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 than 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.
From "What is Infrared Light?" by J.S. Sweitzer, dated 11/22/94.