Colours which appear the prism are to be derived from
the light of the white one.
Newton and the Color Spectrum. The diagram from Sir Isaac Newton's crucial experiment, 1666-72. A ray of light is divided into its constituent colors by the first prism (left), and the resulting bundle of colored rays is reconstituted into white light by the second.
Newton's theory of Light
A year after Italian physicist Francesco Grimaldi's work on diffraction was published, British physicist Isaac Newton bought his first prism in an attempt to disprove French Philosopher Rene Descartes' wave theory of light.
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 was made of particles, which he referred to as corpuscles, as waves do not tend to travel in straight lines.
Newton's crucial experiment
After joining the Royal Society of London in 1672, Newton stated that the 44th trail in a series of experiments he had conducted earlier that year had proven that light is made of particles and not waves.
Newton's crucial experiment. Image credit: Helen Klus.
Advocates of the wave theory had previously stated that light waves are made of white light, and that the colour spectrum, which 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.
In order 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. This showed that the colour spectrum is not caused by glass corrupting the light. Newton claimed that this was a 'crucial experiment'.
A crucial experiment is any experiment 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.
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 as this reflected the Ancient Greek belief that seven is a mystical number, due to the fact that there are seven 'wandering stars' and seven days in a week, a quarter of the time between two full Moons.
Newton showed that every colour has a unique angle of refraction, which 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.
