from approximately 200 μm up to a few mm is often referred to as "submillimeter" in astronomy, reserving far infrared for wavelengths below 200 μm.
Mid-infrared, from 30 to 120 THz (10 - 2.5 μm). Hot objects (black-body radiators) can
radiate strongly in this range, and human skin at normal body temperature radiates strongly

at the lower end of this region. This radiation is absorbed by molecular vibrations, where the
different atoms in a molecule vibrate around their equilibrium positions. This range is
sometimes called the fingerprint region, since the mid-infrared absorption spectrum of a
compound is very specific for that compound.
Near-infrared, from 120 to 400 THz (2,500 - 750 nm). Physical processes that are relevant
for this range are similar to those for visible light. The highest frequences in this region can
be detected directly by some types of photographic film, and by many types of solid
state image sensors for infrared photography and videography.
Visible radiation (light)[edit]
Main article: Visible spectrum
Above infrared in frequency comes visible light. The Sun emits its peak power in the visible
region, although integrating the entire emission power spectrum through all wavelengths shows
that the Sun emits slightly more infrared than visible light.[18] By definition, visible light is the part
of the EM spectrum to which the human eye is the most sensitive. Visible light (and near-infrared
light) is typically absorbed and emitted by electrons in molecules and atoms that move from one
energy level to another. This action allows the chemical mechanisms that underly human vision
and plant photosynthesis. The light which excites the human visual system is a very small
portion of the electromagnetic spectrum. A rainbow shows the optical (visible) part of the
electromagnetic spectrum; infrared (if it could be seen) would be located just beyond the red
side of the rainbow withultraviolet appearing just beyond the violet end.
Electromagnetic radiation with a wavelength between 380 nm and 760 nm (400–790 terahertz)
is detected by the human eye and perceived as visible light. Other wavelengths, especially near
infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred
to as light, especially when the visibility to humans is not relevant. White light is a combination of
lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it
up into the several colors of light observed in the visible spectrum between 400 nm and 780 nm.
If radiation having a frequency in the visible region of the EM spectrum reflects off an object,
say, a bowl of fruit, and then strikes the eyes, this results in visual perception of the scene. The
brain's visual system processes the multitude of reflected frequencies into different shades and
hues, and through this insufficiently-understood psychophysical phenomenon, most people
perceive a bowl of fruit.
At most wavelengths, however, the information carried by electromagnetic radiation is not
directly detected by human senses. Natural sources produce EM radiation across the spectrum,
and technology can also manipulate a broad range of wavelengths. Optical fiber transmits light
that, although not necessarily in the visible part of the spectrum (it is usually infrared), can carry
information. The modulation is similar to that used with radio waves.
Ultraviolet radiation[edit]
Main article: Ultraviolet
The amount of penetration of UV relative to altitude in Earth's ozone
Next in frequency comes ultraviolet (UV). The wavelength of UV rays is shorter than the violet
end of the visible spectrum but longer than the X-ray.
UV in the very shortest range (next to X-rays) is capable even of ionizing atoms
(see photoelectric effect), greatly changing their physical behavior.
At the middle range of UV, UV rays cannot ionize but can break chemical bonds, making
molecules to be unusually reactive. Sunburn, for example, is caused by the disruptive effects of
middle range UV radiation on skin cells, which is the main cause of skin cancer. UV rays in the
middle range can irreparably damage the complex DNA molecules in the cells
producing thymine dimers making it a very potentmutagen.
The Sun emits significant UV radiation (about 10% of its total power), including extremely short
wavelength UV that could potentially destroy most life on land (ocean water would provide some
protection for life there). However, most of the Sun's most-damaging UV wavelengths are
absorbed by the atmosphere and ozone layer before they reach the surface. The higher energy
(shortest wavelength) ranges of UV (called "vacuum UV") are absorbed by nitrogen and, at
longer wavelengths, by simple diatomic oxygen in the air. Most of the UV in the mid-range of
energy is blocked by the ozone layer, which absorbs strongly in the important 200–315 nm
range, the lower part of which is too long to be absorbed by ordinary dioxygen in air. The very
lowest energy range of UV between 315 nm and visible light (called UV-A) is not blocked well by
the atmosphere, but does not cause sunburn and does less biological damage. However, it is
not harmless and does cause oxygen radicals, mutation and skin damage. See ultraviolet for
more information.
