Isaac Newton · Science
Newton’s experiments with the prism, which showed that white light is composed of all the colours of the spectrum - the foundation of the science of optics and a model of experimental method.
For centuries it was believed that white light was the purest, simplest form of light, and that colours were produced when light was somehow modified, darkened, or corrupted - by passing through coloured glass, or by mixing with shadow. Newton overturned this completely. Working in his rooms at Cambridge, he let a narrow beam of sunlight pass through a glass prism, and the prism spread the white light into a band of brilliant colours - red, orange, yellow, green, blue, indigo, violet - the full spectrum of the rainbow, cast upon the wall. The crucial question was: did the prism create these colours, adding them to the pure white light? Or did the white light already contain all these colours, with the prism merely separating them? Newton’s experiments showed, decisively, that white light is not pure and simple at all. It is a mixture - a compound of all the colours of the spectrum - and the prism does not create the colours but separates the components that were there all along. White light, the supposedly purest light, is the most composite. This was a startling and counterintuitive discovery, and Newton proved it with a beautifully designed series of experiments.
Newton explained why the prism separates white light into its colours: because the different colours of light are bent - refracted - by different amounts when they pass through the glass. Newton called this property refrangibility: the degree to which a ray of light is bent when it passes from one medium to another. He discovered that each colour has its own fixed, characteristic degree of refrangibility, unchangeable by any further refraction. Red light is bent the least; violet light is bent the most; and the other colours fall in between, in the familiar order of the spectrum. When white light - a mixture of all colours - enters the prism, each colour is bent by its own amount, so they emerge spread out into a band, sorted by refrangibility, with red at one end and violet at the other. This is the cause of the spectrum: not that the prism dyes or corrupts the light, but that it bends each component colour by a different amount, separating the mixture into its constituents. Newton further showed that each pure colour, once separated, is simple and cannot be broken down further: passing a single pure colour through another prism does not change its colour, only bends it again by its fixed amount. The colours of the spectrum are the simple, primary components; white is the compound.
Newton’s experiments led to a profound conclusion about the nature of colour itself. Colour, he argued, is not a quality added to light, nor a property of objects in themselves, but arises from the refrangibility of the rays of light - each colour corresponding to rays of a particular degree of refrangibility (what we would now describe as a particular wavelength). The rays themselves, he carefully noted, are not coloured; rather, each kind of ray has a disposition to produce the sensation of a particular colour in us. And the colour of an object - why grass looks green or a rose looks red - arises because the object reflects some kinds of rays more than others: a red rose reflects the red-making rays and absorbs the rest, so we see it as red. Colour, then, is bound up with the composition of light: white light contains all the colours, objects appear coloured because they reflect some rays and absorb others, and the prism reveals the hidden composition by separating the rays according to their refrangibility. This was a deep unification - it explained the colours of the spectrum, the colours of objects, the rainbow, and the colours produced by prisms and thin films, all from a single principle: that white light is a mixture of rays of different refrangibility, each disposed to produce a different colour. Newton had penetrated to the physical basis of colour.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned how Newton used the prism to show that white light is composed of all the colours of the spectrum. Explain his key experiment and what it proved about the nature of light and colour.
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