National Aeronautics and Space Administration 

www.nasa.gov
TOUR OF THE 

ELECTROMAGNETIC
 SPECTRUM

Introduction to the Electromagnetic Spectrum 2
 

Anatomy of an Electromagnetic Wave 4
 

Wave Behaviors 6
 

Visualization: From Energy to Image 8
 

Radio Waves 10
 

Microwaves 12
 

Infrared Waves 14
 

Reflected Near Infrared Waves 16
 

Visible Light 18
 

Ultraviolet Waves 20
 

X-Rays 22
 

Gamma Rays 24
 

The Earth’s Radiation Budget 26
 

Credits 28
 
 

 

 

 

 

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2 Tour of the Electromagnetic Spectrum 3 

INTRODUCTION TO THE 

ELECTROMAGNETIC 

SPECTRUM

When you tune your radio, watch TV, send a text message, or pop popcorn 

in a microwave oven, you are using electromagnetic energy. You depend 

on this energy every hour of every day. Without it, the world you know 

could not exist. 

Electromagnetic energy travels in waves and spans a broad spectrum 

from very long radio waves to very short gamma rays. The human eye 

can only detect only a small portion of this spectrum called visible light. 

A radio detects a different portion of the spectrum, and an x ray machine 

uses yet another portion. NASA’s scientific instruments use the full range 

of the electromagnetic spectrum to study the Earth, the solar system, 

and the universe beyond. 

OUR PROTECTIVE ATMOSPHERE 

Our Sun is a source of energy across the full spectrum, and its elec 

tromagnetic radiation bombards our atmosphere constantly. However, 

the Earth’s atmosphere protects us from exposure to a range of higher 

energy waves that can be harmful to life. Gamma rays, x rays, and some 

ultraviolet waves are “ionizing,” meaning these waves have such a high 

energy that they can knock electrons out of atoms. Exposure to these 

high energy waves can alter atoms and molecules and cause damage to 

cells in organic matter. These changes to cells can sometimes be help 

ful, as when radiation is used to kill cancer cells, and other times not, as 

when we get sunburned. 

Introduction to the Electromagnetic Spectrum 

Seeing Beyond our Atmosphere 

NASA spacecraft, such as RHESSI, provide scientists 

with a unique vantage point, helping them “see” at 

higher-energy wavelengths that are blocked by the 

Earth’s protective atmosphere. 

RHESSI

ATMOSPHERIC WINDOWS 

Electromagnetic radiation is reflected or absorbed mainly by several gases in the Earth’s atmosphere, 

among the most important being water vapor, carbon dioxide, and ozone. Some radiation, such as vis 

ible light, largely passes (is transmitted) through the atmosphere. These regions of the spectrum with 

wavelengths that can pass through the atmosphere are referred to as “atmospheric windows.” Some 

microwaves can even pass through clouds, which make them the best wavelength for transmitting 

satellite communication signals. 

While our atmosphere is essential to protecting life on Earth and keeping the planet habitable, it is not 

very helpful when it comes to studying sources of high energy radiation in space. Instruments have 

to be positioned above Earth’s energy absorbing atmosphere to “see” higher energy and even some 

lower energy light sources such as quasars. 
 

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4 Tour of the Electromagnetic Spectrum 5 

ANATOMY OF AN 

ELECTROMAGNETIC WAVE
 Energy, a measure of the ability to do work, comes in many forms and can transform from one type to another. Examples of stored or potential 

energy include batteries and water behind a dam. Objects in motion are examples of kinetic energy. Charged particles such as electrons and 

protons—create electromagnetic fields when they move, and these fields transport the type of energy we call electromagnetic radiation, or light. 

WHAT ARE WAVES? 

Mechanical waves and electromagnetic waves are two important ways 

that energy is transported in the world around us. Waves in water and 

sound waves in air are two examples of mechanical waves. Mechanical 

waves are caused by a disturbance or vibration in matter, whether solid, 

gas, liquid, or plasma. Matter that waves are traveling through is called 

a medium. Water waves are formed by vibrations in a liquid and sound 

waves are formed by vibrations in a gas (air). These mechanical waves 

travel through a medium by causing the molecules to bump into each oth -

er, like falling dominoes transferring energy from one to the next. Sound 

waves cannot travel in the vacuum of space because there is no medium 

to transmit these mechanical waves. 

Classical waves transfer energy without transporting matter through the 

medium. Waves in a pond do not carry the water molecules from place to 

place; rather the wave’s energy travels through the water, leaving the wa 

ter molecules in place, much like a bug bobbing on top of ripples in water. 

ELECTROMAGNETIC WAVES 

Electricity can be static, like the energy that can 

make your hair stand on end. Magnetism can also 

be static, as it is in a refrigerator magnet. A chang 

ing magnetic field will induce a changing electric 

field and vice versa the two are linked. These 

changing fields form electromagnetic waves. 

Electromagnetic waves differ from mechanical 

waves in that they do not require a medium to propagate. This means that 

electromagnetic waves can travel not only through air and solid materials, 

but also through the vacuum of space. 

WAVES OR PARTICLES? YES! 

Light is made of discrete packets of energy called photons. Photons 

carry momentum, have no mass, and travel at the speed of light. All light 

has both particle-like and wave-like properties. How an instrument is 

designed to sense the light influences which of these properties are ob 

served. An instrument that diffracts light into a spectrum for analysis is an 

example of observing the wave like property of light. The particle like na 

ture of light is observed by detectors used in digital cameras individual 

photons liberate electrons that are used for the detection and storage of 

the image data. 

POLARIZATION 

One of the physical properties of light is that it can be polarized. Po 

larization is a measurement of the electromagnetic field’s alignment. In 

the figure above, the electric field (in red) is vertically polarized. Think 

of a throwing a Frisbee at a picket fence. In one orientation it will pass 

through, in another it will be rejected. This is similar to how sunglasses 

are able to eliminate glare by absorbing the polarized portion of the light. 

DESCRIBING ELECTROMAGNETIC ENERGY 

The terms light, electromagnetic waves, and radiation all refer to the 

same physical phenomenon: electromagnetic energy. This energy can be 

described by frequency, wavelength, or energy. All three are related math 

ematically such that if you know one, you can calculate the other two. 

Radio and microwaves are usually described in terms of frequency (Hertz), 

infrared and visible light in terms of wavelength (meters), and x rays and 

gamma rays in terms of energy (electron volts). This is a scientific conven 

tion that allows the convenient use of units that have numbers that are 

neither too large nor too small. 

FREQUENCY 

The number of crests that pass a given point within one second is de 

scribed as the frequency of the wave. One wave—or cycle—per second 

Anatomy of an Electromagnetic Wave 

Electromagnetic Waves 

Electromagnetic waves are formed by the vibra 

tions of electric and magnetic fields. These fields are 

perpendicular to one another in the direction the wave 

is traveling. Once formed, this energy travels at the 

speed of light until further interaction with matter. 

is called a Hertz (Hz), after Heinrich Hertz who established the existence 

of radio waves. A wave with two cycles that pass a point in one second 

has a frequency of 2 Hz. 

WAVELENGTH 

Electromagnetic waves have crests and troughs similar to those of ocean 

waves. The distance between crests is the wavelength. The shortest 

wavelengths are just fractions of the size of an atom, while the longest 

wavelengths scientists currently study can be larger than the diameter 

of our planet! 

ENERGY 

An electromagnetic wave can also be described in terms of its ener 

gy—in units of measure called electron volts (eV). An electron volt is the 

amount of kinetic energy needed to move an electron through one volt 

potential. Moving along the spectrum from long to short wavelengths, 

energy increases as the wavelength shortens. Consider a jump rope with 

its ends being pulled up and down. More energy is needed to make the 

rope have more waves. 
 

 

 

 

 

 

 

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6 Tour of the Electromagnetic Spectrum Wave Behaviors 7

WAVE BEHAVIORS 

Light waves across the electromagnetic spectrum behave in similar ways. Light can be transmitted, reflected, absorbed, refracted, polarized, 

diffracted, and scattered. Specialized instruments onboard NASA spacecraft and airplanes collect data on how waves behave when they interact 

with matter. Scientists use these data to learn about the physical and chemical composition of the different types of matter. 

