Stories from Physics for 11-14 14-16
A legend at Emmanuel College suggests that Young developed his model of interference after observing the ripples generated by a pair of swans on the college pond. Perhaps inspired by the pond, Young later used a thought experiment involving a body of water when responding to a criticism of his theory:
“Suppose a number of equal waves of water to move upon the surface of a stagnant lake, with a certain constant velocity, and to enter a narrow channel leading out of the lake; suppose, then, another similar cause to have excited another equal series of waves, which arrive at the same channel with the same velocity, and at the same time with the first. Neither series of waves will destroy the other, but their effects will be combined; if they enter the channel in such a manner that the elevations of the one series coincide with those of the other, they must together produce a series of greater joint elevations; but if the elevations of one series are so situated as to correspond to the depressions of the other, they must exactly fill up those depressions, and the surface of the water must remain smooth; at least, I can discover no alternative, either from theory or from experiment. Now, I maintain that similar effects take place whenever two portions of light are thus mixed; and this I call the general law of interference of light.”
Before Young’s work on interference, Newton used the notion of superposition to explain a curious tidal effect near Hanoi. An English traveller, Francis Davenport, had published a letter describing the difficulty of accessing the city of Tongkin, now Hanoi, due to the unusual pattern of tides. Edmund Halley had commented on the phenomenon and Newton developed an explanation of the effect in the Principia. He argued that two components of tides, one from “the sea of China”, the other from “the Indian sea” arrived at Tongkin with different time delays, leading to a superposition effect. However, despite this apparent understanding of superposition, he did not transfer the model to other aspects of his work, for example, his explanation of the colours formed on soap bubbles.