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Newton's Prism Experiment

What is Newton’s prism experiment? The discovery of the visible spectrum.

Sir Isaac Newton (1642-1726) made a series of experiments which revealed the properties of complex white light and its simple elements we call colors. A 38-year-old journey that culminated with the book “Optics” in 1704 and changed our understanding of light forever.

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Education

We may say Newton’s journey in science began in June 1661 at the time he was admitted into Trinity College, Cambridge. The college had based its educational system on the classical teachings of Aristotle, however, Newton preferred to spend his time in the books of then-modern philosophers such as Descartes and the astronomers Galileo Galilei and Thomas Street. Trough these readings he also got introduced to Kepler’s work.

After Graduation

In 1665, Newton created the Generalised Binomial Theorem and began the development of a mathematical theory. The same theory that later became known as “Calculus” is now a fundamental part of modern science.

Newton obtained his degree in August 1665 and just in the nick of time. Soon after, the University temporarily closed its doors as a prevention to The Great Plague.

For the same reason Newton returned home to Woolsthorpe for a period of two years, while technically still remaining a Cambridge student. It was during this “exile” when he developed his theories on Calculus, Optics and Law of Gravitation.

Optics

The first research and experiments of Newton involving prisms started in 1666 as an attempt to increase the quality of the glass lenses used in telescopes. It started while Newton was working on Kepler’s ideas on astronomy. He realised the main disadvantage of telescopes was the corrupt image quality, caused by its rainbow-colored outline.

In his attempt to change the situation and improve the image quality, Newton began a series of experiments involving glass prisms.

The most well-known one took place one year after the Italian natural philosopher Francesco Grimaldi published his work on the diffraction of light. A piece of work which greatly involved the use of triangle glass prisms.

With his newly acquired findings, Newton stated that the nature of the laws of reflection and the laws of refraction can be valid only if light was made out of particles since waves do not travel in a straight line.

One of the main topics of conflict in the science community at the time was the real nature of light or more precisely, whether its physics should be described as a wave or particles. The popular opinion on Newton’s experiments was that the glass prism was responsible for the colorization of light, which itself was a combination of light and darkness.

The experiment involved a small circular hole in a window shutter, through which a beam of light can enter a very dark room. A glass prism is then placed on the path of the light beam. After passing through the prism the light is refracted into a vibrant colorful line on the wall. Newton called it the spectrum (greek for image/to see).

Experiment №1

I procured a triangular glass prism, to try therewith the celebrated phaenomena of colours. And for that purpose having darkened my chamber, and made a small hole in the window shuts, to let in a convenient quantity of the sun's light, I place my prism at his entrance, that it might be thereby refracted to the opposite wall.

It was at first a very pleasing diversion to view the vivid and intense colours produced thereby; but after a while applying myself to consider them more circumspectly, I was surprised to see them in an oblong form; which, according to the received laws of refraction, I expected would have been circular.

They were terminated at the sides with straight lines, but at the ends, the decay of light was so gradual, that it was difficult to determine justly what was the figure; yet they seemed semicircular.

Comparing the length of this coloured spectrum with its breadth, I found it about five times greater; a disproportion so extravagant, that it excited me to a more than ordinary curiosity of examining from whence it might proceed.

— Sir Isaac Newton

On Vision and Colors

1816

Experiment №2

And then I suspected, whether by any unevenness in the glass, or other contingent irregularity, these colors might be thus dilated.

And to try this, I took another prism like the former, and so placed it, that the light, passing through them both, might be refracted contrary ways, and so by the latter returned into that course from which the former had diverted it.

For, by this means, I thought the regular effects of the first prism would be destroyed by the second, but the irregular ones more augmented, by the multiplicity of refractions.

The event was, that the light, which by the first prism was diffused into an oblong form, was by the second reduced into an orbicular one, with as much regularity as when it did not at all pass through them.

So that, whatever was the cause of that length, it was not any contingent irregularity.

— Sir Isaac Newton

On Vision and Colors

1816

Spectrum

Defenders of the wave theory had previously argued that a single light wave is made out of pure white light, which the glass prism corrupted into a color. This theory suggested the more glass prisms the light travels through, the more corrupt it will become.

To further defend his unpopular theories, Newton passed a beam of light through two glass prisms. The first one split the passing light into the full spectrum. The second restructured the split spectrum into a single beam of pure white light. This proved the colorization was not a side-effect of the glass prism.

When Newton compared these observations, he came to a conclusion. There are simple colors which do not disperse when they pass through the glass prism. There also complex colors made out of simple colors. Pure white light is such a complex color and can get dispersed into smaller simple colors we together call the visible spectrum.

Despite the spectrum appearing with no distinct boundaries between the colors, Newton chose to divide them into seven pieces. Red, Orange, Yellow, Green, Blue, Indigo and Violet.

The separation of the spectrum into unique color zones is purely verbal. Our eyes can distinguish infinite transitional tints and shades in the spectrum. The transition between colors is seamless and gradual.

Each color in Newton’s color wheel corresponded to a note from the musical scale. He thought Violet was a recurrence of Red in the same manner musical notes recur the octaves apart. This gave him the idea to create an analogy between musical notes and the spectrum.

Newton’s work had impressed the Lucasian professor Isaac Barrow. A man who was more interested in developing his religious and administrative potential. Newton managed to follow into his footsteps and succeed him as a professor in 1669 only one year after obtaining his Master’s Degree. He was later elected a Fellow of the Royal Society in 1672.

In 1704, Newton was again elected, but this time as the President of the Royal Society. He used the opportunity and to publish the book “Opticks”, his most comprehensive theory of light. The essence of this incredible research was published 38 years after his ingenious realisation when he was just 22 years old.

A piece of work that defined the key principles in our model of light: Simple colors - unique wavelengths, characterized by their different refractive properties in a given substance under dispersion; Sunlight - a collection of simple colors, the visible spectrum.

Newton’s main idea was that a ray of light by itself has no color. Light has nothing but a quality to evoke in us our predisposition to see colors. In this sense, color is never an objective physical value that is independent of the eyes.

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