Investigations into the nature of light constitute a venerable tradition. By the eighteenth century, various explanations, none broadly agreed, had existed to explain the phenomenon. (This was, in Kuhnian terms, a preparadigm phase.) Newton’s theory that light came in discrete bits, corpuscles, was pitted against the contradictory idea of Huygens that it was continuous. Dr Thomas Young, the father of physioptics, whom we have already met in connection with acoustic machines and as an influence on Faraday, here made his most fertile contribution to the scientific competencies underpinning modern communications systems by proving Huygens right. The proof was to stand for the next 104 years.
In 1801, Young was studying the patterns thrown on a screen when light from a monochromatic source, sodium, passed through a narrow slit. Areas lit through one slit darkened when a second slit, illuminated by the same sodium light source, opened. This phenomenon—interference—Young explained by assuming that light consists of continuous waves and suggesting that interference was caused when the crests of the waves from one slit were cancelled by the troughs emanating from the second. He was able to measure the wavelengths of different coloured lights, getting close to modern results. The importance of the concept of interference cannot be overstated since it is still current and lies at the heart of holography; but for television Young’s experiment was suggestive because, eventually, it allowed researchers to think of systems which treated light waves as telephony treated sound waves.
Television depends in essence on the photovoltaic (or photoemissive) effect, that is the characteristic possessed by some substances of releasing electrons when struck by light. The observation of this phenomenon is credited to a 19-year-old, Edmond Bequerel, in 1839, but it seems that his father, the savant Antoine Cesar, may have helped him to prepare his account for L’Academie des Sciences. Their