The most common element that is added to collect data when speaking of remote sensing is light. A camera is a good example: a camera without a flash will take a picture of exactly what is there, but you might not get much information from the picture when it is dark. A camera with a flash changes the environment by adding light, but you get more information from the picture.
Light doesn’t have to be used for active remote sensing, however. Another good example of remote sensing is sonar (or SONAR, which stands for SOund Detection And Ranging). Passive sonar is the process of listening to ambient noise to determine what objects may be close by and how far away they are. Active sonar involves making a noise, such as a beep or a click. The sound waves bounce off of objects both near and far, creating echoes. By measuring how long it takes for the echoes to return and from what direction they come, we can determine where objects are. This is considered active sensing because sound has been added to gain more information.
Topics in Remote Sensing: Overview | Active Vs passive | Satellites | Geostationary VS polar orbiting