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Electromagnetic Spectrum
Life on Earth is highly dependent on light. Plants need light for the energy to perform
photosynthesis, and animals need plants for oxygen and food. Animals need light to see their way
around. Have you ever stopped to think about what light actually is? Where does it come from?
How does it move from place to place?
The Electromagnetic Spectrum
The light we see on Earth is a type of electromagnetic radiation. (Radiation refers to energy that
spreads out as it travels.) Electromagnetic radiation is energy made up of special particles called
photons. This energy travels faster than anything else known in the universe: approximately
300,000,000 meters per second (mps).
There are many different types of electromagnetic
radiation, but all types travel as waves. Scientists use
wavelength to classify electromagnetic waves. Wavelength
is a measurement of the length of a single wave of energy.
As this diagram shows, wavelength is measured from the
trough (or very bottom) of one wave to the trough of the
next wave.
All the different wavelengths of electromagnetic
radiation make up the electromagnetic spectrum. The electromagnetic spectrum includes radio
waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-Rays, and gamma rays.
The electromagnetic spectrum is usually organized with the different types of radiation arranged
from longest wavelength to shortest.
In this model of the electromagnetic spectrum, longer wavelengths (such as radio waves and
microwaves) are on the left and shorter wavelengths (such as X-Rays and gamma rays) are on
the right.
Regardless of their wavelengths, all types of electromagnetic radiation travel at about the same
speed of 300,000,000 mps. In addition to wave speed and wavelength, electromagnetic waves can
be measured by frequency. Frequency refers to the number of waves that pass a given point in a
given period of time (usually one second). Electromagnetic waves with longer wavelengths have
lower frequencies: in other words, fewer waves pass a given point each second. Electromagnetic
waves with shorter wavelengths have higher frequencies, which means more waves pass a given
point each second.
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Electromagnetic Spectrum
Electromagnetic radiation is made of
particles called photons. However, it
is important to remember that
photons are a special type of particle.
They are not like other particles you
are familiar with. Unlike protons and
electrons, photons have no mass.
How the energy of the
electromagnetic spectrum behaves
as both particles and waves
simultaneously is one of the great
mysteries of science.
Visible Light and Color
Visible light is the only part of the
electromagnetic spectrum humans
can see.
This model of the electromagnetic spectrum shows the
different colors of visible light. Red light waves, which
have the longest wavelengths and lowest frequencies, are
on the left. Violet light waves, which have the shortest
wavelengths and highest frequencies, are on the right.
Visible light is radiated as white light, which is
made up of different wavelengths of radiation.
Visible light ranges in wavelength from about 400–
700 nanometers (nm). Each of these different
wavelengths corresponds to a different color. We
see the longest wavelengths of visible light as red
light. We see the shortest wavelengths as violet
light.
A prism separates white light into different
colors.
White light can be separated into different colors
using a special lens called a prism. When sunlight
shines through raindrops, the raindrops can act as
a prism and create a rainbow.
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Electromagnetic Spectrum
Studying the Universe Using Visible Light
Most of our knowledge about the universe comes from observations of visible light. Sometimes
we can look directly at an object with our eyes. Sometimes we must use telescopes to aid our
eyes. At other times, a more complex analysis is required. For example, scientists have
discovered the composition of many objects in the universe— including distant stars—simply by
analyzing the visible light these objects emit or reflect. Scientists have done experiments to
determine the colors given off by different elements when they are burned. By studying the
colors given off by a star, scientists can determine which elements make up that star. This is
spectroscopy, the study of the electromagnetic spectrum to determine an object’s composition.
When scientists look at stars and other distant objects in space, they are actually looking back in
time. Electromagnetic radiation travels faster than anything else in the known universe. However,
it still takes some time for the radiation to travel across the vast distances of space. Because of
this, scientists measure these vast distances in space using a unit called the light-year. One lightyear equals the distance light travels in one year: about 9.5 trillion kilometers. The closest star to
Earth other than the Sun is called Proxima Centauri. Proxima Centauri is located about 4.2 lightyears from Earth. In other words, light emitted by Proxima Centauri takes 4.2 years to reach
Earth. When scientists look at Proxima Centauri, they are not observing the star as it is now, but
rather as it appeared 4.2 years ago. This means scientists can observe light from across the
universe that was emitted millions or even billions of years ago. They could even look at a star
that no longer exists!
