What is infra-red (IR)? Infra-red is an electromagnetic wave who wavelength is between 0.75 microns to 1000 microns (1 micron = 1µm). Since infra-red is out of visible light range, we can’t really see IR with naked eye. However, there is a method to “see” IR which will be shown later on. Some of the infra-red applications includes night vision, hyperspectral imaging, and communications. We also use IR daily in our TV remote or any device remote.
From Wikipedia, Passive Infra-Red sensor (PIR sensor) is an electronic sensor that measures infrared (IR) light radiating from objects n its field of view. The term passive refers that PIR sensor does not emit or generate any energy for detection purpose.