Electromagnetic Spectrum

One family, one wavelength axis. Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays are, physically, exactly the same kind of disturbance discussed throughout this unit -- a self-propagating electromagnetic wave, obeying $c=\nu\lambda$ and travelling at the same speed $c$ in vacuum -- arranged purely by WAVELENGTH (equivalently, by frequency, since the two are fixed by the same constant $c$) into what is called the electromagnetic spectrum. There is no sharp physical boundary between one band and the next; the divisions are a matter of convention, based on how a given wavelength range is typically produced, detected, and used. The table for this section lists all seven commonly named bands in order, from the longest-wavelength radio waves down to the shortest-wavelength gamma rays.

Radio waves and microwaves. Radio waves, the longest-wavelength band, are produced by accelerating charges in an oscillating electrical circuit connected to a transmitting antenna -- exactly the mechanism described in Section 8.3 -- and are used for AM/FM radio broadcasting, television, and long-distance communication. Microwaves, a shorter-wavelength sub-band, need special vacuum-tube oscillators (such as a klystron or a magnetron) to generate efficiently at their much higher frequency, and are used in radar, in satellite and mobile-phone links, and, at the specific frequency $2.45\ \text{GHz}$, in the ordinary domestic microwave oven, where the microwaves are absorbed strongly by water molecules in food, heating it from within (Numerical 4 works this example through directly).

Infrared, visible, and ultraviolet. Infrared radiation is emitted by the vibration and rotation of molecules in any object at ordinary or elevated temperatures (all warm bodies, including the human body, radiate strongly in the infrared) and is used in television remote controls, thermal-imaging cameras, and night-vision equipment. Visible light, a narrow band roughly $400$-$700\ \text{nm}$ wide, is produced by electron transitions within atoms of a sufficiently hot or electrically excited source (the Sun's surface, an incandescent filament, an LED) and is, unsurprisingly, the only band the human eye can directly detect. Ultraviolet radiation, produced by even hotter sources or by special discharge lamps, carries enough energy per photon to damage living tissue and DNA in large doses, but is also used constructively in LASIK eye surgery, in sterilising drinking water and surgical instruments, and in detecting forged banknotes under a UV lamp.

X-rays and gamma rays. X-rays are produced when fast-moving electrons, accelerated through a high voltage inside a vacuum tube, are suddenly decelerated on striking a heavy metal target -- their kinetic energy converting directly into high-frequency electromagnetic radiation -- and are used for medical and dental imaging, airport security screening, and studying the atomic structure of crystals. Gamma rays, the shortest-wavelength and highest-frequency band of all, originate not from the ATOM's outer electrons at all but from rearrangements deep INSIDE the atomic NUCLEUS itself (radioactive decay or nuclear reactions), and are used in cancer radiotherapy, in sterilising medical equipment, and in nuclear-medicine imaging, though their very high photon energy also makes them the most biologically hazardous band of the whole spectrum in large, unshielded doses. …