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RADIO WAVES|MICROWAVES|INFRARED|VISIBLE LIGHT|ULTRAVIOLET|X-RAYS|GAMMA RAYS

Radio Waves

Radio waves have the longestwavelengths in theelectromagnetic spectrum.These waves can be longerthan a football field or as shortas a football. Radio waves domore than just bring music toyour radio. They also carrysignals for your television andcellular phones.
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The antennae on your television set receive the signal,in the form of electromagnetic waves, that is

broadcasted from the television station. It is displayedon your television screen.

Cable companies have antennae or dishes whichreceive waves broadcasted from your local TV stations.The signal is then sent through a cable to your house.

Why are car antennae about the same size as TVantennae?

Cellular phones also useradio waves to transmitinformation. Thesewaves are much smallerthat TV and FM radiowaves.

Why are antennae oncell phones smallerthan antennae on your

radio?

How do we "see" using Radio Waves?

Objects in space, such as planets and comets, giant clouds of gas and dust, and stars andgalaxies, emit light at many different wavelengths. Some of the light they emit has verylarge wavelengths - sometimes as long as a mile!. These long waves are in the radio regionof the electromagnetic spectrum.

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Because radio waves are larger thanoptical waves, radio telescopes workdifferently than telescopes that we

use for visible > light (opticaltelescopes). Radio telescopes aredishes made out of conducting metalthat reflect radio waves to a focuspoint. Because the wavelengths ofradio light are so large, a radiotelescope must be physically largerthan an optical telescope to be ableto make images of comparableclarity. For example, the Parkesradio telescope, which has a dish 64meters wide, cannot give us anyclearer an image than a smallbackyard telescope!

In order to make better and moreclear (or higher resolution) radioimages, radio astronomers oftencombine several smaller telescopes,or receiving dishes, into an array.Together, the dishes can act as onelarge telescope whose size equalsthe total area occupied by the array.

The Very Large Array (VLA) is one ofthe world's premier astronomicalradio observatories. The VLAconsists of 27 antennas arranged ina huge "Y" pattern up to 36 km (22miles) across -- roughly one and ahalf times the size of Washington,DC.

The VLA, located in New Mexico, is an interferometer; this means that it operates bymultiplying the data from each pair of telescopes together to form interference patterns. Thestructure of those interference patterns, and how they change with time as the earthrotates, reflect the structure of radio sources in the sky.

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What do Radio Waves show us?

The above image shows the Carbon Monoxide (CO) gases in our Milky Way galaxy.

Many astronomical objects emit radio waves, but that fact wasn't discovered until 1932.Since then, astronomers have developed sophisticated systems that allow them to makepictures from the radio waves emitted by astronomical objects.

Radio telescopes look toward the heavens atplanets and comets, giant clouds of gas and dust,and stars and galaxies. By studying the radiowaves originating from these sources, astronomerscan learn about their composition, structure, andmotion. Radio astronomy has the advantage thatsunlight, clouds, and rain do not affectobservations.

Did you know that radio

astronomy observatories usediesel cars around thetelescopes? The ignition ofthe spark plugs in gasoline-powered cars can interferewith radio observations - justlike running a vacuum caninterfere with your televisionreception!

[NEXT SHORTER WAVELENGTH]RETURN TO THE ELECTROMAGNETIC SPECTRUM

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Microwaves

Microwaves have wavelengths thatcan be measured in centimeters! Thelonger microwaves, those closer to afoot in length, are the waves whichheat our food in a microwave oven.

Microwaves are good for transmittinginformation from one place toanother because microwave energycan penetrate haze, light rain andsnow, clouds, and smoke.

Shorter microwaves are used inremote sensing. These microwavesare used for radar like the dopplerradar used in weather forecasts.Microwaves, used for radar, are justa few inches long.

This microwave tower can transmit information like telephone calls and computer data from one city to another.

How do we "see" using Microwaves?

