Electromagnetic Spectrum

The electromagnetic spectrum is the complete, orderly arrangement of every possible electromagnetic wave, ordered by wavelength or, equivalently, by frequency -- since for any wave $c=f\lambda$, a longer wavelength always corresponds to a lower frequency and vice versa. Running from the longest wavelengths (lowest frequencies, lowest photon energies) to the shortest, the named regions are: radio waves, produced by the accelerated motion of charges in conducting wires, spanning a few Hz up to $10^9$ Hz, used in radio and television broadcasting and in cellular voice communication in the ultra-high-frequency band, and known for showing reflection and diffraction; microwaves, produced by special vacuum-tube devices such as the klystron, magnetron and Gunn diode, spanning $10^9$ Hz to $10^{11}$ Hz, used in radar systems for aircraft navigation and vehicle-speed detection, in microwave ovens for cooking, and for long-distance satellite communication, and capable of being polarised; infrared radiation, produced by hot bodies (also called heat waves) and by molecules undergoing rotational and vibrational transitions, spanning $10^{11}$ Hz to $4\times10^{14}$ Hz, used to power satellites via solar cells, in night-vision and infrared photography, and in industrial inspection of finished metal products for cracks and flaws; visible light, produced by incandescent bodies and by excited atoms in gases, spanning $4\times10^{14}$ Hz to $8\times10^{14}$ Hz, the only part of the spectrum the human eye directly senses, obeying reflection and refraction, showing interference, diffraction and polarisation, exhibiting the photoelectric effect, and used to study molecular structure and atomic electron arrangements; ultraviolet radiation, produced by the Sun, electric arcs and ionized gases, spanning $8\times10^{14}$ Hz to $10^{17}$ Hz, used to sterilise surgical instruments and destroy bacteria, in burglar alarms, to reveal invisible writing and fingerprints, and in atomic-structure research, but largely absorbed by the atmosphere's ozone layer and harmful to the human body in excess; X-rays, produced when high-speed electrons are suddenly stopped at a high-atomic-number target, or by electronic transitions among the innermost atomic orbits, spanning $10^{17}$ Hz to $10^{19}$ Hz, used to study inner atomic electron shells and crystal structures and, medically, to detect bone fractures, diseased organs and stones, having more penetrating power than ultraviolet; and gamma rays, produced by transitions of radioactive atomic nuclei and by the decay of certain elementary particles, at $10^{18}$ Hz and above, causing chemical reactions on photographic plates, fluorescence, ionisation and diffraction, used in radiotherapy for cancer and tumours and to sterilise food by killing pathogenic microorganisms, and possessing the highest penetrating power of any region of the spectrum (though carrying no charge, they remain harmful to the human body). A practical illustration of measuring the wave speed directly is given by the household microwave oven: the standing microwaves set up between its interior walls burn food more at the fixed antinode positions than elsewhere, so mea …