“Are
not the Rays of Light very small Bodies emitted from shining substances ?”
Newton
(1704) Opticks
There were two
contradictory theories regarding the nature of light in the 17th
century. The corpuscular theory regarded light as a stream of tiny particles
travelling at high speed in straight lines. Newton supported this view and it
could be used to explain observed effects such as rectilinear propagation (light
travelling in straight lines), reflection and refraction.
It is worth noting
that Newton carried out a lot of experimental work, especially on refraction,
for which his theory was advanced as an explanation. To explain refraction this
theory required the corpuscles to be attracted to an optically denser medium and
thus travel faster in the denser medium. Newton had arrived at this conclusion
by considering the forces acting on the corpuscles.
Huygens wrote his book “Treatise on Light” in 1678 and published it in 1690. He proposed that light was a wave motion. He was concerned that two light rays could pass through one another without affecting each other. He could not imagine two particles passing through one another without some noticeable effect. He drew a comparison between light and sound and introduced the idea of a medium existing between the Sun and the Earth which he called the ‘ether’. Huygens arrived at a construction in which the medium consists of particles which pass on wave disturbances in a similar way to the compressions and rarefactions of a sound wave. The construction could explain reflection and refraction. However this theory proposed that the speed of light in an optically denser medium would be slower than in air.
‘Tis
true, that from my theory I argue the corporeity of light; but I do it without
any absolute positiveness’
Isaac Newton
One of the main
reasons was Newton’s prestige. Newton’s theory and experimentation could
explain all the facts then known about light, while Huygens theory could not
explain rectilinear propagation. At this time it was not possible to easily and
accurately measure the speed of light, only a crude estimate had been made. This
was the vital piece of evidence that could decide between the theories but it
was just not possible to measure it accurately enough.
‘...the
absurdity of this [Young] writer’s “law of interference” as it pleases him
to call one of the most incomprehensible suppositions that we remember to have
met with...’
Proc. Am. Academy Science 13, 37, 1888
The factor which
decided the argument in favour of the wave theory was a series of experiments by
Young [1804-7]in which he found that under certain conditions light plus light
gave darkness ie. destructive interference. This could not easily be explained
by a corpuscular theory but could be easily explained by a wave theory. Young
went on to carry out experiments on interference in which he obtained values for
the ‘undulations’ of red and violet light which are of the correct order of
magnitude.
In 1850 Jean Foucault
succeeded in accurately measuring the speed of light and discovered that it was
slower in glass than in air.
However many people were still convinced by Newton’s work some 200 years earlier but more and more experimental evidence was emerging which required a wave theory.