2 - Newton's particle theory

Summary

Even the Rays of Light seem to be hard bodies; for otherwise they would not retain different Properties in their different Sides.

Isaac Newton, Query 31, Opticks

Theory building

By now we have a substantial number of observations; we know quite a bit about how light behaves. Let us try our hand at building a theory of light: what light is, and why it behaves as it does.

Our first observation – that light goes in straight lines from a luminous source – suggests that we need the notion of something that moves through space. The simplest thing is a particle, a “little baseball.” Perhaps the luminous source emits little light particles, a stream of them. Figure 2.1 illustrates this notion.

Isaac Newton developed the same idea. He worked on light – experimentally and theoretically – most of his long life. At the start of chapter 1, we noted that Newton's first paper (published in 1672) was on optics, and his interest in the subject goes back at least to his student days at the University of Cambridge, England, perhaps to the year 1663.

Newton could not abide criticism, and so he went to great lengths to avoid even the possibility of it. Because Robert Hooke and he readily came to acrimonious disagreement, Newton withheld the publication of his book on optics until after Hooke's death. Finally, in 1704, Newton published his treatise Opticks.