The Opticks

Newton as a young man skirmished with Hooke and others on the theory of
colors and other aspects of optics. These polemics finally drove him into a silenceof almost thirty years on the subject of optics, with the excuse that he did not
want to be “engaged in Disputes about these Matters.” What persuaded him to
break the silence and publish more of his earlier work on optics, as well as some
remarkable speculations, may have been the death of his chief adversary, Hooke,
in 1703. In any case, Newton published his other masterpiece, the Opticks, in
1704.
The Opticks and the Principia are contrasting companion pieces. The two
books have different personalities, and may indeed reflect Newton’s changing
persona. The Principia was written in the academic seclusion of Cambridge, and
the Opticks in the social and political environment Newton entered after moving
to London. The Opticks is a more accessible book than the Principia. It is written
in English, rather than in Latin, and does not burden the reader with difficult
mathematical arguments. Not surprisingly, Newton’s successors frequently mentioned
the Opticks, but rarely the Principia.
In the Opticks, Newton presents both the experimental foundations, and an
attempt to lay the theoretical foundations, of the science of optics. He describes
experiments that demonstrate the main physical properties of light rays: their
reflection, “degree of refrangibility” (the extent to which they are refracted), “inflexion”
(diffraction), and interference.
The term “interference” was not in Newton’s vocabulary, but he describes
interference effects in what are now called “Newton’s rings.” In the demonstration
experiment, two slightly convex prisms are pressed together, with a thin
layer of air between them; a striking pattern of colored concentric rings appears,
surrounding points where the prisms touch.
Diffraction effects are demonstrated by admitting into a room a narrow beam
of sunlight through a pinhole and observing that shadows cast by this light source
on a screen have “Parallel Fringes or Bands of colour’d Light” at their edges.
To explain this catalogue of optical effects, Newton presents in the Opticks a
theory based on the concept that light rays are the trajectories of small particles.
As he puts it in one of the “queries” that conclude the Opticks: “Are not the
Rays of Light very small Bodies emitted from shining Substances? For such Bodies
will pass through Mediums in right Lines without bending into the Shadow,
which is the Nature of the Rays of Light.”
In another query, Newton speculates that particles of light are affected by optical
forces of some kind: “Do not Bodies act upon Light at a distance, and by
their action bend its Rays; and is not this action strongest at the least distance?”
With particles and forces as the basic ingredients, Newton constructs in the
Opticks an optical mechanics, which he had already sketched at the end of book
1 of the Principia. He explains reflection and refraction by assuming that optical
forces are different in different media, and diffraction by assuming that light rays
passing near an object are more strongly affected by the forces than those more
remote.
To explain the rings, Newton introduces his theory of “fits,” based on the idea
that light rays alternate between “Fits of easy Reflexion, and . . . Fits of easy
Transmission.” In this way, he gives the rays periodicity, that is, wavelike character.
However, he does not abandon the particle point of view, and thus arrives
at a complicated duality.
We now understand Newton’s rings as an interference phenomenon, arising
when two trains of waves meet each other. This theory was proposed by Thomas
Young, one of the first to see the advantages of a simple wave theory of light,almost a century after the Opticks was published. By the 1830s, Young in England
and Augustin Fresnel in France had demonstrated that all of the physical properties
of light known at the time could be explained easily by a wave theory.
Newton’s particle theory of light did not survive this blow. For seventy-five
years the particles were forgotten, until 1905, when, to everyone’s astonishment,
Albert Einstein brought them back. (But we are getting about two centuries beyond
Newton’s story. I will postpone until later [chapter 19] an extended excursion
into the strange world of light waves and particles.)
The queries that close the Opticks show us where Newton finally stood on
two great physical concepts. In queries 17 through 24, he leaves us with a picture
of the universal medium called the “ether,” which transmits optical and gravitational
forces, carries light rays, and transports heat. Query 18 asks, “Is not this
medium exceedingly more rare and subtile than the Air, and exceedingly more
elastick and active? And doth not it readily pervade all Bodies? And is it not (by
its elastick force) expanded through all the Heavens?” The ether concept in one
form or another appealed to theoreticians through the eighteenth and nineteenth
centuries. It met its demise in 1905, that fateful year when Einstein not only
resurrected particles of light but also showed that the ether concept was simply
unnecessary.
In query 31, Newton closes the Opticks with speculations on atomism, which
he sees (and so do we) as one of the grandest of the unifying concepts in physics.
He places atoms in the realm of another grand concept, that of forces: “Have not
the small particles of Bodies certain Powers, Virtues or Forces, by which they
act at a distance, not only upon the Rays of Light for reflecting, refracting, and
inflecting them [as particles], but also upon one another for producing a great
Part of the Phaenomena of Nature?”
He extracts, from his intimate knowledge of chemistry, evidence for attraction
and repulsion forces among particles of all kinds of chemical substances, metals,
salts, acids, solvents, oils, and vapors. He argues that the particles are kinetic
and indestructible: “All these things being considered, it seems probable to me,
that God in the Beginning form’d Matter in solid, massy, hard, impenetrable,
moveable Particles, of such Sizes and Figures, and in such Proportion to Space,
as most conduced to the End for which he form’d them; even so very hard, as
never to wear or break in pieces; no ordinary Power being able to divide what
God himself made one in the First Creation.”