How We Came to Know the Cosmos: Light & Matter

# Chapter 3. Newton’s theory of Light

## 3.1 Newton’s crucial experiment

The English natural philosopher Isaac Newton bought his first prism in 1666, one year after Francesco Grimaldi’s work on diffraction was published.[1]Newton claimed that Grimaldi’s diffraction was simply a new kind of refraction. He argued that the geometric nature of the laws of reflection and refraction could only be explained if light is made of particles, which he referred to as corpuscles, as waves don’t tend to travel in straight lines.

After joining the Royal Society of London in 1672, Newton stated that the 44th trail in a series of experiments he had previously conducted proved that light is made of particles and not waves.[2,3]

Advocates of the wave theory had previously stated that light waves are made of white light and that the colour spectrum that can be seen through a prism is formed because of corruption within the glass. This means that the more glass the light travels through, the more corrupt it will become.

To prove that this was false, Newton passed a beam of white light through two prisms, which were held at such an angle that it split into a spectrum when passing through the first prism and was recomposed, back into white light, by the second prism (as shown in Figure 3.1). This showed that the colour spectrum is not caused by glass corrupting the light. Newton claimed this was a ‘crucial experiment’.

A crucial experiment is an experiment that is devised to decide between two contradictory theories, where the failure of one determines the certainty of the other. Since almost everyone agreed that light must be composed of either particles or waves, Newton used the failure of the wave theory to prove that light is made of particles. Newton concluded that light is composed of coloured particles that combine to appear white.[4]

 Figure 3.1Image credit Newton’s crucial experiment.

## 3.2 Newton’s colour spectrum

Newton introduced the term ‘colour spectrum’ and although the spectrum appears continuous, with no distinct boundaries between the colours, he chose to divide it into seven: red, orange, yellow, green, blue, indigo, and violet. Newton chose the number seven because of the ancient Greek belief that seven is a mystical number.[5]

Newton showed that every colour has a unique angle of refraction that can be calculated using a suitable prism. He saw that all objects appear to be the same colour as the beam of coloured light that illuminates them, and that a beam of coloured light will stay the same colour no matter how many times it is reflected or refracted. This led him to conclude that colour is a property of the light that reflects from objects, not a property of the objects themselves.[2]

## 3.3 Criticism from the Royal Society

Despite Newton’s confidence that his theory had been proven, it still faced several problems and was not accepted straight away. Within a year of his announcement, fellow Royal Society member, the English natural philosopher Robert Hooke, published similar results to Grimaldi. He argued that diffraction is not a new type of refraction, as Newton had claimed, and that it could only be explained by assuming that light is composed of waves.[1,6]

Many other members joined Hooke in criticising Newton’s particle theory. Some denied that Newton’s colour spectrum existed at all and others denied that his 44th trial really proved that light is not composed of waves. Those that tried to replicate Newton’s experiment often failed.

Prisms were still not commonly accepted as scientific instruments. They were sold as simple forms of entertainment, and there was little technical work on their design or improvement. Venetian glass was regarded as the standard against which other glasses were compared but even this was full of air bubbles and flaws. Newton did not help matters by concealing the details of his trials. He did not explain how to produce a spectrum from the first prism or specify the size or geometry of the second.[1]

It was not until 1676, four years after Newton’s initial publication, that he performed more trials and gave people enough information to replicate them. Newton finally stated that the best prisms to use were those made in London, not Italy, as these were the clearest. Newton stated that those who failed to replicate his experiments must have been using bad prisms, but this was often seen as an excuse and Newton eventually withdrew from the debate.[1]

## 3.4 Huygens’ wave theory

In 1678, the Dutch natural philosopher Christiaan Huygens claimed to have disproved Newton’s theory by showing that the laws of reflection and refraction can be derived from his wave theory of light.[7]

Huygens argued that diffraction occurs because of the interference of wavefronts. When light is pushed through a small slit, waves are pushed together at different angles and this creates fringes of light and dark shadows, an interference pattern. This is how water waves behave when they pass through a small gap.

Huygens believed that light waves differ from water waves in one respect: water waves are transverse, the waves move up and down like a sine wave while the water moves forward. Huygens thought that light waves were longitudinal, moving in a parallel direction to the beam.[8]

Sound is an example of a longitudinal wave because sound waves move forward by periodically displacing molecules in the air, but the molecules themselves do not move forward, they simply vibrate.

 Figure 3.2Image credit A longitudinal wave.
 Figure 3.3Image credit A transverse wave.
 Figure 3.4Image credit The direction of oscillation and propagation of longitudinal and transverse waves.

For light to move through space as either type of wave, it must have a medium to move through. This meant that Huygens’ theory, like all theories of light before it, relied on the idea that the universe is filled with Aristotle’s fifth element, the aether.

## 3.5 Bartholin’s calcite crystals

In 1669, three years before Newton first presented his particle theory of light, the Danish natural philosopher Erasmus Bartholin had begun experimenting with transparent calcite crystals, which had been discovered in Iceland. He found that when an image is placed behind a crystal it’s duplicated, with one copy appearing slightly higher than the other.[8,9]

When Bartholin rotated the crystal, he saw that one image disappeared while the other rotated with it. This led him to conclude that something in the crystal had split the beam of light into two different rays. Bartholin referred to this as “one of the greatest wonders that nature has produced”, and believed it provided evidence for Huygens’ wave theory of light.[10]

 Figure 3.5Image credit Double refraction within a calcite crystal.

Huygens suggested that Bartholin’s findings could be explained if the crystal contained two different materials, one that produced spherical waves and one that produced ellipsoidal waves. Huygens continued Bartholin’s experiments and found that if he placed two crystals next to each other, then the number of images varied depending on whether the crystals were placed parallel or perpendicular to each other. This was something that his wave theory of light could not explain[11] (discussed further in Chapter 5).

## 3.6 Newton’s Opticks

Newton did not contribute to the debate until after Hooke’s death, 32 years after his original publication. In 1704, he was elected President of the Royal Society and published Opticks, his most comprehensive theory of light. In the opening sections of the book, Newton showed how to reconstruct his prism experiments in more detail, which led to many more successful reconstructions.[1,5]

Newton interpreted Huygens’ findings in terms of his own particle theory of light. He suggested that the experiments with the calcite crystals showed that light has ‘sides’.

Newton also used the publication of Opticks to defend his stance on diffraction. To do so, he had to appeal to wave-like properties and argued that particles of light create waves in the aether. After the publication of Opticks, Newton’s theory gained considerable popularity but some of his critics remained unconvinced.[1]

There was one way to prove which theory is correct: if light is composed of particles then it should travel faster in a denser medium, but if it’s composed of waves, then a denser medium should slow it down. This experiment would not be conducted for another 150 years, but by the end of the 19th century, both theories would be proven wrong.