Infrared (IR) light means "below" red which is the longest wavelength and lowest frequency of
visible light.
Its wavelength is between 0.7 and 300 micrometers which is greater than 1000 nanometers. It has
a frequency of range of 1 to 430 THz (Terahertz).
Infrared light is used for military and civilian purposes. For military purposes, it can be used as
night vision, target acquisition, surveillance, homing, and tracking. For civilian purposes, it can
be used for weather forecasting, spectroscopy, sensor-equipped telescopes (application for
astronomy), thermography, heating, communications, meteorology, art history and many more.
There are three categories of infrared light:
Near-infrared (near-IR): Wavelengths which range from 0.7 to 1.3 microns. Used by a variety of
electronic devices such as remote controls.
Mid-infrared (mid-IR) Wavelengths which range from 1.3 to 3 microns. Used by a variety of
electronic devices such as remote controls.
Thermal-infrared (thermal-IR) - Wavelengths which range from 3 microns to over 30 microns.
The difference between thermal-IR and the other two is that thermal-IR is emitted by an object
instead of reflected off it. It is emitted by an object because of what is happening at atomic level.
How inanimate objects reveals colors along with animate objects?
When an electron moves to a higher-energy orbit then returns to a ground state, it releases its
energy as a photon. In order to understand the concept of seeing photons being released, when
you use a toaster, you see the metal wires turn hot red, the red color is caused by the release of
red photons. In simple words, the photons emitted has a specific wavelength (color) that depends
on the state of the electron's energy when the photon is released.
When you look at thermal sensor, the hotter the object, the shorter the wavelength of the infrared
photon it releases. It can be programmed to specify which colors to represent each wavelength.
Hence, we see different thermal sensors such as night vision, layers of paintings in art, coldness
of a popsicle from a thermal captured video.
http://electronics.howstuffworks.com/gadgets/other-gadgets/nightvision1.htm