Heinrich Rubens

Scientific contributions Participants at the first Solvay conference 1911. Rubens is the third person from left standing in the back. Already as a student was Rubens fascinated by electromagnetic radiation as theoretically described by Maxwell and experimentally demonstrated by Hertz. Through the influence of Kundt he had become interested in understanding the optical properties of different materials. In his doctoral work he showed that reflection of light increases with increasing wavelengths into the infrared region. As a related result he could present an experimental verification of Maxwell's theory for electromagnetic waves in different media. This effort also turned into a demonstration of the validity of these equations for infrared radiation. Rubens succeeded in this for wavelengths up to 10 μm.

Through the improvements of instruments and invention of new techniques he could measure infrared radiation for larger and larger wavelengths. One of his goals was to better understand the reflexion of radiation by metals and crystals. It was known that this became stronger for wavelengths which were absorbed. This lead him to a new, powerful method by selective reflexion from several crystals to isolate a narrow range of infrared wavelengths from a broader spectrum of radiation. Using such Reststrahlen he could in 1898 detect wavelengths of sizes around 60 μm. After the guests had left Planck managed to derive a new formula for the radiation energy which was consistent with the new results. He wrote it down on a postcard which Rubens received the following day. A few days later Rubens reported back that it seemed to fit all his measurements. On 14 December Planck could present to Deutsche Physikalische Gesellschaft a derivation of his new radiation law based on the idea of quantization of energy. This was the "birthday" of the new quantum physics.

In the following years Rubens could improve his measurements of infrared radiation and reached wavelengths of several hundred micrometres. This enabled him also to make more and more accurate tests of Plancks new radiation theory and related studies of matter which soon could be described by quantum mechanics. He was loved by his students and colleagues for his care and accuracy in all experimental work. In this connection he constructed in 1905 a Rubens tube to illustrate standing sound waves using a flammable gas in a tube. This was probably inspired by his teacher Kundt's tube where fine sand or powder was used for the same purpose.