When light waves encounter an object, 

they are either transmitted through, 

reflected, or absorbed depending on 

the composition of the object and the 

wavelength of the light. When incident 

light (incoming light) hits an object 

and bounces off, this is an example 

of reflected energy. Very smooth sur 

faces such as mirrors reflect almost 

all incident light. 

The color of an object is actually the color of the light reflected while all 

other colors are absorbed. Color, in this case, refers to the different wave 

lengths of light in the visible light spectrum. 

Lasers onboard NASA’s Lunar Reconnaissance Orbiter rely on the re 

flective property of light waves to map the surface of the Moon. The 

instrument measures the time it takes a laser pulse to hit the surface 

and return. The longer the response time, the farther away the surface 

and lower the elevation. A shorter response time means the surface is 

closer or higher in elevation. In this image of the Moon’s southern hemi 

sphere, low elevations are shown as purple and blue, and high elevations 

are shown in red and brown. 

TOPOGRAPHY OF 

THE MOON 

Absorption occurs when photons 

from incident light hit atoms and 

molecules and cause them to 

vibrate. The more an object’s mole 

cules move and vibrate, the hotter it 

becomes. This heat is then emitted 

from the object as thermal energy. 

Some objects, such as darker colored objects, absorb more incident light 

energy than others. For example, black pavement absorbs most visible 

and UV energy and reflects very little, while a light-colored concrete 

sidewalk reflects more energy than it absorbs. Thus, the black pavement 

is hotter than the sidewalk on a hot summer day. Photons bounce around 

during this absorption process and lose bits of energy to numerous mol 

ecules along the way. This thermal energy then radiates in the form of 

longer wavelength infrared energy. 

Thermal radiation from the energy absorbing asphalt and roofs in a 

city can raise its surface temperature by as much as 10° Celsius. The 

Landsat 7 satellite image below shows the city of Atlanta as an island 

of heat compared to the surrounding area. Sometimes this warming of 

air above cities can influence weather, which is called the “urban heat 

island” effect. 

INFRARED IMAGE OF ATLANTA 

Diffraction is the bending and spread 

ing of waves around an obstacle. 

It is most pronounced when a light 

wave strikes an object with a size 

comparable to its own wavelength. 

An instrument called a spectrometer 

uses diffraction to separate light into 

a range of wavelengths a spec 

trum. In the case of visible light, the 

separation of wavelengths through 

diffraction results in a rainbow. 

A spectrometer uses diffraction (and the subsequent interference) of 

light from slits or gratings to separate wavelengths. Faint peaks of 

energy at specific wavelengths can then be detected and recorded. A 

graph of these data is called a spectral signature. Patterns in a spectral 

signature help scientists identify the physical condition and composition 

of stellar and interstellar matter. 

The graph below from the SPIRE infrared spectrometer onboard the 

ESA (European Space Agency) Herschel space telescope reveals strong 

emission lines from carbon monoxide (CO), atomic carbon, and ionized 

nitrogen in Galaxy M82. 

SPECTRAL SIGNATURE of GALAXY M82 

Refraction 

Refraction is when light waves change direction as they pass 

from one medium to another. Light travels slower in air than 

in a vacuum, and even slower in water. As light travels into a 

different medium, the change in speed bends the light. Different 

wavelengths of light are slowed at different rates, which causes 

them to bend at different angles. For example, when the full 

spectrum of visible light travels through the glass of a prism, 

the wavelengths are separated into the colors of the rainbow. 

Scattering occurs when light bounces 

off an object in a variety of directions. 

The amount of scattering that takes 

place depends on the wavelength of 

the light and the size and structure of 

the object. 

The sky appears blue because of this 

scattering behavior. Light at shorter 

wavelengths blue and violet is scattered by nitrogen and oxygen as 

it passes through the atmosphere. Longer wavelengths of light red 

and yellow transmit through the atmosphere. This scattering of light 

at shorter wavelengths illuminates the skies with light from the blue 

and violet end of the visible spectrum. Even though violet is scattered 

more than blue, the sky looks blue to us because our eyes are more 

sensitive to blue light. 

Aerosols in the atmosphere can also scatter light. NASA’s Cloud Aerosol 

Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite 

can observe the scattering of laser pulses to “see” the distributions of 

aerosols from sources such as dust storms and forest fires. The image 

below shows a volcanic ash cloud drifting over Europe from an eruption 

of Iceland’s Eyjafjallajökull Volcano in 2010. 

DISTRIBUTION OF 

VOLCANIC ASH
 

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8 Tour of the Electromagnetic Spectrum 9 

VISUALIZATION: 

FROM ENERGY TO IMAGE 

False color, or representative color, is used to help scientists visualize data from wavelengths beyond the visible spectrum. Scientific instruments 

onboard NASA spacecraft sense regions within the electromagnetic spectrum—spectral bands. The instruments direct the electromagnetic energy 

onto a detector, where individual photons yield electrons related to the amount of incoming energy. The energy is now in the form of “data,” which 

can be transmitted to Earth and processed into images. 

DIGITAL CAMERA 

Digital cameras operate similarly to some scientific instruments. A sensor 


in the camera captures the brightness of red, green, and blue light and 


records these brightness values as numbers. The three sets of data are 


then combined in the red, green, and blue channels of a computer moni
 

tor to create a color image.
 

Red Green Blue Composite 

NATURAL COLOR IMAGES 

Instruments onboard satellites can also capture visible light data to cre
 

ate natural color, or true color, satellite images. Data from visible light 


bands are composited in their respective red, green, and blue channels 


on screen. The image simulates a color image that our eyes would see 


from the vantage point of the spacecraft. 


Red Green Blue Composite 

FALSE COLOR IMAGES 

Sensors can also record brightness values in regions beyond visible light. 


This Hubble image of Saturn was taken at longer infrared wavelengths 


and composited in the red, green, and blue channels respectively. The 


resulting false-color composite image reveals compositional variations 


and patterns that would otherwise be invisible.
 

2.1 μm 1.8 μm 1.0 μm False Color Composite 

Martian Soil 

This false-color infrared image from the Thermal 


Emission Imaging System (THEMIS) camera onboard 


the Mars Odyssey spacecraft reveals the differences 


in mineralogy, chemical composition, and structure of 


the Martian surface. Large deposits of the mineral ol
 

ivine appear in the image as magenta to purple-blue.
 

DATA FROM MULTIPLE SENSORS 

This composite image of the spiral galaxy Messier 101 combines views 

from Spitzer, Hubble, and Chandra space telescopes. The red color shows 

Spitzer’s view in infrared light. It highlights the heat emitted by dust lanes 

in the galaxy where stars can form. The yellow color is Hubble’s view in 

visible light. Most of this light comes from stars, and they trace the same 

spiral structure as the dust lanes. The blue color shows Chandra’s view in 

x-ray light. Sources of x-rays include million-degree gas, exploded stars, 

and material colliding around black holes. 

Spitzer Hubble Chandra 

Such composite images allow astronomers to compare how features are 

seen in multiple wavelengths. It’s like “seeing” with a camera, nightvision goggles, and x-ray vision all at once. 

COLOR MAPS 

Often a data set, such as elevation or temperature data, is best repre 

sented as a range of values. To help scientists visualize the data, the 

values are mapped to a color scale. The color code is arbitrary and thus 

can be chosen according to how the data can best be visualized. The 

sea surface temperature map below uses a scale from dark blue for cold 

temperatures to red for warm temperatures. 

From Energy to Image 

MARS ODYSSEY 

Composite 

Evaporation at the ocean’s surface leaves minerals and salts behind. 

For this and other reasons, the salinity of the ocean varies from place 

to place. This map shows the long term averages of sea surface salinity 

using practical salinity units units used to describe the concentration 

of dissolved salts in water. The white regions have the highest salinity 

and the dark regions have the lowest. 
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Radio Waves 11

RADIO WAVES 

10 Tour of the Electromagnetic Spectrum 

WHAT ARE RADIO WAVES? 

Radio waves have the longest wavelengths in the electromagnetic spec 

trum. They range from the length of a football to larger than our planet. 

