In order to understand night vision, it is important to understand something about light . Of visible light, violet has the most energy, and red has the least. Just next to the visible light spectrum is the infrared spectrum.
Infrared light is a small part of the light spectrum.
The key difference between thermal IR and the other two is that thermal IR is emitted by an object instead of reflected off it. Infrared light is emitted by an object because of what is happening at the atomic level. Atoms are constantly in motion. They continuously vibrate, move and rotate. Even the atoms that make up the chairs that we sit in are moving around.