ELECTROMAGNETIC RADIATION Considerations of electromagnetic radiations are involved in nearly every branch of modern physics, and various aspects of the subject are treated in separate articles, notably ELECTRICITY; LIGHT ; QUANTUM THEORY ; X-RAYS, NATURE OF : Röntgen Rays; SPECTROSCOPY; ELECTRIC WAVES; WIRELESS TELEGRAPHY. The biological effects of radiation are dealt with under the head ing RADIOTHERAPY. In this 'article a general survey is made of the whole range of radiations, with the object of showing how the different types merge into one another, and discussing certain characteristic differences which are associated with different ranges of wave-length.
It may be well to give here the units used for measurement of the shorter wave-lengths. These are: The micron = one-millionth of a meter = cm., denoted by Az The millimicron = one-thousandth of a micron = cm., „ „ The Angstrom unit = ro 8 cm., „ „Au The Siegbahn unit = i cm., „ „ X The millimicron is often used for giving visible and ultra-violet wave-lengths, and the Siegbahn unit for giving X-ray and 7-ray wave-lengths. These two units are not employed in this article, but are inserted for completeness.
Historical.—The fact that there are radiations outside the limits of the visible spectrum was demonstrated 'by Sir William Herschel in 180o. He found 'that a thermometer showed a higher temperature when placed in the red end of the spectrum than in the blue, and a still higher temperature beyond the red end, where nothing could be seen. Two years later Ritter and Wollas ton independently proved the existence of rays beyond the violet end by establishing that the chemical action of light on silver chloride was even stronger there than in the visible violet. These ultra-violet rays were called at the 'time "dark chemical rays." The wave nature of the ultra violet was proved by Young, who with the help of paper impregnated with silver chloride produced a record of Newton's rings formed by these invisible rays, the rings being smaller than those formed by the visible violet, which proved the 'smaller wave-length. The interference prop erties of the infra-red were established by Fizeau and Foucault in 1847. By the use of a very small alcohol thermometer, read
with a microscope, they were able to demonstrate small differ ences of temperature at different points, corresponding to the alternations of intensity in the interference pattern produced in the infra-red by a suitable arrangement of two mirrors, or by diffraction at a straight edge, as with visible light. (See LIGHT.) Fizeau made measurements of wave-length in the infra-red about this time. In 1842 Becquerel photographed part of the ultra violet 'spectrum. By 185o it had already been definitely estab lished that beyond the red end of the spectrum were invisible radiations of longer wave-length, and beyond the violet invisible radiations of shorter wave-length, and that those rays could be reflected, refracted, polarised and made to interfere in just the same way as the visible rays. In 1884 Langley made accurate measurements in the infra-red as far as 5.3,u by means of the diffraction grating, and virtually inaugurated a new branch of spectroscopy. Masoart, in 1864 and 1866, took the first photo graphs of the ultra-violet which were good enough for wave length determination.
The electromagnetic waves, first generated by H. Hertz in 1888, were soon proved to have all the properties of light waves, as was to be anticipated from Maxwell's theory. (See ELECTRIC WAVES; ELECTRICITY; LIGHT.) Hertz himself, using parabolic reflectors to obtain definite beams, established the reflection and refraction of the rays, using a large prism of pitch for the latter purpose. (See ELECTRIC WAVES.) Polarisation was produced by means of screens of parallel wires, which take the place of the Nicol prisms used in experiments with ordinary light, and also by reflection. Interference, and later dispersion, were also demonstrated with Hertzian waves. The velocity of propagation along wires, measured by Blondlot and by Trowbridge and Duane, was found to be equal to that of light. There is, then, no doubt of the essential similarity in nature between Hertzian waves and light.