Heinrich Hertz proved the existence of radio waves in the late 1880s. 

He used a spark gap attached to an induction coil and a separate spark 

gap on a receiving antenna. When waves created by the sparks of the 

coil transmitter were picked up by the receiving antenna, sparks would 

jump its gap as well. Hertz showed in his experiments that these signals 

possessed all the properties of electromagnetic waves. 

You can tune a radio to a specific wavelength or frequency and listen 

to your favorite music. The radio “receives” these electromagnetic radio 

waves and converts them to mechanical vibrations in the speaker to 

create the sound waves you can hear. 

RADIO EMISSIONS IN THE SOLAR SYSTEM 

Radio waves are also emitted by the Sun and planets in our solar system. 

A day of data from the radio astronomy instrument called WAVES on the 

WIND spacecraft recorded emissions from Jupiter’s ionosphere whose 

wavelengths measure about fifteen meters (shown below). The far right 

of this graph shows radio bursts from the Sun caused by electrons that 

have been ejected into space during solar flares moving at 20% of the 

speed of light. 

RADIO EMISSIONS 

In 1932, Karl Jansky at Bell 

Labs revealed that stars and 

other objects in space radi 

ated radio waves. 

RADIO TELESCOPES 

Radio telescopes look toward the heavens to view planets, comets, giant 

clouds of gas and dust, stars, and galaxies. By studying the radio waves 

originating from these sources, astronomers can learn about their com 

position, structure, and motion. Radio astronomy has the advantage that 

sunlight, clouds, and rain do not affect observations. 

Since radio waves are longer than optical waves, radio telescopes are 

made differently than the telescopes used for visible light. Radio tele 

scopes must be physically larger than an optical telescopes in order to 

make images of comparable resolution. But they can be made lighter 

with millions of small holes cut through the dish since the long radio 

waves are too big to “see” them. The Parkes radio telescope, which has 

a dish 64 meters wide, cannot yield an image any clearer than a small 

backyard optical telescope! 

PARKES RADIO 

TELESCOPE 

Radio Waves in Space 

Astronomical objects that have a changing magnetic 

field can produce radio waves. NASA s STEREO 

satellite monitors bursts of radio waves from the 

Sun’s corona. Data pictured here show emissions 

from a variety of sources including radio bursts from 

the Sun, the Earth, and even Jupiter. 

A VERY LARGE TELESCOPE 

In order to make a clearer, or higher resolution, radio image, radio as 

tronomers often combine several smaller telescopes, or receiving dishes, 

into an array. Together, these dishes can act as one large telescope whose 

resolution is set by the maximum size of the area. The National Radio 

Astronomy Observatory’s Very Large Array (VLA) radio telescope in New 

Mexico is one of the world’s premier astronomical radio observatories. 

The VLA consists of 27 antennas arranged in a huge “Y” pattern up to 36 

km across (roughly one-and-one-half times the size of Washington, DC). 

The techniques used in radio astronomy at long wavelengths can some 

times be applied at the shorter end of the radio spectrum—the microwave 

portion. The VLA image below captured 21-centimeter energy emissions 

around a black hole in the lower right and magnetic field lines pulling gas 

around in the upper left. 

RADIO EMISSIONS NEAR BLACK HOLE 

THE RADIO SKY 

If we were to look at the sky with a radio telescope tuned to 408 MHz, the 

sky would appear radically different from what we see in visible light. 

Instead of seeing point like stars, we would see distant pulsars, star 

forming regions, and supernova remnants would dominate the night sky. 

Radio telescopes can also detect quasars. The term quasar is short for 

quasi stellar radio source. The name comes from the fact that the first 

quasars identified emit mostly radio energy and look much like stars. 

Quasars are very energetic, with some emitting 1,000 times as much 

energy as the entire Milky Way. However, most quasars are blocked from 

view in visible light by dust in their surrounding galaxies. 

Astronomers identified the quasars with the help of radio data from the 

VLA radio telescope because many galaxies with quasars appear bright 

when viewed with radio telescopes. In the false color image below, in 

frared data from the Spitzer space telescope is colored both blue and 

green, and radio data from the VLA telescope is shown in red. The qua 

sar bearing galaxy stands out in yellow because it emits both infrared 

and radio light. 

STEREO

QUASAR IN RADIO & INFRARED 
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12 Tour of the Electromagnetic Spectrum 13 

MICROWAVES
 

MICROWAVE OVENS 

Microwave ovens work by using microwaves about 12 centimeters in 

length to force water and fat molecules in food to rotate. The interaction 

of these molecules undergoing forced rotation creates heat, and the food 

is cooked. 

MICROWAVE BANDS 

Microwaves are a portion or “band” found at the higher frequency end 

of the radio spectrum, but they are commonly distinguished from ra 

dio waves because of the technologies used to access them. Different 

wavelengths of microwaves (grouped into “sub-bands”) provide different 

information to scientists. Medium-length (C-band) microwaves penetrate 

through clouds, dust, smoke, snow, and rain to reveal the Earth’s surface. 

L-band microwaves, like those used by a Global Positioning System (GPS) 

receiver in your car, can also penetrate the canopy cover of forests to 

measure the soil moisture of rain forests. Most communication satellites 

use C-, X-, and Ku-bands to send signals to a ground station. 

Microwaves that penetrate haze, light rain and snow, clouds, and smoke 

are beneficial for satellite communication and studying the Earth from 

space. The SeaWinds instrument onboard the Quick Scatterometer 

(QuikSCAT) satellite uses radar pulses in the Ku-band of the microwave 

spectrum. This scatterometer measures changes in the energy of the 

HURRICANE KATRINA 

This Doppler-radar image 

seen on TV weather news uses 

microwaves for local weather 

forecasting. Shown here is 

Hurricane Claudette s eye wall 

making landfall. 

microwave pulses and can determine speed and direction of wind near 

the ocean surface. The ability of microwaves to pass through clouds en 

ables scientists to monitor conditions underneath a hurricane. 

ACTIVE REMOTE SENSING 

Radar technology is considered an active remote sensing system because 

it actively sends a microwave pulse and senses the energy reflected 

back. Doppler Radar, Scatterometers, and Radar Altimeters are examples 

of active remote sensing instruments that use microwave frequencies. 

The radar altimeter onboard the joint NASA/CNES (French space agen 

cy) Ocean Surface Topography Mission (OSTM)/Jason 2 satellite can 

determine the height of the sea surface. This radar altimeter beams 

microwaves at two different frequencies (13.6 and 5.3 GHz) at the 

sea surface and measures the time it takes the pulses to return to the 

spacecraft. Combining data from other instruments that calculate the 

SEA 

SURFACE 

HEIGHT 

Arctic Sea Ice 

The Japanese Advanced Microwave Scanning Radi
 

ometer for EOS (AMSR E) instrument onboard NASA s 


Aqua satellite can acquire high resolution microwave 


measurements of the entire polar region every day,
 

even through clouds and snowfall.
 

spacecraft’s precise altitude and correct for the effect of water vapor 

on the pulse can determine the sea surface height within just a few 

centimeters! 

GLOBAL WEATHER PATTERNS 

Scientists monitor the changes in sea surface height around the world 

to help measure the amount of heat stored in the ocean and predict 

global weather and climate events such as El Niño. Since warm water 

is less dense than cold water, areas with a higher sea surface tend to 

be warmer than lower areas. The sea surface height image (page 12) 

shows an area of warm water in the central and eastern Pacific Ocean 

that is about 10 to 18 centimeters higher than normal. Such conditions 

can signify an El Niño. 

PASSIVE REMOTE SENSING 

Passive remote sensing refers to the sensing of electromagnetic waves 

that did not originate from the satellite or instrument itself. The sensor 

is merely a passive observer collecting electromagnetic radiation. Pas 

sive remote sensing instruments onboard satellites have revolutionized 

weather forecasting by providing a global view of weather patterns and 

surface temperatures. A microwave imager onboard NASA’s Tropical 

Rainfall Measuring Mission (TRMM) can capture data from underneath 

storm clouds to reveal the underlying rain structure. 

