Robert W. Wood was a famous and eccentric physicist who discovered resonance radiation and greatly expa-nded our understanding of ultraviolet light. In last month's history column, Jarus Quinn recounted his experience cleaning out Wood's laboratory—and finding an explosive surprise when he tried to wash out a jar full of sodium with water. In fact, Wood was well known for his work with sodium. As early as 1901, he had published a paper on the anomalous dispersion of sodium vapor, and the next year he reported on its fluorescence and absorption spectra. About 20 of Wood's 220 published papers related to the spectral behavior or the magnetooptics of sodium vapor.
So, in the fall of 1904, when Wood visited with Lord Rayleigh in Essex, England, Rayleigh was of course interested in discussing Wood's experiments with sodium. Wood had journeyed to England to present research at a meeting of the British Association at Cambridge. He stopped along the way at Lord Rayleigh's estate, which was called Terling. Rayleigh had invited Wood along with two visiting German researchers—the distinguished elderly spectroscopist H. Kayser from Bonn and the somewhat pompous Otto Lummer from the Physikalische Technische Reichsanstalt in Berlin—to spend the weekend there before the meeting.
Wood later recounted his visit to a friend and Long Island neighbor, William Seabrook, who in 1941 published a scientific biography of Wood titled Doctor Wood. (Some of the following text is excerpted from that book.)
Wood felt comfortable in Rayleigh's lab, which was filled with homemade mercury air pumps and glass tubes mounted on weather-beaten boards. The room was similar to Wood's own lab space: It was filled to the brim with equipment that had outlived its usefulness. "There was a profuse use of laths, string and sealing wax, which delighted my soul," Wood said, "for I realized that it was with this primitive apparatus that England's foremost physicist had made his most important discoveries."
A caricature of John Strutt, also known as Lord Rayleigh, in his lab. F.T. Dalton for Vanity Fair, 1899
Rayleigh asked Wood if he would be willing to repeat any of his interesting demonstrations with sodium vapor. Wood replied, "Possibly, if I can use your glass-blowing lamp and you have some metallic sodium." While Wood was busy blowing glass bulbs, Lord Rayleigh was hunting for his sodium. There were an abundance of glass cases, and the shelves were covered with cobwebbed bottles of chemicals of very old vintage. Wood joined him in his search.
After searching high and low, Rayleigh was ready to give up. "I have a jarful somewhere, but it seems to have disappeared," he said. But no sooner had he made that statement when Wood spied a glass preserve jar half-full of yellowish liquid with some dark lumps in it. He showed it Rayleigh and said, "I have a feeling that if this was my laboratory I should be inclined to keep my bottle of sodium about here." Lord Rayleigh chuckled. "I believe you have it," he said. "There seems to be nothing about sodium that you can't discover, even its hiding place!"
Wood loaded the bulbs with the metal, pumped out the air, and then sealed them with flame. He formed the colored deposits and showed the remarkable color changes that were produced by local cooling of sodium. He then got his long gas burner,and within half an hour he had set up the demonstration with the long sodium vapor tube showing anomalous dispersion.
As the men walked to lunch, Lord Rayleigh turned to Professor Kayser and said, "Well, we have had a most interesting morning." Kayser agreed. "Yes, indeed," he said, "very, very interesting." Lummer thrust his hand inside the breast of his long frock coat, threw back his head, and sniffed, "Was mich anbetrifft, ich habe nichts Neues gesehen" ("As for me, I have seen nothing new").
Otto Lummer Helps Usher in the Quantum Age
R.W. Wood found Otto Lummer a bit difficult to be around, and he was not alone in that. Nevertheless, Lummer made some important contributions to physics—including helping to confirm some of Rayleigh's own theoretical predictions. He was about eight years older than Wood, and he had studied under Hermann von Helmholtz. In 1884, he became an assistant to Helmholtz, and three years later they both moved to the newly formed Physikalische Technische Reichsanstalt (PTR) in Berlin. By 1894, Lummer was a professor at PTR.
With his colleague Eugen Brodhun, Lummer designed a photometer that was a large improvement over an earlier type by Bunson. With Leon Arons, he helped to design a mercury vapor lamp. In collaboration with Wilhelm Wien, he made a practical blackbody radiator by making a small aperture in a heated hollow sphere.
Wien had already proposed a formula for displacing the temperature of the energy density maximum of a blackbody (which Lummer confirmed), and Wien also suggested a spectral distribution formula for blackbody radiation. Then, in the fall of 1900, Lummer and Ernst Pringsheim made careful measurements of blackbody radiation at various temperatures. Their data confirmed Wien's formula for short wavelengths; however, at longer wavelengths the data better fitted a theoretical prediction by Lord Rayleigh.
Rubens described these conflicting results to the young thermodynamicist Max Planck, who spent the rest of the day pondering whether he could modify the Wien and Rayleigh formulas. By evening he had found an interpolation formula that would reduce to Wien's at short wavelengths and to Rayleigh's at long ones.
It took Planck two months to derive the formula. He found it necessary to assume that the energy stored in the blackbody oscillators is not indefinitely divisible but is actually built up out of a very large number of "bits," or "quanta" of energy. He concluded that the energy of each quantum is a multiple of the quantum energy hf, where f is the frequency of the oscillator and h is now universally known as "Planck's constant" or "Planck's quantum of action." Planck presented his derivation at a meeting of the German Physical Society at Berlin on December 14, 1900, and physics entered the quantum era.
John N. Howard is the founding editor of Applied Optics and retired chief scientist of the Air Force Geophysics Laboratory.