# 9.2.1 Newton’s corpuscular theory of light

“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’ wave theory of light

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.

## Why was Newton’s theory preferred over Huygens ?

‘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.