The Doppler Effect
Visible light has provided scientists with much
information about the objects in the universe.
Scientists have also studied visible light to
understand the nature of the universe itself. A
scientist named Edwin Hubble used visible light to
provide evidence that the universe is expanding. He
did this through his observations of the Doppler
effect.
When an object moves toward you,
waves from the object appear
compressed. When an object moves
away from you, waves appear
stretched. This is the Doppler effect.
The Doppler effect describes how wavelengths of
energy shift as objects move. If an object emitting
electromagnetic or sound waves is moving toward you,
the waves become compressed. In other words, the
wavelengths appear to become smaller. If an object
emitting electromagnetic or sound waves is moving
away from you, the waves become stretched. In other
words, the wavelengths appear to become larger.
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Electromagnetic Spectrum
The Doppler effect causes sounds to increase or decrease in pitch. It also affects visible light
waves. Recall that we see shorter wavelengths as blue light and longer wavelengths as red
light. When a star is moving toward
Earth, the light waves emitted by the star
become compressed. The shorter
wavelengths cause the star’s light to
appear blue. This phenomenon is called
blueshift. When a star is moving away from
Earth, the light waves emitted by the star
become stretched. The longer wavelengths
cause the star’s light to appear red. This
phenomenon is called redshift.
Edwin Hubble noticed that all the galaxies
we can see from Earth experience redshift.
Therefore, they must be moving away from
Earth.
If all of the objects in the universe are
moving away from each other, the universe
must be expanding.
As the object moves away from the man and
toward the woman, it gives off light waves
(numbered 1–4 in this diagram). Each observer
experiences these light waves differently.
Not only is the universe expanding, it appears to be expanding at an increasing rate. Hubble
noticed that the farther an object is from Earth, the more dramatic the redshift appears. In other
words, these objects are moving even faster. By measuring the intensity of a star’s redshift,
scientists can determine the speed at which a star is moving and use this to calculate its distance
from Earth.
Humans cannot directly perceive any part of the electromagnetic spectrum other than
visible light. Most of what we know about the universe and the objects in it comes from
the study of visible light. Do you think scientists can use the rest of the electromagnetic
spectrum to study the universe? If so, how do you think this is possible?
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Electromagnetic Spectrum
Using High-Energy Electromagnetic Radiation to Study the Universe
When objects burn, they give off visible light. Hotter, more energetic objects and events in the
universe give off high-energy radiation with wavelengths that are shorter than those of the visible
light spectrum. These include ultraviolet radiation, X-Rays, and gamma rays. Scientists can use
special instruments, called remote sensing instruments, to detect this radiation and study the
objects emitting it. For example, ultraviolet radiation can be used to study very hot areas of the
universe, such as the centers of some galaxies where many stars are clustered together.
X-Rays are given off by extremely hot gases, so they can be used to study events that are
even hotter and more energetic than burning stars. These include supernova
explosions caused by the deaths of old stars. After some
supernova explosions, the extremely dense, hot core of the dead
star remains as a neutron star. Neutron stars emit X-Rays that
scientists can detect and study. After other supernova explosions,
a black hole is created. Black holes cannot be studied using
visible light. This is because black holes are so dense, not even
light can escape their gravitational pull. However, scientists can
use X-Rays emitted by a supernova explosion to determine where
black holes are likely to be created or located.
This image shows the
remains of a supernova
explosion. The lighter areas
are X-Rays detected by a
remote sensing telescope.
Gamma rays have the shortest wavelengths, highest frequencies,
and highest energies of the entire electromagnetic spectrum. Bursts
of energy from the Sun, called solar flares, generate gamma rays,
as do other high- energy events in the universe. High-energy
processes on Earth, such as nuclear power generation, can also
emit gamma radiation.
Using Low-Energy Electromagnetic Radiation to Study the Universe
Scientists can also use the low-energy parts of the electromagnetic spectrum to study the
universe. This low-energy radiation—including radio waves, microwaves, and infrared radiation—
has lower frequencies and longer wavelengths. Special telescopes can pick up these waves from
across the universe, allowing scientists to discover distant objects that are impossible to detect
through higher-energy waves. Some of these objects are hidden by dust in space. Others are too
cool to emit higher-energy waves.
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Electromagnetic Spectrum
Because all objects give off infrared radiation in the form
of heat, infrared waves are particularly useful for detecting
cooler objects. Special infrared cameras can even be
used to detect objects on Earth’s surface.
