Electromagnetic Radiation.  In the electromagnetic spectrum, short wavelengths correlate with high frequency, such as gamma rays and X-rays, and long wavelengths correlate with low frequency such as radio waves and microwaves.  In the middle of the spectrum lies ultraviolet radiation (10-400 nm wavelength), visible light (400-700 nm wavelength), and infrared radiation (700-1000 nm wavelength).  The energy in this portion of the spectrum is captured in aerial and satellite imagery to classify surface materials based on the signature of what energy is reflected or absorbed by the material.