UNDULATORY THEORY OF LIGHT, or WAVE THEORY OF LIGHT, the theory which accounts for the phenomena of light and radiant heat, by interpreting them as the re sults of vibratory movements or undulations, in an all-pervading medium called the aether" (q.v.). There are many optical phenomena which admit of a ready mathematical explana tion in this manner, without making any very definite assumptions concerning the nature of the ether, or the kinds of motion that occur in it. In order to explain the phenomena of polarization, however, it is necessary to assume that when a light-ray passes through the ether, the ether is disturbed in such a manner that its particles oscillate back and forth in paths which are perpendicular to the direction in which the light-ray is traveling. In this respect the motion of the ether is very different from that which prevails in air when a wave of sound traverses that medium; the motions of the air particles in the case of a sound-wave being to and fro along lines that are parallel to the direction in which the wave is progress ing. The motion of the ether when a light ray traverses it is supposed, in fact, to be very similar to the motion by which a vibra tory disturbance is propagated from one part of an incompressible elastic solid to another part; and for this reason the undulatory theory of lien is often called the "elastic-solid" theory.
Christian Huyghens was the first to give the undulatory theory of light a definite form, and the origin of that theory may fairly be said to date from a paper written by him for the French Academy of Sciences, in 1678. Sir Isaac Newton was well aware of the possibility of accounting for many of the phenomena of light, by assuming that light consists of a kind of wave-motion in an all-pervading ether; hut he objected to the undulatory theory, because he could not understand how a body could cast a sharp shadow if that theory were correct. He was of the opinion that the waves would necessarily sweep around obstacles, and close in behind them in such a way as to render sharp shadows impossible. Newton, therefore,
adopted the alternative °corpuscular theory,'" which teaches that light consists of storms of tiny material particles, or "corpuscles," which tend, by reason of their inertia, to travel in straight lines. He developed this theory with characteristic power and ingenuity, so that physicists were long divided between his teach ings and those of Huyghens. In 1801, Dr. Thomas Young, an English physicist, solved the difficulty which led Newton to reject the undulatory theory; for he showed that when the dimensions of an opaque object are very great in comparison with the wave-length of light, the light-waves which fall upon the ob ject are prevented from sweeping around and closing up behind it, by means of the phe nomenon known as °interference; those waves which tend to pass around behind such an obstruction becoming compounded together in such a manner as to neutralize one another. Young's discovery stimulated investigation so that within the next 20 years great advances were made in the development of the undula tory theory. Young and Fresnel, for exam ple, exolained the phenomena of polarization, and Fresnel developed his beautiful theory of the optical phenomena manifested by crystals. The objections raised by Newton had been en tirely disposed of, and the undulatory theory had been placed upon an apparently firm foundation. Moreover, there are two things about the corpuscular theory which appeared to be absolutely fatal to it. The first is, that it does not lend itself readily to the explanation of interference. It is easy to understand that two waves in the ether may come together so as to neutralize each other; for an analogous phenomenon may be seen any day at the sea shore, in ocean waves. It is not at all easy to understand, however, how two streams of light producing corpuscles can produce darkness, when either stream alone will produce light.