The sun emits energy in the form of electromagnetic radiation or waves. These waves are characterized by their wavelength and their mathematical inverse, the wave frequency. Either property can be used to characterize a wave, but in Remote Sensing, it is common to distinguish electromagnetic waves by their wavelengths (measured in micro- or nanometers). Solar radiation consists of waves with wavelengths across a wide electromagnetic spectrum. By general agreement, common names have been given to certain (weakly defined) ranges of this spectrum, as in ultraviolet (UV) light, visible light, infrared (IR), microwaves, etc.