HURRICANE KATRINA 

Microwaves 

AQUA

CLUES TO THE BIG BANG 

In 1965, using long, L band microwaves, Arno Penzias and Robert 

Wilson, scientists at Bell Labs, made an incredible discovery quite by 

accident: they detected background noise using a special low noise an 

tenna. The strange thing about the noise was that it was coming from 

every direction and did not seem to vary in intensity much at all. If this 

static were from something on our planet, such as radio transmissions 

from a nearby airport control tower, it would come only from one direc 

tion, not everywhere. The Bell Lab scientists soon realized that they had 

serendipitously discovered the cosmic microwave background radiation. 

This radiation, which fills the entire universe, is a clue to its beginning, 

known as the Big Bang. 

The image below from the Wilkinson Microwave Anisotropy Probe 

(WMAP) shows a detailed, all sky picture of the infant universe at 

380,000 years of age. This light, emitted 13.7 billon years ago, is 2.7 

Kelvin today. The observed +/ 200 microKelvin temperature fluctuations, 

shown as color differences in the image, are the seeds that grew to 

become clusters of galaxies. 

MICROWAVE BACKGROUND 
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14 Tour of the Electromagnetic Spectrum 15 

INFRARED WAVES 

INFRARED ENERGY 

A remote control uses light waves just beyond the visible spectrum of 

light—infrared light waves—to change channels on your TV. This region 

of the spectrum is divided into near-, mid-, and far-infrared. The region 

from 8 to 15 microns (μm) is referred to by Earth scientists as thermal 

infrared since these wavelengths are best for studying the longwave 

thermal energy radiating from our planet. 

DISCOVERY OF INFRARED 

In 1800, William Herschel conducted an experiment measuring the differ 

ence in temperature between the colors in the visible spectrum. When he 

noticed an even warmer temperature measurement just beyond the red 

end of the spectrum, he had discovered infrared light! 

THERMAL IMAGING 

We can sense some infrared energy as heat. Some objects are so hot 

they also emit visible light such as a fire does. Other objects, such as 

humans, are not as hot and only emit only infrared waves. Our eyes cannot 

see these infrared waves but instruments that can sense infrared ener 

gy such as night vision goggles or infrared cameras allow us to “see” 

the infrared waves emitting from warm objects such as humans and ani 

mals. The temperatures for the images below are in degrees Fahrenheit. 

THERMAL IMAGING 

Thermometers placed within 

each color of the visible spec 

trum reveal an increase in 

temperature from blue to red. 

The light energy just beyond 

the visible spectrum, infrared, 

is even warmer. 

COOL ASTRONOMY 

Many objects in the universe are too cool and faint to be detected in 

visible light but can be detected in the infrared. Scientists are begin 

ning to unlock the mysteries of cooler objects across the universe such 

as planets, cool stars, nebulae, and many more, by studying the infrared 

waves they emit. 

The Cassini spacecraft captured this image of Saturn’s aurora using in 

frared waves. The aurora is shown in blue, and the underlying clouds are 

shown in red. These aurorae are unique because they can cover the entire 

pole, whereas aurorae around Earth and Jupiter are typically confined by 

magnetic fields to rings surrounding the magnetic poles. The large and 

variable nature of these aurorae indicates that charged particles stream 

ing in from the Sun are experiencing some type of magnetism above 

Saturn that was previously unexpected. 

SATURN S AURORA IN IR 

SEEING THROUGH DUST 

Infrared waves have longer wavelengths than visible light and can pass 

through dense regions of gas and dust in space with less scattering and 

absorption. Thus, infrared energy can also reveal objects in the universe 

that cannot be seen in visible light using optical telescopes. The James 

Webb Space Telescope (JWST) has three infrared instruments to help 

study the origins of the universe and the formation of galaxies, stars, 

and planets. 

A pillar composed of gas and dust in the Carina Nebula is illuminated by 

the glow from nearby massive stars shown below in the visible light im 

age from the Hubble Space Telescope. Intense radiation and fast streams 

of charged particles from these stars are causing new stars to form 

within the pillar. Most of the new stars cannot be seen in the visible-light 

image (left) because dense gas clouds block their light. However, when 

the pillar is viewed using the infrared portion of the spectrum (right), it 

practically disappears, revealing the baby stars behind the column of gas 

and dust. 

CARINA NEBULA 

Infrared Waves 

Seeing the Unseen 

When we look up at the constellation Orion, we see 

only the visible light. But NASA s Spitzer space tele 

scope was able to detect nearly 2,300 planet-forming 

disks in the Orion nebula by sensing the infrared glow 

of their warm dust. Each disk has the potential to 

form planets and its own solar system. 

JAMES WEBB SPACE

TELESCOPE

MONITORING THE EARTH 

To astrophysicists studying the universe, infrared sources such as plan 

ets are relatively cool compared to the energy emitted from hot stars 

and other celestial objects. Earth scientists study infrared as the thermal 

emission (or heat) from our planet. As incident solar radiation hits Earth, 

some of this energy is absorbed by the atmosphere and the surface, 

thereby warming the planet. This heat is emitted from Earth in the form 

of infrared radiation. Instruments onboard Earth observing satellites can 

sense this emitted infrared radiation and use the resulting measurements 

to study changes in land and sea surface temperatures. 

There are other sources of heat on the Earth’s surface, such as lava flows 

and forest fires. The Moderate Resolution Imaging Spectroradiometer (MO 

DIS) instrument onboard the Aqua and Terra satellites uses infrared data 

to monitor smoke and pinpoint sources of forest fires. This information 

can be essential to firefighting efforts when fire reconnaissance planes 

are unable to fly through the thick smoke. Infrared data can also enable 

scientists to distinguish flaming fires from still-smoldering burn scars. 

FOREST FIRES IN 

NORTHERN CALIFORNIA 

Visible Light Infrared 
 

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16 Tour of the Electromagnetic Spectrum 

NEAR-INFRARED WAVES 

Near-Infrared Waves 17

Helping Farmers 

Near-infrared data collected by the Landsat 7 

satellite, such as this image of Minnesota, can help 

farmers assess the health of their crops. Shades 

of red in this image indicate good crop health, and 

yellow colors reveal where crops are infested. 

LANDSAT 7

NEAR INFRARED RADIATION 

A portion of radiation that is just beyond the visible spectrum is referred 

to as near infrared. Rather than studying an object’s emission of infra 

red, scientists can study how objects reflect, transmit, and absorb the 

Sun’s near infrared radiation to observe health of vegetation and soil 

composition. 

HEALTHY VEGETATION 

Our eyes perceive a leaf as green because wavelengths in the green 

region of the spectrum are reflected by pigments in the leaf, while the 

other visible wavelengths are absorbed. In addition, the components in 

plants reflect, transmit, and absorb different portions of the near-infrared 

radiation that we cannot see. 

Reflected near infrared radiation can be sensed by satellites, allowing 

scientists to study vegetation from space. Healthy vegetation absorbs 

blue and red light energy to fuel photosynthesis and create chlorophyll. 

A plant with more chlorophyll will reflect more near infrared energy than 

an unhealthy plant. Thus, analyzing a plants spectrum of both absorption 

and reflection in visible and in infrared wavelengths can provide infor 

mation about the plants’ health and productivity. 

INFRARED FILM 

Color Infrared film can record near infrared energy and can help sci 

entists study plant diseases where there is a change in pigment and 

cell structure. These two images show the difference between a color 

infrared photo and a natural color photo of trees in a park. 

SPECTRAL SIGNATURES OF VEGETATION 

Data from scientific instruments can provide more precise measurements 

than analog film. Scientists can graph the measurements, examine the 

unique patterns of absorption and reflection of visible and infrared en 

ergy, and use this information to identify types of plants. The graph below 

shows the differences among the spectral signatures of corn, soybeans, 

and Tulip Poplar trees. 

ASSESSING VEGETATION FROM SPACE 

Data and imagery from the U.S. Geological Service (USGS) and NASA 

Landsat series of satellites are used by the U.S. Department of Agriculture 

to forecast agricultural productivity each growing season. Satellite data 

can help farmers pinpoint where crops are infested, stressed, or healthy. 