Radar is an acronym for "radio detection and ranging". Radar was developed to detect objects and determine their range (or position) bytransmitting short bursts of microwaves. The strength and origin of "echoes" received from objects that were hit by the microwaves is then
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recorded.

Because radar senses electromagneticwaves that are a reflection of an active

transmission, radar is considered an activeremote sensing system. Passive remotesensing refers to the sensing ofelectromagnetic waves which did notoriginate from the satellite or sensor itself.The sensor is just a passive observer.

What do Microwaves show us?

Because microwaves can penetrate haze, light rain andsnow, clouds and smoke, these waves are good forviewing the Earth from space.

The ERS-1 satellite sends out wavelengths about 5.7cm long (C-band). This image shows sea ice breakingoff the shores of Alaska.

The JERS satellite uses wavelengths about 20 cm inlength (L-band). This is an image of the Amazon Riverin Brazil.
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This is a radar image acquired from the Space Shuttle.It also used a wavelength in the L-band of the

microwave spectrum. Here we see a computerenhanced radar image of some mountains on the edgeof Salt Lake City, Utah.

In the 1960's a startling discovery was made quite by accident. A pair of scientists at BellLaboratories detected background noise using a special low noise antenna. The strangething 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 world, like radiotransmissions from a nearby airport control tower, it would only come from one direction, noeverywhere. The scientists soon realized they had discovered the cosmic microwavebackground radiation. This radiation, which fills the entire Universe, is believed to be a clueto it's beginning, something known as the Big Bang.

The image above is a Cosmic Background Explorer (COBE) image of the cosmic microwavebackground, the pink and blue colors showing the tiny fluctuations in it.

Did you know that if you had a sensitive microwave telescope in yourhouse that you would detect a faint signal leaking out of your

microwave oven, and from various other man-made sources, but alsoa faint signal coming from all directions that you pointed it? This is theCosmic Microwave Background!
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The Infrared

Infrared light lies between the visible and microwave portions of the electromagneticspectrum. Infrared light has a range of wavelengths, just like visible light has wavelengthsthat range from red light to violet. "Near infrared" light is closest in wavelength to visiblelight and "far infrared" is closer to the microwave region of the electromagnetic spectrum.The longer, far infrared wavelengths are about the size of a pin head and the shorter, nearinfrared ones are the size of cells, or are microscopic.

Far infrared waves are thermal. In other words, weexperience this type of infrared radiation every day in theform of heat! The heat that we feel from sunlight, a fire, a

radiator or a warm sidewalk is infrared. The temperature-sensitive nerve endings in our skin can detect thedifference between inside body temperature and outsideskin temperature

Infrared light is even used to heat food sometimes - special lamps that emit thermal infrared waves are often used in fast foodrestaurants!

Shorter, near infrared waves

are not hot at all - in fact youcannot even feel them. Theseshorter wavelengths are theones used by your TV'sremote control.
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How can we "see" using the Infrared?

Since the primary source of infrared radiation is heat or thermal radiation, any object which has a temperature radiates in the infrared.Even objects that we think of as being very cold, such as an ice cube, emit infrared. When an object is not quite hot enough to radiatevisible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiatio

which we feel as heat. The warmer the object, the more infrared radiation it emits.

Humans, at normal body temperature, radiatemost strongly in the infrared at a wavelength ofabout 10 microns. (A micron is the termcommonly used in astronomy for a micrometeror one millionth of a meter.) This image ( whichis courtesy of the Infrared Processing andAnalysis Center at CalTech), shows a manholding up a lighted match! Which parts of this

image do you think have the warmesttemperature? How does the temperature of thisman's glasses compare to the temperature ofhis hand?

To make infrared pictures like theone above, we can use specialcameras and film that detectdifferences in temperature, and thenassign different brightnesses or false

colors to them. This provides apicture that our eyes can interpret.

