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When Thomas Young (1773 to 1829) discovered interference as a result of his “double-slit” experiment early in the nineteenth century, he could obtain no better than an estimate of the wavelengths of light. He had no “benchmarks” to guide him and could only conclude, “the undulations constituting the extreme red light must be supposed to be, in air, about one 36-thousandths of an inch, and those of the extreme violet about one 60-thousandths.” While these values are quite accurate, they permit but limited quantitative calculations of optical phenomena. Within about twenty years, however, means were found to provide the benchmarks necessary for quantitative determinations.
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