Robert W Wood

Life

Born in Concord, Massachusetts, Wood attended The Roxbury Latin School with the initial intent of becoming a priest. But he decided to study optics instead when he witnessed a rare glowing aurora one night and believed the effect to be caused by "invisible rays". In his pursuit to find these "invisible rays", Wood studied and earned numerous degrees from Harvard, MIT and the University of Chicago. From 1894 to 1896 he worked with Heinrich Rubens at the Berlin University.

Returned to the USA he taught briefly at the University of Wisconsin and eventually became a full-time professor of "optical physics" at Johns Hopkins University from 1901 until his death. He worked closely with Alfred Lee Loomis at Tuxedo Park, New York.

He wrote many articles on spectroscopy, phosphorescence and diffraction. He is best known for his work in ultra-violet light.

Another claim to fame was his debunking of N-rays in 1904. French physicist Prosper-René Blondlot claimed to have discovered a new form of radiation similar to X-rays, which he named N-rays. Some physicists reported having successfully reproduced his experiments; others reported that they had failed. Visiting Blondlot's laboratory at the behest of the journal Nature, Wood surreptitiously removed an essential prism from Blondlot's apparatus during a demonstration. The effect did not vanish, showing that N-rays had always been self-deception on Blondlot's part.

Wood identified a very low ultraviolet albedo (reflectivity, that is most of the ultraviolet is absorbed) region in the Aristarchus Plateau region of the Moon which he suggested was due to high sulphur. The area continues to be called Wood's Spot. In 1909, Wood constructed the first practical liquid mirror astronomical telescope, by spinning mercury to form a paraboloidal shape, and investigated its benefits and limitations. Wood has been described as the "father of both infrared and ultraviolet photography". Though the discovery of electromagnetic radiation beyond the visible spectrum and the development of photographic emulsions capable of recording them pre-date Wood, he was the first to intentionally produce photographs with both infrared and ultraviolet radiation. In 1903 he developed a filter, Wood's glass, that was opaque to visible light but transparent to both ultraviolet and infrared, and is used in modern-day black lights. He used it for ultraviolet photography but also suggested its use for secret communication. He was also the first person to photograph ultraviolet fluorescence. He also developed an ultraviolet lamp, which is widely known as the Wood's lamp in medicine. The slightly surreal glowing appearance of foliage in infrared photographs is called the Wood effect.

Wood also authored non-technical works. In 1915, Wood co-authored a science fiction novel, The Man Who Rocked the Earth, with Arthur Train; a sequel, The Moon Maker, was published the next year. He also wrote and illustrated two books of children's verse, How to Tell the Birds from the Flowers (1907) and Animal Analogues (1908).

Wood participated in the investigation of several crimes including the Wall Street bombing.

Wood married in 1892 in San Francisco, Gertrude Hooper Ames, daughter of Pelham Warren and August Wood (Hooper) Ames, and granddaughter of William Northey Hooper, and Massachusetts Supreme Court judge, Hon. Seth Ames. He died in Amityville, New York.

Contributions to ultrasound

Although physical optics and spectroscopy were Wood's main areas of study, he made substantial contributions to the field of ultrasound as well. His main contributions were photographing sound waves and working with Alfred Lee Loomis in the development of power ultrasonics.

Photography of sound waves

His first contribution to the field of ultrasonics was from the photography of sound waves. Wood's primary research area was physical optics, but he found himself confronted with the problem of demonstrating to his students the wave nature of light without resorting to mathematical abstractions, for which he cared little. He therefore resolved to photograph the sound waves given off by an electric spark as an analogy to light waves. An electric spark was used because it produces not a wave train, but a single wavefront, making it much simpler to study and visualize. Although he did not pioneer that method, an honour belonging to August Toepler, he did more detailed studies of the shock waves and their reflections than Toepler.