The image at the left (courtesy of SE-IR Corporation, Goleta, CA) shows acat in the infrared. The orange areasare the warmest and the white-blueareas are the coldest. This imagegives us a different view of a familiaranimal as well as information that we

could not get from a visible lightpicture.

Humans may not be able to see infrared light, but did you know that snakes in the pit viperfamily, like rattlesnakes, have sensory "pits", which are used to image infrared light? Thisallows the snake to detect warm blooded animals, even in dark burrows! Snakes with 2sensory pits are even thought to have some depth perception in the infrared! (Thanks to
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NASA's Infrared Processing and Analysis Center for help with the text in this section.)

Many things besides people and animals emit infrared light - the Earth, the Sun, and faraway things like stars and galaxies do also! For a view from Earth orbit, whether we are

looking out into space or down at Earth, we can use instruments on board satellites.

Satellites like GOES 6 andLandsat 7 look at the Earth.Special sensors, like thoseaboard the Landsat 7 satellite,record data about the amountof infrared light reflected or

emitted from the Earth'ssurface.

Landsat 7

Other satellites, like the Infrared Astronomy Satellite (IRAS) look up into space andmeasure the infrared light coming from things like large clouds of dust and gas, stars, andgalaxies!

What does the Infrared show us?

This is an infrared image of the Earth taken by theGOES 6 satellite in 1986. A scientist usedtemperatures to determine which parts of theimage were from clouds and which were land andsea. Based on these temperature differences, hecolored each separately using 256 colors, giving

the image a realistic appearance.

Why use the infrared to image the Earth? While itis easier to distinguish clouds from land in thevisible range, there is more detail in the clouds inthe infrared. This is great for studying cloudstructure. For instance, note that darker clouds are

Space Science and Engineering Center,University of Wisconsin-Madison,Richard Kohrs, designer
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warmer, while lighter clouds are cooler. Southeastof the Galapagos, just west of the coast of SouthAmerica, there is a place where you can distinctlysee multiple layers of clouds, with the warmer

clouds at lower altitudes, closer to the ocean that'swarming them.

We know, from looking at an infrared image of a cat, that many things emit infrared light.But many things also reflect infrared light, particularly near infrared light. Near infraredradiation is not related to the temperature of the object being photographed - unless theobject is very, very hot.

Infrared film 'sees' the object because the Sun (or some other light source) shines infraredlight on it and it is reflected or absorbed by the object. You could say that this reflecting or

absorbing of infrared helps to determine the object's 'color' - its color being a combination ored, green, blue, and infrared!

This image of a building with a tree and grassshows how Chlorophyll in plants reflect nearinfrared waves along with visible light waves.Even though we can't see the infrared waves,they are always there. The visible light waves

drawn on this picture are green, and the infraredones are pale red.

This image was taken with special film that candetect invisible infrared waves. This is a false-color image, just like the one of the cat. False-color infrared images of the Earth frequently usea color scheme like the one shown here, whereinfrared light is mapped to the visible color of red.

This means that everything in this image thatappears red is giving off or reflecting infraredlight. This makes vegetation like grasa and treesappear to be red. The visible light waves drawnon this picture are green, and the infrared onesare darker red.

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This is an image of Phoenix, Arizonashowing the near infrared data collected

by the Landsat 5 satellite. The light areasare areas with high reflectance of nearinfrared waves. The dark areas show littlereflectance. What do you think the blackgrid lines in the lower right of this imagerepresent?

This image shows the infrared data(appearing as red) composited withvisible light data at the blue and greenwavelengths. If near infrared is reflectedoff of healthy vegetation, what do youthink the red square shaped areas are inthe lower left of the image?

Instruments on board satellites can also take pictures of things in space. The image belowof the center region of our galaxy was taken by IRAS. The hazy, horizontal S-shapedfeature that crosses the image is faint heat emitted by dust in the plane of the SolarSystem.

Infrared Processing and Analysis Center, Caltech/JPL
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