Wave theory
and corpuscular theory
The Newtonian view of the universe may be described as a
mechanistic
interpretation. All components of the universe, small or large, obey the
laws of mechanics
, and all phenomena are in the last analysis based on matter in motion. A conceptual difficulty in
Newtonian mechanics
, however, is the way in which the
gravitational
force between two massive objects acts over a distance across empty space. Newton did not address this question, but many of his contemporaries hypothesized that the gravitational force was mediated through an invisible and frictionless medium which
Aristotle
had called the
ether
(or aether). The problem is that everyday experience of natural phenomena shows mechanical things to be moved by forces which make contact. Any
cause and effect
without a discernible contact, or
“action at a distance,” contradicts common sense and has been an unacceptable notion since antiquity. Whenever the nature of the transmission of certain actions and effects over a distance was not yet understood, the
ether
was resorted to as a conceptual solution of the transmitting medium. By necessity, any description of how the ether functioned remained vague, but its existence was required by common sense and thus not questioned.
In Newton’s day,
light
was one phenomenon, besides gravitation, whose effects were apparent at large distances from its source.
Newton
contributed greatly to the scientific knowledge of light. His experiments revealed that
white
light is a composite of many colours, which can be dispersed by a
prism
and reunited to again yield white light. The
propagation
of light along straight lines convinced him that it consists of tiny particles which emanate at high or
infinite
speed from the light source. The first observation from which a finite
speed of light
was deduced was made soon thereafter, in 1676, by the Danish astronomer
Ole Rømer
(
see below
Speed of light
).
Observations of two phenomena strongly suggested that light
propagates
as waves. One of these involved interference by thin films, which was discovered in England independently by
Robert Boyle
and
Robert Hooke
. The other had to do with the
diffraction
of light in the geometric shadow of an
opaque
screen. The latter was also discovered by Hooke, who published a
wave
theory of light in 1665 to explain it.
The Dutch scientist
Christiaan Huygens
greatly improved the wave theory and explained
reflection
and
refraction
in terms of what is now called
Huygens’ principle.
According to this principle (published in 1690), each point on a wave front in the
hypothetical
ether or in an optical medium is a source of a new spherical light wave and the wave front is the envelope of all the individual wavelets that originate from the old wave front.
In 1669 another Danish scientist,
Erasmus Bartholin
, discovered the
polarization
of light by
double refraction
in
Iceland spar
(
calcite
). This finding had a profound effect on the
conception
of the nature of light. At that time, the only
waves
known were those of
sound
, which are longitudinal. It was inconceivable to both Newton and Huygens that light could consist of
transverse waves
in which vibrations are perpendicular to the direction of propagation. Huygens gave a satisfactory account of double refraction by proposing that the asymmetry of the structure of Iceland spar causes the secondary wavelets to be ellipsoidal instead of spherical in his
wave front
construction. Since Huygens believed in longitudinal waves, he failed, however, to understand the phenomena associated with polarized light. Newton, on the other hand, used these phenomena as the bases for an additional argument for his corpuscular theory of light. Particles, he argued in 1717, have “sides” and can thus exhibit properties that depend on the directions perpendicular to the direction of motion.
It may be surprising that Huygens did not make use of the phenomenon of interference to support his wave theory; but for him waves were actually pulses instead of periodic waves with a certain
wavelength
. One should bear in mind that the word wave may have a very different
conceptual
meaning and convey different images at various times to different people.
It took nearly a century before a new wave theory was formulated by the physicists
Thomas Young
of England and
Augustin-Jean Fresnel
of France. Based on his experiments on
interference
, Young realized for the first time that light is a transverse wave. Fresnel then succeeded in explaining all optical phenomena known at the beginning of the 19th century with a new wave theory. No proponents of the corpuscular light theory remained. Nonetheless, it is always satisfying when a competing theory is discarded on grounds that one of its principal predictions is contradicted by experiment. The corpuscular theory explained the
refraction
of light passing from a medium of given
density
to a
denser
one in terms of the attraction of light particles into the latter. This means the light
velocity
should be larger in the denser medium. Huygens’ construction of wave fronts waving across the boundary between two optical media predicted the opposite?that is to say, a smaller light velocity in the denser medium. The measurement of the light velocity in air and water by
Armand-Hippolyte-Louis Fizeau
and independently by
Leon Foucault
during the mid-19th century decided the case in favour of the wave theory (
see below
Speed of light
).
The transverse wave nature of light implied that the ether must be a solid elastic medium. The larger velocity of light suggested, moreover, a great elastic stiffness of this medium. Yet, it was recognized that all celestial bodies move through the ether without encountering such difficulties as friction. These conceptual problems remained unsolved until the beginning of the 20th century.
Hellmut Fritzsche