X-rays[edit]
Main article: X-rays
After UV come X-rays, which, like the upper ranges of UV are also ionizing. However, due to
their higher energies, X-rays can also interact with matter by means of the Compton effect. Hard
X-rays have shorter wavelengths than soft X-rays. As they can pass through most substances
with some absorption, X-rays can be used to 'see through' objects with thicknesses less than
equivalent to a few meters of water. One notable use in this category is diagnostic X-ray images
in medicine (a process known as radiography). X-rays are useful as probes in high-energy
physics. In astronomy, the accretion disks around neutron stars and black holes emit X-rays,
which enable them to be studied. X-rays are also emitted by the coronas of stars and are
strongly emitted by some types of nebulae. However, X-ray telescopes must be placed outside
the Earth's atmosphere to see astronomical X-rays, since the atmosphere of Earth is a radiation
shield with areal density of 1000 grams per cm2, which is the same areal density as 1000
centimeters or 10 meters thickness of water.[19] This is an amount sufficient to block almost all
astronomical X-rays (and also astronomical gamma rays—see below).
Gamma rays[edit]
Main article: Gamma rays
After hard X-rays come gamma rays, which were discovered by Paul Villard in 1900. These are
the most energetic photons, having no defined lower limit to their wavelength. Inastronomy they
are valuable for studying high-energy objects or regions, however like with X-rays this can only
be done with telescopes outside the Earth's atmosphere. Gamma rays are useful to physicists
thanks to their penetrative ability and their production from a number of radioisotopes. Gamma
rays are also used for the irradiation of food and seed for sterilization, and in medicine they are
occasionally used in radiation cancer therapy. More commonly, gamma rays are used for
diagnostic imaging in nuclear medicine, with an example being PET scans. The wavelength of
gamma rays can be measured with high accuracy by means of Compton scattering. Gamma
rays are first and mostly blocked by Earth's magnetosphere then by the atmosphere.
Electromagnetic Spectrum
The electromagnetic (EM) spectrum is the range of all types of EM radiation.
Radiation is energy that travels and spreads out as it goes – the visible
light that comes from a lamp in your house and the radio waves that come
from a radio station are two types of electromagnetic radiation. The other
types of EM radiation that make up the electromagnetic spectrum
are microwaves, infrared light,ultraviolet light, X-rays and gamma-rays.
You know more about the electromagnetic spectrum than you may think. The
image below shows where you might encounter each portion of the EM
spectrum in your day-to-day life.
The electromagnetic spectrum from lowest energy/longest wavelength (at the top) to highest
energy/shortest wavelength (at the bottom). (Click image for a larger version.)
Radio: Your radio captures radio waves emitted by radio stations, bringing
your favorite tunes. Radio waves are also emitted by stars and gases in
space.
Microwave: Microwave radiation will cook your popcorn in just a few minutes,
but is also used by astronomers to learn about the structure of
nearby galaxies.
Infrared: Night vision goggles pick up the infrared light emitted by our skin
and objects with heat. In space, infrared light helps us map the dust between
stars.
Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit
visible light.
Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason skin
tans and burns. "Hot" objects in space emit UV radiation as well.
X-ray: A dentist uses X-rays to image your teeth, and airport security uses
them to see through your bag. Hot gases in theUniverse also emit X-rays.
Gamma ray: Doctors use gamma-ray imaging to see inside your body. The
biggest gamma-ray generator of all is the Universe.
Is a radio wave the same as a gamma ray?
Are radio waves completely different physical objects than gamma-rays? They
are produced in different processes and are detected in different ways, but
they are not fundamentally different. Radio waves, gamma-rays, visible light,
and all the other parts of the electromagnetic spectrum are electromagnetic
radiation.
Electromagnetic radiation can be described in terms of a stream of mass-less
particles, called photons, each traveling in a wave-like pattern at the speed of
light. Each photon contains a certain amount of energy. The different types of
radiation are defined by the the amount of energy found in the photons. Radio
waves have photons with low energies, microwave photons have a little more
energy than radio waves, infrared photons have still more, then visible,
ultraviolet, X-rays, and, the most energetic of all, gamma-rays.
Measuring electromagnetic radiation
Electromagnetic radiation can be expressed in terms of energy, wavelength,
orfrequency. Frequency is measured in cycles per second, or Hertz.
Wavelength is measured in meters. Energy is measured in electron volts.
Each of these three quantities for describing EM radiation are related to each
other in a precise mathematical way. But why have three ways of describing
things, each with a different set of physical units?
Comparison of wavelength, frequency and energy for the electromagnetic spectrum. (Click image for
a larger version.)
The short answer is that scientists don't like to use numbers any bigger or
smaller than they have to. It is much easier to say or write "two kilometers"
than "two thousand meters." Generally, scientists use whatever units are
easiest for the type of EM radiation they work with.
