The story of light is still going, with some very surprising twists, as later posts will show. Light itself can even be twisted, like the fibers in a rope, and also used to lift small objects and move them around. When Newton published his book Opticks in 1704, however, everything was still up for grabs.
Newton still thought of light as “fiery particles” – an unfortunate analogy, considering that his first manuscript caught fire when he carelessly left it near a burning candle in his room while he went to chapel. Another story says that his pet dog Diamond knocked the candle over. Whatever the truth, the result was that he had to rewrite the whole thing. It was the first major scientific book written in English, and it is still a good read. But if Newton had remembered something from his earlier Principia, written in Latin, he might have found a different analogy.
That something was his explanation of the curious tides in Hanoi harbour, a hundred miles up the Red River in Vietnam. The entrance was blocked by a sandbank, which ships could cross only at the highest tides. But these only occurred, not once or twice a day, but every second week!
Edmund Halley, the discoverer of Halley’s comet, tried to fit the complicated tidal pattern to an equation, which is another way that scientists try to make sense of data. When Newton finally got access to the data (Halley had kept it to himself for years), he spotted a simpler explanation – that there must be two tides, coming from different directions. One, he suggested, came from the ‘Sea of China’, with a delay of six hours. The other came from the ‘Indian Sea’, with a delay of twelve hours. If both were high when they reached Hanoi, they would add together, and the enormous tide would let ships pass over the sandbank safely. If one was high and the other was low, however, they would cancel, and there would be no tide at all.
Newton was so pleased by his idea that he wrote it up in the Principia, alongside his new theory of gravity. But he didn’t realize its significance when it came to the behaviour of light. If he had used the wave analogy when thinking about light passing through a small gap, he might have realized that the dark lines in the middle of the gap could form by a similar mechanism to the high and low tides in Hanoi harbour, with light in the form of waves being scattered from the two edges, and a dark line being formed in the middle where the waves cancelled each other out. As it was, it took nearly a hundred years for the wave picture of light to see the light of day. And it took a man who had read the Bible twice from cover to cover by the time he was four years old to figure it out. But that’s the next Mini Story.
Image: Nineteenth century engraving of the “dog” story. Wikimedia Commons.