396 ENERGY MAY BE PROPAGATED BY VIBRATIONS teristic absorption pattern, which is identified by matching it with patterns of known substances previously determined. Ultrashort Radio Waves Have Many Applications. Radio waves of less than ten meters in wave length are called ultrashort radio waves. Previous to World War II they were used for a two-way wireless telephone between France and England which employed radio waves 18 cm. in length. The shortest of these waves have been investigated only since 1928. They were first produced by oscillating electric sparks, but later vacuum tubes were used, because the waves produced by sparks are a mixture of wave lengths. The smallest waves originally produced with vacuum tubes were 30 cm. in length. When oscillations of sufficiently high frequencies to produce shorter waves were generated, the materials of the tube broke down because of the intensity of the resulting electric fields. Later it was found that these frequencies could be reinforced by proper tuning to produce much shorter waves. Ultrashort waves can be focused and transmitted in the form of narrow beams less than a square centimeter in cross section. This concentration of energy presents the advantage that no energy is wasted. An overwhelming proportion of the energy of longer radio waves is wasted in space without being received. The ultrashort radio telephone between France and England, mentioned above, requires an energy output of only a fraction of a watt. Short radio waves, about seven meters in length, with a frequency of 42,000,000 cycles per second, are now used to kill eggs, larvae, and pupae of insects concealed in grain. Weevils, worms, mites, and other infestations of cereals, cocoa beans, spices, tobacco, nuts, and similar products are thus killed without injury to the products themselves. It has been found that paresis, a disease due to syphilis, can be cured by inducing malaria fever. In 1930 Whitney (1868- ) found that a fever could be induced and controlled by short radio-wave transmitters. Since that time many cases of paresis have been cured by this method, although the old malaria method is still preferred by some physicians. Many remarkable experiments can be carried out with powerful short-wave transmitters. For example, electric-light globes can be made to glow several feet from the transmitter without the use of any conductor. Popcorn can be popped in a cake of ice; meat can be cooked in mid-air. So far the results obtained with such transmitting devices have been more novel than valuable, but many possibilities of practical applications are open. Radiations of various kinds bring about profound changes in the
LIGHT IS A FORM OF RADIANT ENERGY 397 growth of living things, particularly seeds or primary cells; for example, onions treated with two-meter waves bloom ten days before nonexposed plants. Ultrashort radio waves transmit energy more efficiently than do longer radio waves. STUDY QUESTIONS 1. Upon what evidence did Young base his conclusions concerning the nature of light? 2. How do light quanta differ from one another? 3. List the most important types of electromagnetic waves. 4. State two fundamental properties of wave motion. 5. Outline briefly the two light theories. What is their status today? 6. How should the speculations and theories of scientists be treated by the average individual? 7. Discuss the value of infrared photography. 8. Discuss the physiological effects of ultraviolet rays. 9. Compare the different electromagnetic rays as to their ability to penetrate matter and their physiological effect.