Astronomers who study radio waves tend to use wavelengths or frequencies.
Most of the radio part of the EM spectrum falls in the range from about 1 cm to
1 km, which is 30 gigahertz (GHz) to 300 kilohertz (kHz) in frequencies. The
radio is a very broad part of the EM spectrum.
Infrared and optical astronomers generally use wavelength. Infrared
astronomers use microns (millionths of a meter) for wavelengths, so their part
of the EM spectrum falls in the range of 1 to 100 microns. Optical astronomers
use bothangstroms (0.00000001 cm, or 10-8 cm) and nanometers (0.0000001
cm, or10-7 cm). Using nanometers, violet, blue, green, yellow, orange, and red
light have wavelengths between 400 and 700 nanometers. (This range is just
a tiny part of the entire EM spectrum, so the light our eyes can see is just a
little fraction of all the EM radiation around us.)
The wavelengths of ultraviolet, X-ray, and gamma-ray regions of the EM
spectrum are very small. Instead of using wavelengths, astronomers that
study these portions of the EM spectrum usually refer to these photons by
their energies, measured in electron volts (eV). Ultraviolet radiation falls in the
range from a few electron volts to about 100 eV. X-ray photons have energies
in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then are all the
photons with energies greater than 100 keV.
Show me a chart of the wavelength, frequency, and energy regimes of the
spectrum
Why do we put telescopes in orbit?
The Earth's atmosphere stops most types of electromagnetic radiation from space from reaching
Earth's surface. This illustration shows how far into the atmosphere different parts of the EM
spectrum can go before being absorbed. Only portions of radio and visible light reach the surface.
(Credit: STScI/JHU/NASA)
Most electromagnetic radiation from space is unable to reach the surface of
the Earth. Radio frequencies, visible light and some ultraviolet light makes it to
sea level. Astronomers can observe some infrared wavelengths by putting
telescopes on mountain tops. Balloon experiments can reach 35 km above
the surface and can operate for months. Rocket flights can take instruments
all the way above the Earth's atmosphere, but only for a few minutes before
they fall back to Earth.
For long-term observations, however, it is best to have your detector on an
orbitingsatellite and get above it all!
Updated: March 2013
Lesson: Visible
Light and the Electromagnetic Spectrum
Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado
Boulder
Summary
During this lesson, the electromagnetic
spectrum is explained and students learn that
visible light makes up only a portion of this
wide spectrum. Students also learn that
engineers use electromagnetic waves for
many different applications.
Engineering Connection
Relating science and/or math concept(s)
to engineering
Engineers use many types of electromagnetic
waves. Gamma radiation is emitted by fuel
rods in nuclear power plants, x-rays are used
to see inside our bodies, ultraviolet light can
be used to sanitize things, microwaves are
used to cook, and radio waves allow us to
communicate over large distances.
Contents
1. Pre-Req Knowledge
2. Learning Objectives
3. Introduction/Motivation
4. Background
5. Vocabulary
6. Associated Activities
7. Lesson Closure
8. Attachments
9. Assessment
Figure 1. The electromagnetic spectrum.
copyright
10. Extensions
11. References
Grade Level: 4 (3-5)
Lessons in this Unit: 12345678
Time Required: 20 minutes
Lesson Dependency
:None
Keywords: light, electromagnetic spectrum, radiation, visible spectrum, transverse wave
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Educational Standards
:

Colorado: Science

International Technology and Engineering Educators Association: Technology
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Next Generation Science Standards: Science
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Pre-Req Knowledge (Return to Contents)
This lesson follows Lessons 1-6 of the Sound and Light Unit.
Learning Objectives (Return to Contents)
After this lesson, students should be able to:

Explain that light can be considered an electromagnetic wave.
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Give two examples of how engineers use electromagnetic waves.
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Explain that we can only see a small portion of all electromagnetic waves.
Introduction/Motivation (Return to Contents)
*Note: These concepts might be very difficult to visualize; if possible, create transparencies of the lesson
figures for students to look at during class discussions.
Let's think about our brave adventurers for a minute – who remembers what Angie and Harmon need to
do next to find the treasure? That's right – they want to use lights underwater to find the sunken treasure
ship. But first, they need to learn something about light waves so that they can choose the right kind of
lights to use in their exploration.
When we learned about sound, we discovered that sound travels in waves. Light can also be thought of
as a wave – light is a form of anelectromagnetic wave. That's a big word, so let's write it on the board.
