Why is a PIR motion detector referred to as "passive"?

A PIR (Passive Infrared) motion detector is termed "passive" because it detects infrared radiation emitted by objects (typically humans or animals) in its field of view without contributing any energy of its own to the process. Instead of actively sending out energy signals, which can be the case with other types of sensors, the PIR sensor simply receives and responds to the infrared radiation from warm bodies. This inherent characteristic allows it to be energy efficient and thus makes it classified as passive, as it only reacts to existing energy rather than creating its own signals.

The distinction between active and passive sensors lies fundamentally in their operation mode; active sensors emit energy (like laser beams or radio waves) and then measure the reflected signals, whereas passive sensors only detect and measure the energy that is naturally present in the environment. Thus, the correct answer highlights the PIR motion detector's role as one that does not emit energy but only senses it.