Young's interference experiment

Young's interference experiment is any one of a number of optical experiments described or performed at the beginning of the nineteenth century by Thomas Young to demonstrate the wave [theory of light]. These experiments played a major role in the acceptance of the wave theory of light. One such experiment was the original version of the modern double-slit experiment.

Background

In the second half of the 17th century two hypothesis for the nature of light were discussed. Robert Hooke, Christiaan Huygens advocated a wave theory, while Isaac Newton, who did many experimental investigations of light, developed his corpuscular theory of light according to which light is emitted from a luminous body in the form of tiny particles. By the end of the century Newton's reputation as the preeminent physicist gave the emission theory a wide lead. Even the famous Leonhard Euler who supported the wave theory was unable to encourage its discussion.

Young's work on wave theory

While studying medicine at Göttingen in the 1790s, Young wrote a thesis on the human voice. To practice medicine in England Young was required to spend three years at an English university. He used that time at Cambridge to work on the physical and mathematical properties of sound. The work dealt with superposition of sound waves, the way two independent waves combine, and interference, the consequence of combination. Superposition was understood before Young because it was known that two sound waves could pass through each other. Interference was less well understood because the frequency of the two waves affects result of the combination.
In 1800, he presented a paper to the Royal Society where he argued that light was also a wave motion. His idea was greeted with a certain amount of skepticism because it contradicted Newton's corpuscular theory.
Nonetheless, he continued to develop his ideas. He believed that a wave model could much better explain many aspects of light propagation than the corpuscular model:
Young presented the Royal Society Bakerian prize lecture in 1800, 1801, and 1803. The 1801 lecture, "On the Theory of Light and Colours" described various interference phenomena and was published in 1802. In these lectures, Young demonstrated interference of mechanical water waves using a ripple tank, consisting of a candle illuminating the bottom of a glass tank with a 45-degree mirror overhead. His published lectures included a sketch of the interference pattern from two sources of equal frequency mechanical waves.
The first published account of what Young called his 'general law' of interference appeared in January 1802, in his book A Syllabus of a Course of Lectures on Natural and Experimental Philosophy:

But the general law, by which all these appearances are governed, may be very easily deduced from the interference of two coincident undulations, which either cooperate, or destroy each other, in the same manner as two musical notes produce an alternate intension and remission, in the beating of an imperfect unison.

The first of Young's Bakerian lectures was published in the spring of 1802.

Relation to the double-slit experiment

In 1803, Young described an experiment with two slits. In modern times this experiment is considered an important classic proof of the wave theory of light. However it is not clear which experiments Young performed and which ones he described as thought experiments.
The first version of Young's experiment reflects sunlight through a small hole, and splits the thin beam in half using a paper card.
He also mentions the possibility of passing light through two slits in his description of the experiment:

Criticism

In the years 1803–1804, a series of unsigned attacks on Young's theories appeared in the Edinburgh Review. The anonymous author succeeded in undermining Young's credibility among the reading public sufficiently that a publisher who had committed to publishing Young's Royal Institution lectures backed out of the deal. This incident prompted Young to focus more on his medical practice and less on physics.

Acceptance of the wave theory of light

In 1817, the corpuscular theorists at the French Academy of Sciences which included Siméon Denis Poisson were so confident that they set the subject for the next year's prize as diffraction, being certain that a particle theorist would win it. Augustin-Jean Fresnel submitted a thesis based on wave theory and whose substance consisted of a synthesis of the Huygens' principle and Young's principle of interference.
Being a supporter of the particle theory of light, Poisson studied Fresnel's theory in detail for a way to prove it wrong. Poisson thought that he had found a flaw when he argued that a consequence of Fresnel's theory was that there would exist an on-axis bright spot in the shadow of a circular obstacle blocking a point source of light, where there should be complete darkness according to the particle-theory of light. Fresnel's theory could not be true, Poisson declared: surely this result was absurd. However, the head of the committee, Dominique-François-Jean Arago thought it was necessary to perform an experiment. He molded a 2-mm metallic disk to a glass plate with wax and observed the predicted spot. This convinced most scientists of the wave-nature of light. In the end, Fresnel won the competition. Arago later noted that the phenomenon had already been observed by Joseph-Nicolas Delisle