An electromagnetic wave is a type of wave that can travel through empty space... Yes, you heard
correctly, empty space. Unlike sound waves, which need "something" to travel through (for example,
water or air), electromagnetic waves are able to travel through "emptiness" or a vacuum.
This picture (show Electromagnetic Transparency #1– in the Attachments section) shows different kinds
of electromagnetic waves. Engineers use electromagnetic waves for many different purposes. Gamma
rays (nuclear power plant radiation), x-rays, light, microwaves, and radio waves (including cell phone
waves) are all electromagnetic waves. What makes all these waves different from each other are their
wavelengths and frequencies.
Who would like to remind the class what frequency is? That's right! The frequency of a wave is the
number of times a crest occurs each second. Some waves have really big — or even really small —
frequencies. If a wave has a higher frequency (many waves in a certain amount of time), it has more
energy. And, if a wave has a smaller frequency (fewer waves in a certain amount of time), it has less
energy.
Let's look at another picture (show Electromagnetic Transparency #2) to see if we can figure out which
waves have the most energy. Which waves do you think are the most powerful? That's right! gamma
waves have very high frequencies and, consequently, have a lot of energy. This extreme amount of
energy is one reason why gamma waves are very dangerous if improperly used.
Have any of you ever had an x-ray? X-rays are not as strong as gamma rays, but they are still very
powerful. A sunburn? Have any of you ever burned your skin when out in the hot sun (or overcast as well)
too long? Sunburns come from ultraviolet light, which we cannot see, but can still burn our skin. Radio
waves and microwaves have a smaller frequency, so they are much less powerful than x-rays or
ultraviolet light. Waves are fascinating, that's for sure!
We know that waves with high frequencies have a lot of energy. And, the waves that have smaller
frequencies have less energy — think of these wave types as energetic waves that move very fast and
lazy waves that move slow. Did you know that we cannot see most electromagnetic waves? The small
section of the spectrum with the waves that we can see is called the visible spectrum, and the
wavelengths that we can see allow us to see the colors of red, orange, yellow, green, blue, indigo, violet
— the colors of the rainbow! We do not usually think of visible light (the visible spectrum) as being an
electromagnetic wave, but it is. Figure 2 shows where visible light falls on the electromagnetic spectrum.
Figure 2. The electromagnetic spectrum in detail.
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Engineers use electromagnetic waves in all sorts of different inventions. We cannot see most
electromagnetic waves, but you do observe these waves every day when you see visible light. Today we
are going to build a fascinating invention so that we can see the different colors that are a part of visible
light. Let's get started!
Lesson Background & Concepts for Teachers (Return to Contents)
A wave is a change (a variation) that travels through a substance (or medium). You can often see the
change, such as the increased height of a traveling ocean, but what is important to understand is that the
medium itself does not travel with the wave.
Ripples in a pond are good examples of waves. If no wind, a pond is smooth until a rock is thrown in and
disturbs the water. Then ripples, "disturbances" in the pond, travel to the edges. The medium in this case
is the water, through which the ripples travel. The water is not actually moving, but the waves (ripples)
are.
Waves move in two ways: longitudinally and transversely. Transverse waves oscillate (move back and
forth) in a direction perpendicular to their motion. Our pond ripples, for instance, oscillate up and down but
move horizontally towards the edge of the pond. Because the ripples oscillate perpendicular to their
horizontal motion towards the edge, they can be classified as transverse waves.
An electromagnetic wave is a transverse wave that can travel through empty space or a vacuum.
Literally, electromagnetic waves are able to travel through "emptiness," unlike sound waves, which need
"something" to travel through (for example, water or air). Electromagnetic waves have two parts to them:
electric and magnetic. Both of these parts
are considered transverse waves.
Waves with high frequencies have a lot of
energy, and, waves with smaller frequencies
have less energy. Most electromagnetic
waves are not visible. However, a small
section of the spectrum includes waves that
we can see — it is called the visible
spectrum (see Figure 3). These visible
wavelengths allow us to see the colors of
the rainbow: red, orange, yellow, green,
blue, indigo and violet. We do not usually
think of visible light (the visible spectrum) as
being an electromagnetic wave, but it is.
The electromagnetic spectrum can be
considered from another perspective — in
nanometers — specifically from 400 nm to
700 nm. Each color in this visual range has
a different wavelength. Red has the longest
wavelength (700 nm) and violet has the
shortest wavelength (400 nm). Green
occurs near the middle at about 550 nm. A
prism divides light into the wavelengths that
make it up. Seen together, color waves
make white light. White light is especially
dramatic because many different colors of
the visual spectrum can combine to make
white light. Two "white" light sources can
have very different spectral compositions.