SOIL COMPOSITION 

Near infrared data can also help identify types of rock and soil. This im 

age of the Saline Valley area in California was acquired by the Advanced 

Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on 

board NASA’s Terra satellite. 

Data from ASTER’s visible and near infrared bands at 0.81 μm, 0.56 μm, 

and .66 μm are composited in red, green, and blue creating the false 

color image below. Vegetation appears red, snow and dry salt lakes are 

white, and exposed rocks are brown, gray, yellow, and blue. Rock colors 

mainly reflect the presence of iron minerals and variations in albedo 

(solar energy reflected off the surface). 

PLANETS IN NEAR-INFRARED 

This false-color composite of Jupiter combines near-infrared and visiblelight data of sunlight reflected from Jupiter’s clouds. Since methane gas 

in Jupiter’s atmosphere limits the penetration of sunlight, the amount of 

reflected near-infrared energy varies depending on the clouds’ altitude. 

The resulting composite image shows this altitude difference as different 

colors. Yellow colors indicate high clouds; red colors are lower clouds; 

and blue colors show even lower clouds in Jupiter’s atmosphere. The 

Near Infrared Camera and Multi-Object Spectrometer (NICMOS) onboard 

NASA’s Hubble Space Telescope captured this image at the time of a rare 

alignment of three of Jupiter’s largest moons—Io, Ganymede, and Cal 

listo—across the planet’s face. 

REFLECTED 

HEALTHY LEAF 

SALINE VALLEY, CALIFORNIA JUPITER IN 

NEAR IR 

Infrared Film Color Film 
 

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18 Tour of the Electromagnetic Spectrum 19 

VISIBLE LIGHT 

WAVELENGTHS OF VISIBLE LIGHT 

All electromagnetic radiation is light, but we can only see a small portion 

of this radiation—the portion we call visible light. Cone-shaped cells in 

our eyes act as receivers tuned to the wavelengths in this narrow band of 

the spectrum. Other portions of the spectrum have wavelengths too large 

or too small and energetic for the biological limitations of our perception. 

As the full spectrum of visible light travels through a prism, the wave 

lengths separate into the colors of the rainbow because each color is 

a different wavelength. Violet has the shortest wavelength, at around 

380 nanometers, and red has the longest wavelength, at around 700 

nanometers. 

THE SUN’S CORONA 

The Sun is the dominant source for visible-light waves our eyes receive. 

The outer-most layer of the Sun’s atmosphere, the corona, can be seen 

in visible light. But it is so faint it cannot not be seen except during a 

total solar eclipse because the bright photosphere overwhelms it. The 

photograph below was taken during a total eclipse of the Sun where the 

photosphere and chromosphere are almost completely blocked by the 

Isaac Newton’s experiment in 1665 

showed that a prism bends visible 

light and that each color refracts at 

a slightly different angle depending 

on the wavelength of the color. 

moon. The tapered patterns—coronal streamers—around the Sun are 

formed by the outward flow of plasma that is shaped by magnetic field 

lines extending millions of miles into space. 

COLOR AND TEMPERATURE 

As objects grow hotter, they radiate energy dominated by shorter wave 

lengths, changing color before our eyes. A flame on a blow torch shifts 

from reddish to bluish in color as it is adjusted to burn hotter. In the same 

way, the color of stars tells scientists about their temperature. 

Our Sun produces more yellow light than any other color because its 

surface temperature is 5,500°C. If the Sun’s surface were cooler say 

3,000°C it would look reddish, like the star Betelgeuse. If the Sun were 

hotter say, 12,000°C it would look blue, like the star Rigel. 

TEMPERATURE OF STARS 

Victoria Crater on Mars 

The High Resolution Imaging Science Experiment 


(HiRISE) camera onboard the Mars Reconnaissance 


Orbiter (MRO) captured this spectacular visible light 


image of Victoria Crater.
 

SPECTRA AND SPECTRAL SIGNATURES 

Close examination of the visible-light spectrum from our Sun and other 

stars reveals a pattern of dark lines—called absorption lines. These 

patterns can provide important scientific clues that reveal hidden prop 

erties of objects throughout the universe. Certain elements in the Sun’s 

atmosphere absorb certain colors of light. These patterns of lines within 

spectra act like fingerprints for atoms and molecules. Looking at the 

Sun’s spectrum, for example, the fingerprints for elements are clear to 

those knowledgeable about those patterns. 

Patterns are also evident in a graph of an object’s reflectance. Elements, 

molecules, and even cell structures have unique signatures of reflec 

tance. A graph of an object’s reflectance across a spectrum is called a 

spectral signature. Spectral signatures of different Earth features within 

the visible light spectrum ARE shown below. 

SPECTRAL SIGNATURES OF 

EARTH FEATURES
 

Visible Light 

MARS 

RECONNAISSANCE

ORBITER

ACTIVE REMOTE SENSING—ALTIMETRY 

Laser altimetry is an example of active remote sensing using visible light. 

NASA’s Geoscience Laser Altimeter System (GLAS) instrument onboard 

the Ice, Cloud, and land Elevation Satellite (ICESat) enabled scientists to 

calculate the elevation of Earth’s polar ice sheets using lasers and ancil 

lary data. Changes in elevation over time help to estimate variations in 

the amount of water stored as ice on our planet. The image below shows 

elevation data over the West Antarctic Ice Streams. 

Laser altimeters can also make unique measurements of the heights and 

characteristics of clouds, as well as the top and structure of the vegeta 

tion canopy of forests. They can also sense the distribution of aerosols 

from sources such as dust storms and forest fires. 

ANTARCTIC ICE STREAMS 

TOTAL SOLAR ECLIPSE 
 

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20 21 

SOLAR DYNAMICS 

OBSERVATORY

Tour of the Electromagnetic Spectrum 

ULTRAVIOLET WAVES

ULTRAVIOLET LIGHT FROM OUR SUN 

Ultraviolet (UV) light has shorter wavelengths than visible light. Although 

UV waves are invisible to the human eye, some insects, such as bumble 

bees, can see them. This is similar to how a dog can hear the sound of a 

whistle just outside the hearing range of humans. 

The Sun is a source of the full spectrum of ultraviolet radiation, which 

is commonly subdivided into UV-A, UV-B, and UV-C. These are the clas 

sifications most often used in Earth sciences. UV-C rays are the most 

harmful and are almost completely absorbed by our atmosphere. UV-B 

rays are the harmful rays that cause sunburn. Exposure to UV-B rays 

increases the risk of DNA and other cellular damage in living organisms. 

Fortunately, about 95 percent UV-B rays are absorbed by ozone in the 

Earth’s atmosphere. 

Scientists studying astronomical objects commonly refer to different sub 

divisions of ultraviolet radiation: near ultraviolet (NUV), middle ultraviolet 

(MUV), far ultraviolet (FUV), and extreme ultraviolet (EUV). NASA’s SDO 

spacecraft captured the image below in multiple wavelengths of extreme 

ultraviolet (EUV) radiation. The false-color composite reveals different 

gas temperatures. Reds are relatively cool (about 60,000 Celsius) while 

blues and greens are hotter (greater than one million Celsius). 

OUR SUN IN UV 

Johann Ritter s experiment 

was designed to expose 

photographic paper to light 

just beyond the visible spec 

trum and prove the existence 

of light beyond violet 

ultraviolet light. 

DISCOVERY OF ULTRAVIOLET 

In 1801, Johann Ritter conducted an experiment to investigate the exis 

tence of energy beyond the violet end of the visible spectrum. Knowing 

that photographic paper would turn black more rapidly in blue light than 

in red light, he exposed the paper to light beyond violet. Sure enough, the 

paper turned black, proving the existence of ultraviolet light. 

ULTRAVIOLET ASTRONOMY 

Since the Earth’s atmosphere absorbs much of the high-energy ultra 

violet radiation, scientists use data from satellites positioned above the 

atmosphere, in orbit around the Earth, to sense UV radiation coming from 

our Sun and other astronomical objects. Scientists can study the formation 

of stars in ultraviolet since young stars shine most of their light at these 

wavelengths. This image from NASA’s Galaxy Evolution Explorer (GALEX) 

spacecraft reveals new young stars in the spiral arms of galaxy M81. 

