Most of Newton’s theories only became known later in his life and after the publication of the 'Principia'. His theories regarding optics, however, were made public right at the start of his career; with his first written work on optics being published just after his election to the Royal Society.
This paper was the first of many to outline Newton’s theory that white light is a “Heterogeneous mixture of differently refrangible rays”, where ‘heterogeneous’ refers to something that is diverse in content, and ‘refrangible’ refers to matter that is able to be refracted. In other words, Newton held that white light is composed of colours that can be considered primary; a statement that he proved in what is now known as ‘The Crucial Experiment’, which incorporated the use of prisms to separate the white light and to isolate a given colour, as each colour has a unique angle of refraction.
Newton’s book, ‘The Opticks’, published in 1704, provides further explanation for his theory regarding white light and colour, and instructs on how to perform the prism experiment for oneself. It also discusses his particle theory, contradicting the theory of the time: that light, like sound, is made up of waves. Newton reasoned that light, in fact, is composed of discrete particles, arguing that these particles of light create waves.