Introduction

In 1704 Sir Isaac Newton published Optiks, which detailed his theories on the properties of white light and the distinct 'refrangibility' of what he called each primary colour. Newton was not the first to form theories on the nature of white light, but due to his stature as a physicist and the fact that he published, his theories became widely accepted.

Fig 1 This photograph of a natural Rainbow shows only 5 visible colours, Red, Yellow, Green, Blue and Violet. The colours are clearly visible in contrast to the tree background. Photo: Jacobs, Murichison, New Zealand, 20/6/2003

Recently, many online articles have been published supplying differing views regarding Newton's theories, discussing the merits of the implications he drew from what he found through his experiments. Such sites are Newtons colors(9), Isaac Newton (5) and Color Music in the New age: De-mystifying DE Clario(4), where we find opposing view points regarding the ways Newton developed his theories of light. One observation made in both Isaac Newton and Color Music in the New age: De-mystifying DE Clario, looks at the absence of Indigo in the real spectrum observed by the eye and to some extent of Orange, opposing the ROYGBIV order presented by Newton. These opposing viewpoints pose an interesting research question regarding the legitimacy of both Newton's spectrum derivation and his assignment of seven spectral colours. An interesting way to investigate these notions is by studying rainbows. While not identical to a refractive prism, raindrops give the same effect, a rainbow, which is in fact a colour spectrum.

Studying the separation of colours in rainbows allows a focused look at the real spectrum, allowing a comparison between Newton's rendition of the spectrum and that of an actual, real life spectrum. A simulation program created by Les Cowley creates predicted rainbows and fogbows for rain drops of specified sizes, thus allowing the analysis of rainbow colours in different conditions. Using these simulations the idea of the absence of Indigo in the composition of white light can be investigated.

Does a rainbow showing the seven colours proposed by Newton and still used in many elementary textbooks to describe the composition of light, actually exist or does his theory represent a more subjective result based on his personal reasoning?

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