A device that gives an output by detecting the changes in quantities or events can be defined as a sensor. In general, sensors are termed as the devices that generate an electrical signal or optical output signal corresponding to the variations in the level of inputs.
Sensors are how a product senses anything in the real-world, which are then converted into an electrical signal.
There exist mainly two types of remote sensing sensors and those are active and passive sensors.
These generate energy to scan the things and locations and then a sensor identifies and calculates the amount of reflected radiation from the target object. The examples of active sensors are RADAR where the time difference that is in between the emission process and return process is calculated by determining the area, speed, and object direction.
These sensors collect radiation which is either radiated or reflected by the surrounding locations or object. The most crucial example of a passive sensor is reflected sunlight and other example is film camera work.