390 ENERGY MAY BE PROPAGATED BY VIBRATIONS the number of stones thrown per minute. If the wave front moves out twenty feet in one second and ten stones are thrown per second, there will be ten waves produced, each two feet in length. The length of the wave times the frequency (i.e., the number of waves per second) will equal the velocity of the wave, or the distance covered by the wave front in a second. The Wavelike Nature of Light Is Confirmed by the Nicety with Which It Fits into the Electromagnetic Spectrum. When it is not in our power to discern what is ultimately true, we should subscribe to what is most probable. — Descartes. Clerk Maxwell (1831-1879) worked out equations which summarized the knowledge of his time concerning electricity and magnetism, that included the concept of ether and seemed to apply to light. Michael Faraday had already suggested that light and electromagnetic forces might be phenomena of ether. On the basis of these equations Maxwell advanced the hypothesis that light is an electromagnetic wave. In ISSS Heinrich Hertz showed that electromagnetic waves sent out from an electric spark have all the properties of light rays and thus verified Maxwell's hypothesis. All radiant waves are now considered to be produced by the motion of electromagnetic lines of force, and hence the radiation spectrum is properly called the "electromagnetic spectrum." Electromagnetic lines of force will be discussed in Unit VII. Although the nature of light is not well understood, it fits into the electromagnetic spectrum so beautifully that there is little doubt concerning the wavelike nature of light. The wave length of any type of electromagnetic radiation can be measured, and the electromagnetic spectrum was constructed on the basis of such measurements. The accompanying insert shows the electromagnetic spectrum as it is known today. Our knowledge of radiant energy is still very limited, and many portions of the electromagnetic spectrum remain unexplored. Ultraviolet Radiations Are of Great Importance. The biological effects and chemical effects of light become conspicuous in the shorter wave lengths just beyond the limit of visibility to the eye, which fails to respond to any wave lengths shorter than 3800 angstroms. The atmosphere on a clear, dry day transmits light of wave lengths as low as 2900 angstroms. In higher altitudes the ultraviolet light is transmitted more than in lower altitudes, so that a sunburn is rapidly obtained even on cold days at high altitudes. At
LIGHT IS A FORM OF RADIANT ENERGY 391 lower altitudes, when there are considerable smoke and humidity, the ultraviolet rays below 3200 angstroms may be absorbed. Inasmuch as the radiations between 2900 and 3100 angstroms are most effective biologically, it can readily be understood why ultraviolet lamps are of distinct value to people who live in cities where smoke vies with clouds to screen out most of the biologically useful ultraviolet light during the winter months. There seems to be little doubt that the widespread popularity of out-of-door life and exposure to sunlight has a sound physical and physiological basis. Ultraviolet lamps have been used as a substitute for sunlight in producing a healthy tan and in producing vitamin D in the human body, as well as in stimulating various body functions. Ultraviolet radiations are now widely used to irradiate milk, cereal, and other products, for such irradiation produces vitamin D in these foods. Ordinary window glass does not permit the passage of these healthgiving ultraviolet rays, but quartz transmits them freely. Certain types of glass are now manufactured which do permit the passage of ultraviolet light. Ultraviolet radiations are of special interest to the chemist because they affect the outer electrons in atoms. Recent studies show that plants thrive best under red light and that ultraviolet light has a definite retarding effect upon plant growth. The Shorter Wave-length Radiations Likewise Activate Chemical Changes. X rays, whose production will be discussed in the next Unit, are more penetrating than ultraviolet rays and involve the displacement of inner electrons within atoms. Short-wave radiations, especially X rays, modify the chromosomes in living cells. Thus even the sex of flies has been changed by X rays. A lintless cotton has been grown from X-rayed seed. Gamma rays, produced by radioactive disintegration, are still more The penetrating and displace electrons in the central parts of atoms. applications of X rays and gamma rays will be studied in the next Unit. Cosmic rays, studied by Millikan and many others, are the shortest of all waves and are exceedingly penetrating. A high percentage of cosmic rays consists of high-speed particles, electrons or protons rather than photons, which possess enormous energies. While it is true that a high proportion of cosmic rays are particles rather than waves, the term "cosmic ray" is still used, just as cathode rays, beta rays, and alpha rays are used to refer to streams of particles.