Passive remote sensing is done by recording the naturally-occurring interaction between the object of study and the source of electromagnetic radiation, such as photography. Electromagnetic energy has distinctive properties. The most familiar form of electromagnetic energy is visible light. For practical purposes, we can consider the speed of light a constant. The electromagnetic spectrum is the name we give to the continuum of energy that travels through the universe at the speed of light.
Matter radiates a range electromagnetic energy.
Electromagnetic energy that interacts with matter is called incident radiation. The intensity, direction, polarization, wavelength and phase of electromagnetic energy can change when the energy interacts with matter. The electromagnetic energy can be transmitted, absorbed, emitted, scattered or reflected from the study object. The physical properties of the matter control what interaction will take place, and how the properties of the electromagnetic energy will be changed. Force fields include things like gravity , and magnetic fields .