GALAXY M81 IN UV 

THE OZONE “HOLE” 

Chemical processes in the upper atmosphere can affect the amount of 

atmospheric ozone that shields life at the surface from most of the Sun’s 

harmful UV radiation. Each year, a “hole” of thinning atmospheric ozone 

expands over Antarctica, sometimes extending over populated areas of 

South America and exposing them to increased levels of harmful UV rays. 

The Dutch Ozone Monitoring Instrument (OMI) onboard NASA’s Aura sat 

ellite measures amounts of trace gases important to ozone chemistry 

and air quality. The image above shows the amount of atmospheric ozone 

in Dobson Units—the common unit for measuring ozone concentration. 

These data enable scientists to estimate the amount of UV radiation 

reaching the Earth’s surface and forecast high-UV-index days for public 

health awareness. 

ULTRAVIOLET LIGHT FROM STARS 

The Lyman Alpha Mapping Project (LAMP) onboard the Lunar Reconnais 

sance Orbiter can peer into permanently shaded craters on the moon by 

sensing the faint reflections of UV light coming from distant stars. 

MAPPING THE MOON IN UV 

Ultraviolet Waves 

Solar Prominence 

NASA s Solar Dynamics Observatory 

(SDO) spacecraft captured this view of 

a dense loop of plasma erupting on the 

Sun’s surface—a solar prominence. 

The plasma is seen flowing along a 

magnetic field 

AURORAE 

Aurorae are caused by high-energy waves that travel along a planet’s 

magnetic poles, where they excite atmospheric gases and cause them 

to glow. Photons in this high-energy radiation bump into atoms of gases 

in the atmosphere causing electrons in the atoms to excite, or move to 

the atom’s upper shells. When the electrons move back down to a lower 

shell, the energy is released as light, and the atom returns to a relaxed 

state. The color of this light can reveal what type of atom was excited. 

Green light indicates oxygen at lower altitudes. Red light can be from 

oxygen molecules at a higher altitude or from nitrogen. On Earth, aurorae 

around the north pole are called the Northern Lights. 

JUPITER’S AURORA 

The Hubble Space Telescope captured this image of Jupiter’s aurora in 

ultraviolet wrapping around Jupiter’s north pole like a lasso. 

JUPITER S AURORA IN UV 
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22 Tour of the Electromagnetic Spectrum 23 

X-RAYS
 

X-RAYS AND ENERGY 

X-rays have much higher energy and much shorter wavelengths than 

ultraviolet light, and scientists usually refer to x-rays in terms of their 

energy rather than their wavelength. This is partially because x-rays have 

very small wavelengths, between 0.03 and 3 nanometers, so small that 

some x-rays are no bigger than a single atom of many elements. 

DISCOVERY OF X-RAYS 

X-rays were first observed and documented in 1895 by German scientist 

Wilhelm Conrad Roentgen. He discovered that firing streams of x-rays 

through arms and hands created detailed images of the bones inside. 

When you get an x-ray taken, x-ray sensitive film is put on one side of 

your body, and x-rays are shot through you. Because bones are dense 

and absorb more x-rays than skin does, shadows of the bones are left on 

the x-ray film while the skin appears transparent. 

Our Sun’s radiation peaks in the visual range, but the Sun’s corona is much 

hotter and radiates mostly x rays. To study the corona, scientists use data 

collected by x ray detectors on satellites in orbit around the Earth. Japan’s 

Hinode spacecraft produced these x-ray images of the Sun that allow 

scientists to see and record the energy flows within the corona. 

TEMPERATURE AND COMPOSITION 

The physical temperature of an object determines the wavelength of the 

radiation it emits. The hotter the object, the shorter the wavelength of 

peak emission. X-rays come from objects that are millions of degrees 

Celsius—such as pulsars, galactic supernovae remnants, and the accre 

tion disk of black holes. 

From space, x-ray telescopes collect photons from a given region of the 

sky. The photons are directed onto the detector where they are absorbed, 

and the energy, time, and direction of individual photons are recorded. 

Such measurements can provide clues about the composition, tempera 

ture, and density of distant celestial environments. Due to the high energy 

and penetrating nature of x-rays, x-rays would not be reflected if they hit 

the mirror head on (much the same way that bullets slam into a wall). 

X-ray telescopes focus x-rays onto a detector using grazing incidence 

mirrors (just as bullets ricochet when they hit a wall at a grazing angle). 

NASA’s Mars Exploration Rover, Spirit, used x-rays to detect the spectral 

signatures of zinc and nickel in Martian rocks. The Alpha Proton X-Ray 

Spectrometer (APXS) instrument uses two techniques, one to determine 

structure and another to determine composition. Both of these tech 

niques work best for heavier elements such as metals. 

CHANDRA

Galatic Center & the Solar Cycle 

This mosaic of several Chandra X-ray Observa
 

tory images of the central region of our Milky 


Way galaxy reveals hundreds of white dwarf 


stars, neutron stars, and black holes. Separately,
 

the Solar and Heliophysics Observatory (SOHO) 


captured these images of the Sun representing 


an entire solar cycle from 1996 through 2006.
 

SUPERNOVA 

Since Earth’s atmosphere blocks x-ray radiation, telescopes with x-ray 

detectors must be positioned above Earth’s absorbing atmosphere. The 

supernova remnant Cassiopeia A (Cas A) was imaged by three of NASA’s 

great observatories, and data from all three observatories were used to 

create the image shown below. Infrared data from the Spitzer Space Tele 

scope are colored red, optical data from the Hubble Space Telescope are 

yellow, and x-ray data from the Chandra X-ray Observatory are green 

and blue. 

The x-ray data reveal hot gases at about ten million degrees Celsius that 

were created when ejected material from the supernova smashed into 

surrounding gas and dust at speeds of about ten million miles per hour. 

By comparing infrared and x-ray images, astronomers are learning more 

about how relatively cool dust grains can coexist within the super-hot, 

x-ray producing gas. 

X-Rays 

EARTH’S AURORA IN X-RAYS 

Solar storms on the Sun eject clouds of energetic particles toward Earth. 

These high-energy particles can be swept up by Earth’s magnetosphere, 

creating geomagnetic storms that sometimes result in an aurora. The 

energetic charged particles from the Sun that cause an aurora also ener 

gize electrons in the Earth’s magnetosphere. These electrons move along 

the Earth’s magnetic field and eventually strike the Earth’s ionosphere, 

causing the x-ray emissions. These x-rays are not dangerous to people 

on the Earth because they are absorbed by lower parts of the Earth’s at 

mosphere. Below is an image of an x-ray aurora by the Polar Ionospheric 

X-ray Imaging Experiment (PIXIE) instrument aboard the Polar satellite. 

OUR SUN IN X RAY SUPERNOVA CAS A EARTH S X RAY AURORA 
 

 

 

 

 

 

 

 

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24 Tour of the Electromagnetic Spectrum 25 

GAMMA RAYS
 

SOURCES OF GAMMA RAYS 

Gamma rays have the smallest wavelengths and the most energy of any 

wave in the electromagnetic spectrum. They are produced by the hottest 

and most energetic objects in the universe, such as neutron stars and 

pulsars, supernova explosions, and regions around black holes. On Earth, 

gamma waves are generated by nuclear explosions, lightning, and the less 

dramatic activity of radioactive decay. 

DETECTING GAMMA RAYS 

Unlike optical light and x rays, gamma rays cannot be captured and 

reflected by mirrors. Gamma-ray wavelengths are so short that they can 

pass through the space within the atoms of a detector. Gamma-ray de 

tectors typically contain densely packed crystal blocks. As gamma rays 

pass through, they collide with electrons in the crystal. This process is 

called Compton scattering, wherein a gamma ray strikes an electron and 

loses energy, similar to what happens when a cue ball strikes an eight 

ball. These collisions create charged particles that can be detected by 

the sensor. 