High-powered ultrasound

After having made these contributions, Wood returned to physical optics, his interest in "supersonics" lying dormant for quite some time. With America's entry into the First World War, Wood, as with many other scientists, was asked to help with the war effort. After a handful of other ideas, Wood requested to devote his attentions to the work of Paul Langevin, who was investigating ultrasound as a method for detecting submarines. While in Langevin's lab, he observed high powered ultrasound cause the formation of air bubbles in water, and how fish would be killed or a hand suffer searing pain if put into the beam line. This piqued his interest in high powered ultrasound. Still later, in 1926, Wood recounted Langevin's experiments to Loomis, and the two of them collaborated on high intensity ultrasound experiments, which would turn out to be Wood's primary contribution to the field of ultrasonics.

The experimental setup was impressive, and it was driven by a 2 kW oscillator that had been designed for a furnace, allowing for the generation of very high output powers. The frequencies they used ran from 100 kHz to 700 kHz. The most impressive display of the output power of the setup is perhaps how strongly the output sound waves can fight even against gravity. When the quartz plate transducer was suspended in oil, it would make a mound of oil up to 7 centimetres (3 in) higher than the rest of the surface of the oil. While at low powers, the mound was low and lumpy, at high powers, it would rise up to the full 7 cm, "its summit erupting oil drops like a miniature volcano." These drops could reach heights of 30–40 centimetres (12–16 in). Similarly, when an 8-centimetre (3 in) diameter glass plate was placed on the surface of the oil, up to 150 grams (5 oz) of external weight could be placed on top of the glass plate, and supported by the strength of the ultrasound waves alone. This was achieved by the waves reflecting and re-reflecting between the transducer and the glass plate, allowing each generated wave to impart its momentum to the glass plate multiple times.

When attempting to take the temperature of the mound of erupting oil with a glass thermometer, Wood and Loomis discovered another set of effects quite serendipitously. They note that although the mercury in the thermometer only read 25 °C (77 °F), the glass was so hot that it was painful to touch, and they noticed that the pain became unbearable if they tried to squeeze the thermometer tightly. Even if very fine thread of glass only 0.2 millimetres (0.01 in) in diameter and 1 metre (3 ft 3 in) long was put in the oil at one end, holding a bulge in the glass at the other end still resulted in a groove being left in the skin and the skin being seared, with painful and bloody blisters forming that lasted several weeks, showing the ultrasound generated was quite powerful indeed. In a similar vein, when a glass rod was put lightly in contact with dried woodchips, the rod would burn the wood and cause it to smoke, or if pressed against a woodchip, it would quickly burn through the chip, leaving behind a charred hole. All the while the glass rod remained cool, with the heating confined to the tip. When a glass rod is pressed lightly against a glass plate, it etches the glass plate, while if it is pressed, it bores right through the plate. Microscopic examinations showed that the debris given off includes finely powdered glass and globules of molten glass.

Wood and Loomis also investigated heating liquids and solids internally with high intensity ultrasound. While the heating of liquids was relatively straightforward, they were able to heat an ice cube such that the centre melted before the outside. The ability to heat or damage objects internally is now the basis of modern therapeutic ultrasound. Turning their attention to the effects of high intensity ultrasound on living matter, Wood and Loomis observed ultrasound tearing fragile bodies to pieces. Cells were generally torn apart at sufficiently high exposure, although very small ones, like bacteria, managed to avoid destruction. Frogs, mice or small fish were killed after 1–2 minutes of exposure, replicating Langevin's earlier observation.

In addition to the above, Wood and Loomis also investigated the formation of emulsions and fogs, crystallization and nucleation, chemical reactions, interference patterns, and standing waves in solids and liquids under high intensity ultrasound. After completing this broad array of experiments, Wood returned to optics, to never really touch ultrasonics in any depth again. Loomis, however, would go on to advance the science further with other collaborators.

Miscellaneous

Wood was the last person who took photographs of the flying Otto Lilienthal only one week before his fatal crash.

Wood is sometimes credited as the inventor of tear gas.

Honours

Legacy