WHAT IS INFRARED LIGHT?
J. S. Sweitzer
11/22/94
We experience infrared light every time we feel the heat of the sun
on our skin or the warmth of a camp fire. Technically, what we are experiencing
in these instances is thermal infrared light. Scientists call waves of
electromagnetic energy that travel through space light. We tend to think
of light as only the radiant energy that we see with our eyes. But the
light we can detect with our eyes is a very tiny fraction of all the types
of light that exist. Infrared light is one type of light that is invisible
to us. Gamma rays, X-rays, ultraviolet light, microwaves and radio waves
are other types of invisible light. All of these rays and waves are the
same type of electromagnetic energy. They are different only because the
length of their waves are different.
When we see something by visible light our eyes allow us to distinguish
the light of different waves as different colors. The waves that are short
are blue and the waves that are long appear as red. We can't really see
the waves, we only see the colors that our mind creates to help us interpret
the world. Every color has a distinct wavelength. For example, violet
light can be seen at light wavelengths of around 0.4 micron* and yellow
light is made up of waves that are 0.6 microns long. Reddish colors begin
at wavelengths of about 0.65 microns. Your eyes cannot see light that
has a wavelength longer that 0.7 microns.
Light with wavelengths from 0.7 micron to about 0.1 millimeter is called
infrared light. The band of infrared light is a thousand times wider than
that of visible light. All of it is invisible to our eyes. Infrared films
and normal video cameras are sensitive to what is called very near infrared
light (0.7 to 0.9 microns). This is also the type of light that the remote
control for your TV uses. (Try shining your remote control at your video
camera.) Beyond those waves are the near infrared waves at 2.4 microns
that the South Pole Infrared Explorer (SPIREX) telescope observes from
the South Pole. SPIREX uses a very special video still-frame camera that
can detect and make images of stars that emit this "color" of light. Longer
wavelength infrared light is emitted by hot objects in our world. So,
although we can't see the thermal infrared light from a hot piece of metal
like a soldering iron, we can feel it on our skin when we bring our hand
close. Scientists use many types of devices to detect and measure infrared
light. Even if we can't see it with our eyes through a telescope our specialized
astronomical cameras can.
We know that infrared radiation is light just like visible light because
it has the same properties as visible light. Infrared can be focused and
reflected like visible light. Infrared light can also be "aligned" like
regular light and therefore polarized. This means we can make infrared
telescopes that look and work the way normal visible light telescopes
do.
- A micron is defined as a millionth of a meter. That is also one thousandth
of a millimeter.
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