COMPTON SCATTERING 

GAMMA RAY BURSTS 

Gamma-ray bursts are the most energetic and luminous electromagnetic 

events since the Big Bang and can release more energy in 10 seconds 

than our Sun will emit in its entire 10-billion-year expected lifetime! Gam 

ma-ray astronomy presents unique opportunities to explore these exotic 

objects. By exploring the universe at these high energies, scientists can 

search for new physics, testing theories and performing experiments that 

are not possible in Earth-bound laboratories. 

If we could see gamma rays, the night sky would look strange and un 

familiar. The familiar view of constantly shining constellations would be 

replaced by ever changing bursts of high energy gamma radiation that 

last fractions of a second to minutes, popping like cosmic flashbulbs, 

momentarily dominating the gamma ray sky and then fading. 

NASA’s Swift satellite recorded the gamma-ray blast caused by a black 

hole being born 12.8 billion light years away (below). This object is among 

the most distant objects ever detected. 

GAMMA RAY BURST 

Brighter colors in the Cygus 

region indicate greater num 

bers of gamma rays detected 

by the Fermi gamma ray 

space telescope. 

COMPOSITION OF PLANETS 

Scientists can use gamma rays to determine the elements on other 

planets. The Mercury Surface, Space Environment, Geochemistry, and 

Ranging (MESSENGER) Gamma-Ray Spectrometer (GRS) can measure 

gamma rays emitted by the nuclei of atoms on planet Mercury’s surface 

that are struck by cosmic rays. When struck by cosmic rays, chemical 

elements in soils and rocks emit uniquely identifiable signatures of en 

ergy in the form of gamma rays. These data can help scientists look for 

geologically important elements such as hydrogen, magnesium, silicon, 

oxygen, iron, titanium, sodium, and calcium. 

The gamma-ray spectrometer on NASA’s Mars Odyssey Orbiter detects 

and maps these signatures, such as this map (below) showing hydrogen 

concentrations of Martian surface soils. 

HYDROGEN ON MARS 

Gamma Rays 

FERMI

GAMMA RAY SKY 

Gamma rays also stream from stars, supernovas, pulsars, and black hole 

accretion disks to wash our sky with gamma-ray light. These gamma-ray 

streams were imaged using NASA’s Fermi gamma-ray space telescope to 

map out the Milky Way galaxy by creating a full 360-degree view of the 

galaxy from our perspective here on Earth. 

A FULL-SPECTRUM IMAGE 

The composite image below of the Cas A supernova remnant shows 

the full spectrum in one image. Gamma rays from Fermi are shown in 

magenta; x-rays from the Chandra Observatory are blue and green. The 

visible light data captured by the Hubble space telescope are displayed in 

yellow. Infrared data from the Spitzer space telescope are shown in red; 

and radio data from the Very Large Array are displayed in orange. 

SUPERNOVA 

Gamma Ray Visible and UV 
 

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THE EARTH’S 

RADIATION BUDGET

The energy entering, reflected, absorbed, and emitted by the Earth system are the components of the Earth’s radiation budget. Based on the physics 

principle of conservation of energy, this radiation budget represents the accounting of the balance between incoming radiation, which is almost entirely 

solar radiation, and outgoing radiation, which is partly reflected solar radiation and partly radiation emitted from the Earth system, including the atmo 

sphere. A budget that’s out of balance can cause the temperature of the atmosphere to increase or decrease and eventually affect our climate. The units 

of energy employed in measuring this incoming and outgoing radiation are watts per square meter (W/m2). 

Reflected Shortwave Incoming Shortwave
 

Radiation Radiation
 

INCOMING SOLAR 

RADIATION 

Incoming ultraviolet, visible, and a limited 

Reflected by portion of infrared energy (together some 

Clouds	 times called “shortwave radiation”) from 

the Sun drive the Earth’s climate system. 

Some of this incoming radiation is reflect 

ed off clouds, some is absorbed by the 

atmosphere, and some passes through to 

the Earth’s surface. Larger aerosol par Absorbed by ticles in the atmosphere interact with and Atmosphere absorb some of the radiation, causing the 

atmosphere to warm. The heat generated 

by this absorption is emitted as longwave 

infrared radiation, some of which radiates 

out into space. 

Reflected by 

ABSORBED ENERGY Surface 

The solar radiation that passes through 

Earth’s atmosphere is either reflected off 

snow, ice, or other surfaces or is absorbed 

by the Earth’s surface. 

Absorbed Energy 

RADIATION AND THE CLIMATE SYSTEM 

For scientists to understand climate change, they must also determine 

what drives the changes within the Earth’s radiation budget. The Clouds 

and the Earth’s Radiant Energy System (CERES) instrument aboard NASA’s 

Aqua and Terra satellites measures the shortwave radiation reflected and 

longwave radiation emitted into space accurately enough for scientists 

to determine the Earth’s total radiation budget. Other NASA instruments 

monitor changes in other aspects of the Earth’s climate system—such 

as clouds, aerosol particles, and surface reflectivity—and scientists are 

examining their many interactions with the radiation budget. 

GREENHOUSE EFFECT 

Greenhouse gases in the atmosphere (such as water vapor and carbon 

dioxide) absorb most of the Earth’s emitted longwave infrared radiation, 

which heats the lower atmosphere. In turn, the warmed atmosphere 

emits longwave radiation, some of which radiates toward the Earth’s 

surface, keeping our planet warm and generally comfortable. Increasing 

concentrations of greenhouse gases such as carbon dioxide and meth 

ane increase the temperature of the lower atmosphere by restricting the 

outward passage of emitted radiation, resulting in “global warming,” or, 

more broadly, global climate change. 

EMITTED LONGWAVE RADIATION 

Heat resulting from the absorption of incoming shortwave radiation is 

emitted as longwave radiation. Radiation from the warmed upper at 

mosphere, along with a small amount from the Earth’s surface, radiates 

out to space. Most of the emitted longwave radiation warms the lower 

atmosphere, which in turn warms our planet’s surface. 

The Earth’s Radiation Budget 

Emitted Longwave 

Energy TERRA

Emitted by the 

Atmosphere 

Atmospheric 

Window 

Emitted by Lower Emitted Surface 


Atmosphere Radiation
 
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CREDITS 

IMAGE CREDITS 

Inside Cover: Waves, Flavio Takemoto; Page 2 3: Communication Tower, Mihai Andoni; Radio, Peter Huys; Microwave Oven, Kriss Szkurlatowski; Remote Control, Bartek Ambrozik; 

Eye, Flavio Takemoto; Page4: Waves black background, Jana Kollarova; Waterdrop, iStock Photo; Page 6: Topography of the Moon, NASA/Goddard; Atlanta in Infrared, NASA images 

by Marit Jentoft Nilsen, based on Landsat 7 data; Ice background, Elisabeth Sophia Fuchs; Page 7: Spectrograph of Galaxy 3C 326, Troy Benesch; Volcanic Ash Distribution, NASA/ 

GSFC/LaRC/JPL, MISR Team; Page 8: Balloon Photo, iStock Photo; Saturn Visible Light, NASA and The Hubble Heritage Team (STScI/AURA) Acknowledgment: R.G. French (Wellesley 

College), J. Cuzzi (NASA/Ames), L. Dones (SwRI), and J. Lissauer (NASA/Ames); Saturn False Color, NASA/JPL/STScI; Page 9: Messier 101 Galaxy, NASA, ESA, CXC, JPL, Caltech and 

STScI; Sea Surface Temperature and Sea Surface Salinity, NASA/Goddard Space Flight Center Scientific Visualization Studio; Page 10: Karl Jansky s Radio Telescope, Image courtesy 

of NRAO/AUI; Solar Radio Bursts, NASA/GSFC Wind/Wave, Michael L. Kaiser; Parkes Radio Telescope, Ian Sutton; Page 11: Radio Emission Near Black Hole, VLA & NRAO, Farhad Yusef 

Zedeh et al. Northwestern; Quasar in Radio & Infrared, NASA/JPL Caltech/A. Martinez Sansigre (Oxford University); Page 12: Hurricane Katrina Sea Winds, NASA image courtesy 

the QuikSCAT Science Team at the Jet Propulsion Laboratory; Doppler Radar, NOAA; Sea Surface Height, NASA/JPL Ocean Surface Topography Team; Page 13: Arctic Sea Ice from 