Low-energy waves have even been used to
understand the origin of the entire universe. The most
widely accepted theory to explain the origin of the
universe is the Big Bang theory. The Big Bang theory
states that everything in the universe exploded and
expanded out of a single point during a single event
about 13.7 billion years ago. This massive explosion of
energy and matter must have given off
electromagnetic radiation.
Special cameras reveal the infrared radiation emitted by things,
including people!
Scientists have been able to detect some of this radiation left over from the Big Bang. This is
called cosmic microwave background (CMB) radiation. CMB radiation can be detected
everywhere in the universe, using a special instrument called the Wilkinson Microwave
Anisotropy Probe (WMAP).
The image produced by
WMAP shows CMB radiation.
Slight differences in
temperature, represented
by different colors, indicate
areas of the universe with
more or less matter.
Everyday Life: Uses of the Electromagnetic Spectrum
You need visible light to see, but did you know you use other parts of the electromagnetic spectrum
in your everyday life? Every time you use a cell phone or listen to the radio, you are using radio
waves. Radio waves can travel through the air to transmit information, such as music or a
conversation. A microwave oven gets its name because it uses microwave radiation to energize the
atoms that make up your food. This energy heats up cold food. Doctors and dentists use X-Rays to
examine your teeth and bones. X-Rays can pass through your skin and muscles to create images
of your bones and teeth. Sometimes electromagnetic radiation can hurt you. When you get a
sunburn, it is because of ultraviolet (UV) radiation given off by the Sun. The UV rays can destroy
skin cells, causing painful blisters and, in some cases, cancers.
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Electromagnetic Spectrum
What do you know?
The electromagnetic spectrum is made up of seven types of radiation of varying wavelength. In the
diagram below, place each type of radiation in order from longest wavelength to shortest. Then,
decide which characteristic in the following box describes each type of radiation. Write each
characteristic beside the appropriate type of electromagnetic radiation.
Characteristics of Electromagnetic Radiation
•  Can be used to transmit information
•  Can be used to study supernovae,
across distances
neutron stars, and black holes
•  The only part of the electromagnetic
spectrum humans can see
•  Emitted by all objects in the form of heat
energy
•  Left over from the Big Bang throughout
the universe
•  Can result in a sunburn
•  Can be emitted by nuclear power
generation
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Electromagnetic Spectrum
Exploring the Doppler Effect
To help your child learn more about the electromagnetic spectrum, try experimenting with the
Doppler effect together. Sound waves are not a part of the electromagnetic spectrum. However,
they are shifted by the Doppler effect the same way that electromagnetic waves are. The unaided
human eye is not capable of directly observing the Doppler shift of light on Earth, but the unaided
ear can easily detect the Doppler shift of sound waves.
A sound’s pitch is related to the frequency of the waves that produce that sound. Sound waves with
higher frequencies produce sounds with higher pitches, while sound waves with lower frequencies
produce sounds with lower, deeper pitches. The Doppler effect creates an apparent shift in the
frequencies of sound waves as the source of the sound moves toward or away from an observer.
This is easy to demonstrate at home.
First, experiment with different sounds and discuss their frequencies with your child. Try playing
notes on a piano or listening to some music together. Listen to the different notes, and ask your
child to pick out the sounds that are produced by higher- frequency waves and lower-frequency
waves. Then, experiment with the Doppler effect by choosing an object that creates a repetitive
sound that has only one pitch.
This can be an alarm clock, a beeping timer, a cell phone with a repetitive ring tone, or
a portable stereo playing a recording of a repetitive sound. Have your child stand in one spot in a
wide, open area, preferably outside. Hold the object producing the sound, and stand several feet
away from your child. Tell your child to listen carefully to the sound, paying careful attention to the
pitch. Then, quickly run back and forth past your child a few times. Instruct your child to pay
attention to the way the pitch changes as you run back and forth. Discuss the phenomenon of the
changing pitch as the sound moves closer and farther away.
Here are some questions to discuss with your child:
•  How does the pitch of this sound relate to the frequency of the sound waves?
•  How does the pitch of the sound change as I move toward you?
•  How does the pitch of the sound change as I move away from you?
•  How can you explain this changing pitch?
•  Sound waves are not a part of the electromagnetic spectrum. How does this experiment relate
to what you have learned about the electromagnetic spectrum and the way it is used to study
the universe?
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