Welcome to part two, of exploring the Aether. In the first part we looked at the first scientific
use of the term aether and the initial ideas on what light was. In this episode we will
continue this journey by exploring the additions Robert Hooke, Newton and Huygens made to our
understanding of the behaviour of light.
Robert Hooke had begun his scientific career as the assistant of Robert Boyle. Hooke had
already made 2 discoveries which were of importance. The first was the observation of the iridescent
colours which are seen when light falls on a thin layer of air between two glass plates
or lenses. The second was that light in air does not travel in straight lines.
Hooke did not like Descartes mechanical concept of what light was. He felt that the idea that
light was a tendency to motion rather than actual motion was wrong. Moreover since some
bodies shine for a considerable time without wasting away, it follows that whatever is
in motion is not permanently lost to the body, and therefore that the motion must be of a
to and fro vibratory character. The amplitude of these vibrations must be exceedingly small.
He then set about trying to understand by what motion light travelled. He came up with
a series of statements regarding the movement of light from a source through a medium to
the eye
If the medium allows for the vibration motion of light through it it is called transparent.
The part of this medium must be homogenous or of the same type
That the light pulse can travel great distances through this medium and very short time frames.
He did however feel that this need not be instant
The motion is propagated in every direction by direct or straight lines extending from
the source like rays from the centre of a sphere
In a homogenous medium this motion is propagated in all directions with equal velocity creating
expanding spheres
So although Hook open criticised Descartes there are many similarities, both are mechanical
and require a medium as the vehicle for light. Where Descartes had the movement as a pressure
wave, Hooke instead had a rapid vibration motion of small amplitude.
Next he attempted to explain colour. He thought that white light was the simplest type of
disturbance and that colour was generated by the distortions to which this disturbance
is subjected in the process of refraction. Blue was an impression on the retina of an
oblique and confused pulse of light, whose weakest part precedes and strongest follows.
Red was effectively the opposite.
His theory on colour was completely overthrown within a few years of its publication by Newton
in one of his earliest discoveries. Newton had obtained a prism and was fascinated by
the rainbow colours it produced. He had noticed that the length of the spectrum was five times
its breadth. The then accepted law of refraction should have made it appear circular.
Newton set about trying to understand this by conducting various experiments. Through
this he discovered the fact that white light is simply a mixture of rays of every variety
of colour and that the elongation of the spectrum was due to the differences in the refractive
power of the glass for these different rays.
Newton's concept that coloured light was an intrinsic property that could not be changed
was in direct contrast to Hooke’s theory. This would start a feud between the two, Hooke
at the end of his career and Newton at the start.
Newton tended to reject the idea of constructing the universe from its foundations like Descartes
had and aspired to nothing more than a formulation of the laws which directly govern the actual
phenomena.
He was however not able to connect the phenomena of light into a coherent and reasoned whole
without having falling back on hypotheses.
His view was that all space is permeated by an elastic medium or aether, which is capable
of propagating vibrations in the same way as the air propagates the vibrations of sound,
but with far greater velocities.
This Aether pervades the pores of all material bodies, and is the cause of their cohesion.
Its density varies from one body to another, being greatest in the free interplanetary
spaces. It is not necessarily a single uniform substance: but just as air contains aqueous
vapour, so the aether may contain various ‘ethereal spirits’ adapted to produce
the phenomena of electricity, magnetism and gravitation.
The vibrations of the aether he felt could not account for light. Light was something
of a different kind, propagated from lucid bodies. Possibly a multitude of unimaginable
small and swift particles of various sizes, springing from shining bodies at great distances
one after another. Their motion is that of acceleration first then they meet resistance
from the aether and the acceleration stops and they continue at a constant velocity.
He felt that light and aether were capable of mutual interactions. The aether was an
intermediary between light and the physical matter.
The lights path would be affected by changes in density in the aether.
Although the regular vibrations of Newton’s aether were not supposed to constitute light,
the turbulence in the medium seems to represent the concept of heat fairly well. He supposed
that when light is absorbed by a material body, vibrations are set up in the aether,
and are recognisable as the heat which is always generated in these cases.
Colour he viewed to be an inherent characteristic of light, and inferred that it must be associated
with some definitie quality of the particles or aether vibrations. The particles corresponding
to different colours would excite vibrations of different types in the aether. He viewed
these vibrations similar to how sound can be created. The colours were the individual
notes but could be played together to produce composite sounds or mixed colours.
This was the first mention of the great principle that homogenous light is essentially periodic
in its nature and that the difference of period corresponds to differences in colour.
It was Huygens who would solidify the wave theory of light. The question arose as to
whether the motion was of the medium, as Hooke had supposed, or whether it was particles
being emitted, like Newton's concept. In the end Huygens decided to go with Hooke’s concept
of a medium. Light consisted of disturbances propagated with great velocities, in a highly
elastic medium composed of a subtle matter.
His concept of wave propagation would allow him to succeed where Hooke had failed in attempting
to explain both refraction and reflection. Refraction was essentially caused by light
travelling at a different speed in different materials.
Huygens offered a physical explanation of the variations in the velocity of light in
one medium to another, by supposing that transparent bodies consist of hard particles which interact
with the aethereal matter, modifying its elasticity. The opacity of metals he explained by an extension
of the same idea, supposing that some of the particles of metal are hard, accounting for
the reflection and the rest soft; the later destroy the luminous motions by dampening
it.
In the next part of our journey we must look back at the origin of the electric and magnetic
science.