Electromagnetic Spectrum
. The electromagnetic spectrum refers to the entire range of frequencies or wavelengths of electromagnetic waves (see Electromagnetic Radiation). Light traditionally refers to the range of frequencies that can be seen by humans. The frequencies of these waves are very high, about one-half to three-quarters of a million billion (5 x 1014 to 7.5 x 1014) Hz. Their wavelengths range from 400 to 700 nm. X rays have wavelengths ranging from several thousandths of a nanometer to several nanometers, and radio waves have wavelengths ranging from several meters to several thousand meters. Waves with frequencies a little lower than the range of human vision (and with wavelengths correspondingly longer) are called infrared. Waves with frequencies a little higher and wavelengths shorter than human eyes can see are called ultraviolet. About half the energy of sunlight at Earth’s surface is visible electromagnetic waves, about 3 percent is ultraviolet, and the rest is infrared. Each different frequen