AMSR E, NASA/Goddard Space Flight Center Scientific Visualization Studio; Sea Surface Height, NASA/JPL Ocean Surface Topography Team; WMAP Microwave Background, NASA/ 

WMAP Science Team; Page 14: Thermal Imaging, Courtesy NASA/JPL Caltech; Hershel s Experiment, Troy Benesch; Saturn s Aurora in IR, Cassini VIMS Team, JPL, ESA, NASA; Page 

15: Spitzer image of Orion in Infrared & James Web Telescope, Thomas Megeath (Univ. Toledo) et al., JPL, Caltech, NASA; Carina Nebula, NASA, ESA, and the Hubble SM4 ERO Team; 

Forest Fires in Northern California, NASA image by Jeff Schmaltz, MODIS Rapid Response Team; Page 16: E. coli, Rocky Mountain Laboratories, NIAID, NIH; Healthy Leaf, Jeff Carns; 

Infrared and Natural Color Film photos, Ginger Butcher; Page 17: Minnesota Crops, NASA image created by Jesse Allen, using Landsat data provided by the United States Geological 

Survey; Saline Valley California, NASA, GSFC, MITI, ERSDAC, JAROS, and the U.S./Japan ASTER Science Team; Jupiter in Near Infrared, NASA and E. Karkoschka (University of Arizona); 

Page 18: Soap bubble, iStock; Sun s Corona, Courtesy of Miloslav Druckmüller, Martin Dietzel, Peter Aniol, Vojtech Rušin; Sun Image, Courtesy of SOHO/[instrument] consortium. 

SOHO is a project of international cooperation between ESA and NASA; Prism Image, Troy Benesch; Page 19: Victoria Crater on Mars, NASA/JPL/University of Arizona; Antarctic Ice 

Streams, NASA/Goddard Space Flight Center; Page 20: Rhinovirus, iStock; Ultraviolet waves, NASA/SDO/AIA; Our Sun in UV, NASA/Goddard/SDO AIA Team; Galaxy M81 in UV, NASA/ 

JPL Caltech; Ritter s Experiment, Troy Benesch; Page 21: Solar Prominence, NASA image and animation from the Goddard Space Flight Center Scientific Visualization Studio and 

the Solar Dynamics Observatory; Mapping the Moon in UV, Ernest Wright, LRO LAMP; Ozone Hole, NASA; Jupiter s Aurora in UV, John Clarke (University of Michigan) and NASA; Page 

22: Our Sun in X Ray, Hinode JAXA/NASA/PPARC; Mars Rover, Jeff Carns; Page 23: Galactic Center, NASA/UMass/D.Wang et al.; Sun images from SOHO EIT Consortium: NASA/ 

ESA; Supernova, NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL Caltech/Steward/O.Krause et al.; Earth s X Ray Aurora, POLAR, PIXIE, NASA; Page 24: Gamma Ray Burst, 

NASA/Swift/Stefan Immler, et al.; Page 25: Fermi Gamma Ray Sky and Fermi spacecraft, NASA/DOE/International LAT Team; Hydrogen on Mars, NASA/Goddard Space Flight Center 

Scientific Visualization Studio; Supernova, NASA/DOE/Fermi LAT Collaboration, CXC/SAO/JPL Caltech/Steward/O. Krause et al., and NRAO/AUI; Back Cover: Waves, Flavio Takemoto 

COMPANION VIDEO CREDITS 

Animation Team: Troy Benesch, Mike Brody, Jeff Carns, Ron Mochinski 

Script Writing Team: Ginger Butcher, Kendall Haven 

Storyboards: Jeff Carns 

Art Direction: Troy Benesch 

Narration: Jack Elias 

Project Management Team: Jim Consalvi, Gamble Gilbertson, Tim Smith 

Subject Matter Experts: Jeannette Allen, Steve Graham, Britt Griswold, 

Dr. Jeffrey Hayes, John Leck, Dr. Marilyn Lindstrom, Dr. Jennifer Rumburg, 

Todd Toth, Dr. Lisa Wainio 

Key Science Advisors and Editors: Dr. Claire Parkinson, Dr. Edward J. Wollack 

Special Thanks to Ruth Netting 

Additional Thanks to: Dr. Denis Bogan, Dr. Eric Brown de Colstoun, Anita Davis, 

Dr. Marcianna P. Delaney, Dr. James Irons, Bela Kekesi, John Keller, Dr. Anita 

Krishnamurthi, Elaine Lewis, Dr. David Lindley, Dr. James 

Lochner, Lou Mayo, Jeffrey Morisette, Carolyn Ng 

Videos available at: 

http://missionscience.nasa.gov/ems 

28 Tour of the Electromagnetic Spectrum Credits 29

BOOK CREDITS 

Author: Ginger Butcher 

Design and illustration: Jenny Mottar 

Copyediting: C. Claire Smith 

Special Thanks to NASA Science Mission Directorate and Ruth Netting 

Created under the HITSS contract to NASA Headquarters by InDyne, Inc. 

and V! Studios, Inc. 

Key Science Advisors and Editors: Dr. Claire L. Parkinson, Dr. Edward J. Wollack 

Reviewers: Jeannette E. Allen, Max Bernstein, Dr. Marcianna P. Delaney, Britt Griswold, 

Dr. Hashima Hasan, Dr. J. E. Hayes, Dr. Paul Hertz, Dr. Lisa Wainio, and Greg Williams 

Additional Thanks to: Dr. Eric Brown de Colstoun, Scott Gries, Dr. David Lindley, 

Dr. Christopher A. Shuman, Todd E. Toth, and George Varros 

UNIT CONVERSION SCALE 

As noted on page 5, electromagnetic waves can be described by their frequency, wavelength, or energy. Radio and microwaves are usually described 

in terms of frequency (Hertz), infrared and visible light in terms of wavelength (meters), and x rays and gamma rays in terms of energy (electron volts). 

This is a scientific convention that allows the use of the units that are the most convenient with numbers not too large or too small. For comparison, the 

various wavelengths described in this booklet are all expressed in meters in this table. 

FRONT COVER 

Cover image shows the Chandra Spacecraft, the Ozone hole image by Aura’s OMI instrument, the Blue Marble image of Earth from Terra’s MODIS 

instrument, and pictured left to right Arno Penzias, Wilhelm Roentgen and Issac Newton. 

BACK COVER 

Missions and instruments mentioned in the text are displayed in this chart. 

Discipline Units Alternate Unit Length (meters) Sci Notation (meters) Size Comparison 

10 kHz 30 kilometer 30000 3.0E+04 

100 kHz 3.0 kilometer 3000 3.0E+03 

1 MHz 0.3 kilometer 300 3.0E+02 

1 GHz 30 centimeter 0.3 3.0E-01 

1.4 GHz 21 centimeter 0.21 2.1E-01 

1000 micrometer 1.0 millimeter 0.001 1.0E-03 

100 micrometer 100 micrometer 0.0001 1.0E-04 

50 micrometer 50 micrometer 0.00005 5.0E-05 

20 micrometer 20 micrometer 0.00002 2.0E-05 

1.0 micrometer 1.0 micrometer 0.000001 1.0E-06 

5000 Angstrom 500 nanometer 0.0000005 5.0E-07 

1000 Angstrom 100 nanometer 0.0000001 1.0E-07 

500 Angstrom 50 nanometer 0.00000005 5.0E-08 

100 Angstrom 10 nanometer 0.00000001 1.0E-08 

1 kev 12 Angstrom 0.0000000012 1.2E-09 

10 kev 1.2 Angstrom 0.00000000012 1.2E-10 

100 kev 0.12 Angstrom 0.000000000012 1.2E-11 

1 Mev 1200 femtometer 0.0000000000012 1.2E-12 

1 Gev 1.2 femtometer 0.0000000000000012 1.2E-15 Diameter/nucleus 

Diameter of an atom 

Diameter of a soap 

bubble membrane 

Diameter of a 

human hair 

Height of the Statue 

of Liberty 

Size of a baseball 

Winner 

22 

AWARDS 
NASA MISSIONS & THE ELECTROMAGNETIC SPECTRUM 
 

NP-2010-07-664-HQ