What is Active and Passive Remote Sensing?

Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on-site observation. Remote sensing is used in numerous fields, including geography, land surveying and most Earth Science disciplines (for example, hydrology, ecology, oceanography, glaciology, geology); it also has military, intelligence, commercial, economic, planning, and humanitarian applications. There are two types of remote sensing technology, active and passive remote sensing. Difference between these two is quite small, however, leaves great impact on their functionality. As we learnt in primary school, sun constantly emits light energy and is only source of natural light for earth. Moreover, this energy is not observed by any component in solar system. Instead it is reflected to somewhere else. Just like the moon, it shines despite of not possessing its own light source.