388 ENERGY MAY BE PROPAC.AIED BY VIBRATIONS Roger Bacon (1210-1292) studied the general phenomena of refraction and described the laws of reflection. In 1678 Christian Iluygens, a Dutchman, developed the wave theory of light which was first suggested by Leonardo da Vinci. A few years later Sir Isaac Newton advanced a different theory. Newton's work on light and optics alone was sufficient to place his name on the roster of the world's greatest scientists. The Pythagoreans believed that light consisted of particles projected into the eye. Newton likewise believed that light must consist of a stream of corpuscles, for he could not account for sharp shadows in terms of wave motion. Water waves bend around obstacles and do not produce shadows. The same thing is true in the case of sound waves, except that the shorter wave lengths of sound will produce shadows. A hundred years after Newton it was shown that the extreme smallness of the wave lengths of light compared with the dimensions of the objects placed in their path explains the sharpness of the shadows produced. Strangely enough, the colored rings produced when light is passed through a polished glass plate in close contact with a lens of small curvature, called "Newton's rings," are now accepted as one of the best proofs of the wave theory of light. It remained for Thomas Young (1773-1829) to revive Huygens' wave theory of light, which had been rejected for so long, largely on account of Newton's great prestige. In 1801 he concluded, as the result of studies in interference, that light was propagated in the form of waves rather than particles. Water waves show interference; thus, when two waves meet so that the crest of one coincides with the trough of another, they neutralize each other. When Young passed a narrow beam of light through two narrow slits in a screen, the rays from the two holes overlapped on a second screen, producing a series of brilliantly colored bands. He reasoned that the light waves from one slit have to travel farther than those from the other slit, so that, at points where the crests of one wave coincide with the troughs of another, bands of darkness are produced. At other points two crests would coincide to produce light bands of double the intensity produced by the light from either hole. When the light source was of a single wave length, the bands were thus alternately dark and bright; but when white light was used, the bands were colored, because white light consists of a mixture of wave lengths. If any one wave length is extracted from white light, the mixture of wave lengths left produces a complementary ^ color. Young measured the wave lengths of different colored lights in this way; the dimensions of his apparatus and the breadth of the light bands > Complementary colors are those colors which produce white light when mixed with each other in the proper proportions.
LIGHT IS A FORM OF RADIANT ENERGY 389 produced provided the necessary data for his calculations. These light waves were found to be very short, about 1/50,000 inch. Recently Newton's corpuscular theory has found favor again, although the modern concept of energy packets, called quanta, diiifers considerably from Newton's corpuscles. Rapidly moving electrons and protons have been shown to exhibit some of the properties of waves and to produce interference patterns, while photons, or light corpuscles, are accepted as realities today. At present, therefore, light is considered to be both wavelike and corpuscular in nature, but the wave metaphor and the particle metaphor are used in quite different contexts. Roughly speaking, when we want to know or account for where a beam of light goes, we pretend it is a wave motion ; whereas when we want to account for what it does when it gets there we pretend it is somehow corpuscular. Of course this dualism is unsatisfactory — it constitutes a problem to be solved. Radiant-energy units are now generally referred to as photons. The amount of energy of photons varies with the frequency of the radiations. Photons of red light contain less energy than those of violet. As the frequencies become greater, the energy of the photons becomes greater. Thus, for X rays the photons are so large that they show atomic characteristics. A. H. Compton of Chicago has shown that X rays falling on electrons scattered as if they consisted of material particles. How Waves Transfer Energy. Huygens proposed the existence of a hypothetical medium for the waves to travel in, which he called "ether." Ether is a hypothetical medium that is supposed to occupy otherwise unoccupied space. ^ The concept of ether is useful, but it must be kept in mind that there is no experimental evidence for its existence. It is not known how light really travels, but an exact account of its path can be given if it is regarded as a train of waves. Light waves are supposed to be propagated through the ether in a manner somewhat analogous to the propagation of water waves. If a stone is thrown into a pool of water, the surface will be depressed momentarily, and a series of circular waves will spread from that point. Finally the waves reach the shore and move small objects. The energy used to do work on these objects is transmitted by the waves from the stone. Now, if stones are thrown into a pool at the same rate in rapid succession, the waves formed will move at the same rate, but the distance from crest to crest and from trough to trough will depend upon ' Eddington defines ether as follows: "Ether is the subject of the verb, to undulate."