BACKGROUND
Rainbows, Newton and the colours of the spectrum
Figure 1. Primary rainbow (bottom) and secondary rainbow (top).
When you look in the sky on a rainy day, and the Sun is also out, you may be lucky enough to see a rainbow. To find a rainbow, the Sun needs to be behind your back.
If you look carefully, you may also be able to see a faint double or a secondary rainbow, above the primary rainbow (Figure 1).
Notice the colour order for the rainbow: red is on the top, and violet is on the bottom. If you look closely at the secondary rainbow, you will see that the colour order is reversed; violet is on the top and red is on the bottom (Figure 2).
Figure 2a. Identifying the colour-order in the rainbow.
Figure 2b. Detail.
The colours of the rainbow entranced the British physicist Isaac Newton, even as a young boy. He knew that when a light beam passes through a piece of triangular shaped glass – a prism – the white light beam is transformed into a rainbow of colours. In 1665-66, Isaac Newton performed a series of experiments using light, glass lenses and prisms, and revolutionized the way we think about colour (Figure 3). Prior to Newton, ancient and Medieval philosophers thought that colour was a fundamental property of an object, like its weight or texture. After Newton performed his experiments, he realized that colour was a fundamental property of light, not objects. Newton coined the term spectrum for the set of colours we can see in the rainbow.
Figure 3. Historical depiction of Isaac Newton directing a beam of sunlight through a prism and creating a spectrum.
Figure 4. Visualization for how white light enters a prism from the left and a coloured spectrum exits the prism at the right. Note the colour order in the exiting light beam. (Source: https://en.wikipedia.org/wiki/Isaac_Newton#/media/File:Dispersive_Prism_Illustration.jpg )
Newton recognized that the colour order was fixed, and assigned seven principal colours to the spectrum: Red, Orange, Yellow, Green, Blue, Indigo and Violet, although he acknowledged that there was a continuous range of colours in the spectrum, not just seven. When he wrote his initial letters to the scientific community, he described only five colours in the spectrum: red, yellow, green, blue and violet. He later added two more – orange and indigo – in order to have a similar relationship between the divisions between colours in the spectrum and the intervals between notes in a musical octave. Newton felt that since light and sound were both natural phenomena, they would have similar relationships. (Although we know today that this is not the case.) In the Examining the Rainbow exercise, you will notice that there are many more colours in the spectrum than the seven identified with ROYGBIV. Newton recognized that the number of colours in the spectrum depended upon how closely you looked, and could indeed be ’indefinite’.
When a white light beam enters a glass prism, it slows down and it changes direction (or refracts - Figure 4). Newton recognized that each colour of the spectrum has its path changed by a different amount by the prism: violet-coloured light bends the most through the prism, while red-coloured light bends the least.
Newton also recognized that the light rays themselves were not coloured. Instead, colour is what we perceive when light rays enter our eye.
“For the rays to speak properly are not coloured. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that Colour.”
— Isaac Newton in Opticks, 1704
Find out more:
An in-depth account of the historical identification of colours, spectral colours and how they relate to the hue circle can be found in David Briggs’ article on “From Aristotle to Newton”.
A good video which shows reconstructions of Isaac Newton’s experiments on light and colour, and discusses the impact of his discoveries can be found in the BBC video “The Beauty of Diagrams: Newton’s Prism”
