RADIO WAVES
| MICROWAVES | INFRARED
| VISIBLE LIGHT | ULTRAVIOLET
| X-RAYS | GAMMA RAYS
The Infrared
Infrared light lies between the visible and microwave
portions of the electromagnetic spectrum. Infrared light has a range of
wavelengths, just like visible light has wavelengths that range from red
light to violet. "Near infrared" light is closest in wavelength to
visible light 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, near infrared ones are the size of cells, or are
microscopic.
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Far infrared waves are thermal. In other words, we experience this type of
infrared radiation every day in the form 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 the
difference between inside body temperature and outside skin
temperature.
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Infrared light is even used to heat food sometimes - special lamps that emit
thermal infrared waves are often used in fast food restaurants!
Shorter, near infrared waves are not hot
at all - in fact you cannot even feel them. These shorter wavelengths are the ones
used by your TV's remote 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 radiate visible 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 radiation which we feel as
heat. The warmer the object, the more infrared radiation it emits.
Humans, at normal body temperature, radiate most strongly in the infrared
at a wavelength of about 10 microns. (A micron is the term commonly used in
astronomy for a micrometer or one millionth of a meter.) This image (
which is courtesy of
the Infrared Processing and Analysis Center at CalTech), shows
a man holding up a lighted match! Which parts of this image
do you think have the warmest temperature? How does the temperature of
this man's glasses compare to the temperature of his hand?
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To make infrared pictures like the one above, we can use special cameras and
film that detect differences in temperature, and then assign different brightnesses
or false colors to them.
This provides a picture that our eyes can interpret.
The image at the left (courtesy of SE-IR Corporation, Goleta, CA) shows a cat in the
infrared. The orange areas are the warmest and the white-blue areas are
the coldest. This image gives us a different view of a familiar animal as
well as information that we could not get from a visible light picture.
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Humans may not be able to see infrared light, but did you know that
snakes in the pit viper family, like rattlesnakes, have sensory "pits",
which are used to image infrared light? This allows the snake to detect
warm blooded animals, even in dark burrows!
Snakes with 2 sensory pits are even thought to have some
depth perception in the infrared! (Thanks to
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 far away 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 and Landsat 7 look at the
Earth. Special sensors, like those aboard the Landsat 7 satellite,
record data about the amount of infrared light reflected or emitted from
the Earth's surface.
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Landsat 7
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Other satellites, like the Infrared Astronomy
Satellite (IRAS) look up into space and measure the infrared light coming
from things like large clouds of dust and gas, stars, and galaxies!
What does the Infrared show us?
This is an infrared image of the Earth taken by the GOES 6 satellite in
1986. A scientist used temperatures to determine which parts of the image
were from clouds and which were land and sea. Based on these temperature
differences, he colored each separately using 256 colors, giving the image
a realistic appearance.
Why use the infrared to image the Earth? While it is easier to
distinguish clouds from land in the visible range, there is more detail in the clouds
in the infrared. This is great for studying cloud structure. For
instance, note that darker clouds are warmer, while lighter clouds are cooler.
Southeast of the Galapagos, just west of the
coast of South America, there is a place where you can distinctly see multiple
layers of clouds, with the warmer clouds at lower altitudes, closer to the
ocean that's warming them.
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Space Science and Engineering Center, University of
Wisconsin-Madison, Richard Kohrs, designer
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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 infrared radiation is not related to the
temperature of the object being photographed - unless the object is very,
very hot.
Infrared film 'sees' the object because the Sun (or some other light source)
shines
infrared light 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 of red, green, blue, and
infrared!
This image of a building with a tree and grass shows how
Chlorophyll in plants reflect near infrared
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 infrared ones are
pale red.
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This image was taken with special film that can detect invisible
infrared waves. This is a false-color
image, just like the one of the cat. False-color infrared images of the
Earth frequently use a color scheme like the one shown here, where infrared
light is mapped to the visible color of red. This means that everything in
this image that appears red is giving off or reflecting infrared light.
This makes vegetation like grasa and trees appear to be red.
The visible light waves drawn on this picture are green, and the
infrared ones are darker red.
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This is an image of Phoenix, Arizona showing the
near infrared data collected by the Landsat 5 satellite. The light
areas are areas with high reflectance of near infrared waves. The
dark areas show little reflectance. What do you think the black
grid lines in the lower right of this image represent? |
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This image shows the infrared data (appearing as
red) composited with visible light data at the blue and green wavelengths.
If near infrared is reflected off of healthy vegetation, what do
you think the red square shaped areas are in the lower left of the
image?
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Instruments on board satellites can also take pictures of things in space.
The image below of the center region of our galaxy was taken by IRAS. The
hazy, horizontal S-shaped feature that crosses the image is faint heat
emitted by dust in the plane of the Solar System.
Infrared Processing and Analysis Center, Caltech/JPL
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