When white light shines on a prism, the
Figure 3. The visual spectrum occurs between 400 nm (violet) and 700 nm
colors in white light separate from each
(red).
other because they refract at different
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angles depending on their wavelength (see
Figure 4). Water droplets in the air refract sunlight to create rainbows.
Light can be absorbed, reflected (or diffused) and refracted. Some
materials can affect how light bends in more than one way, refracting
and reflecting at the same time. Objects made of more than one
substance usually have different reflective, refractive and absorptive
properties.
Light reflects at a predictable angle: the angle of the light that strikes
a surface equals the angle of the light that bounces off the surface.
Rough surfaces scatter — or diffuse — light, which can cause glare,
blur an image or prevent us from seeing an image.
Light changes speed and direction — refracts — when it moves from
one transparent medium to another. The refractive property of
transparent materials can be used to make lenses that focus light (for
Figure 4. White light shining through a prism
example, cameras, eyeglasses, telescopes).
breaks apart into the colors of the visible
light spectrum.
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Since visible light is the part of the electromagnetic spectrum that our
eyes can see (remember, we cannot see most electromagnetic
waves), our whole world is oriented around it and the colors that are produced through this visible
spectrum. Understanding these visible electromagnetic waves has enabled engineers to develop many
instruments that can see farther and more clearly than our eyes. That is why we use satellites to look at
the Earth, and telescopes to look at the Sky!
Vocabulary/Definitions (Return to Contents)
electromagnetic
wave:
visual spectrum:
A transverse wave with electric and magnetic parts that travels at 300,000 km/sec. Examples are
gamma waves, x-rays, light waves, microwaves, and radio waves.
The range of the electromagnetic spectrum — between 400 nm and 700 nm — that our eyes can
see.
Associated Activities (Return to Contents)

Building a Fancy Spectrograph - Students create and decorate their own spectrographs using
simple materials and holographic diffraction gratings. A holographic diffraction grating acts like a
prism, showing the visual components of light. After building the spectrographs, students observe
the spectra of different light sources as homework.
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The Visual Spectrum - Students make simple spectroscopes (prisms) to look at different light
sources. Using the spectroscopes, students see differing spectral distributions of different light
sources
Lesson Closure (Return to Contents)
You all did a great job today of being good listeners and thinking hard about light! We learned that light
can be thought of as a wave, just as sound is a wave. We also learned that our eyes can only see some
wavelengths of electromagnetic radiation. What do we call the set of wavelengths that our eyes can see ?
That's right---the visible spectrum. We also discovered that engineers use many different electromagnetic
waves for many different applications. In our next lesson, we are going to learn about rainbows, and
continue to follow the adventures of Angie and Harmon. What do you think will happen to them next?
Attachments (Return to Contents)
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Electromagnetic Transparency #1 (doc)
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Electromagnetic Transparency #1 (pdf)
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Electromagnetic Transparency #2 (doc)
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Electromagnetic Transparency #2 (pdf)
Assessment (Return to Contents)
Pre-Lesson Assessment
Discussion Question: Turn on a radio, and tune it to a station. Next turn on a lamp. Ask the students what
the two devices (radio and lamp) have in common. (Answer: They both use electromagnetic waves.) Tell
students that today we are going to learn about a few different types of electromagnetic waves.
Post-Introduction Assessment
Fill in the Table: On the left side of the classroom board, list the types of electromagnetic waves. Then
work with the students to fill in the right side with ways that engineers use each type of wave. Examples
of waves and their uses are:
 Gamma waves – emitted by nuclear power plants
 X-rays – used for healthcare
 Light waves – for seeing visible light
 Microwaves – for cooking
 Radio waves – for communication
Lesson Summary Assessment
What I Learned Today: At lesson end, give students time to think about what they learned. Invite a few
students to volunteer something new that they learned through the lesson. (If students do not mention it,
remind them that they learned that visible light is an electromagnetic wave and the only electromagnetic
wave we can see; also it is only a small portion of the overall spectrum of electromagnetic waves.)
Lesson Extension Activities (Return to Contents)
Bring in small prisms for students to experiment with and explore how they break light into different
wavelengths/colors.
Have students create an electromagnetic wave journal of different electromagnetic waves they
experience throughout the week (for example, microwaving food, seeing a rainbow, listening to the radio,
etc.).
Invite an engineer to visit the class and talk about how s/he uses electromagnetic waves in his/her
research or work.