# Refraction in Newton's Corpuscular Theory of Light [duplicate]

Newton's theory of light stated that a light travelled in a straight line as small particles. When these particles travelled in a medium, they experienced an attractive force with the particles of the surrounding medium. This attractive force was equal in all directions, and hence the particle travelled in a straight line. He extended this to explain refraction by saying that when light entered a denser medium, the particles would experience differing levels of attractive forces, as the denser medium would have more particles. This would lead it to bend towards/away from the normal.

However, in this case, how did he explain why different colours of light refracted at different angles?

• Hi, welcome! Please provide references which substantiate your description of Newton's views on propagation of light. – Carl Witthoft May 7 '19 at 13:06
• There is a 37 paged article in the History of Archives of Exact Sciences which deals with your question. Keep in mind that Netwon was not the first one to see refraction. Snell's law already existed. Newton's search for a mechanistic model of colour dispersion: A suggested interpretation (Here is the link link.springer.com/article/10.1007%2FBF00357436?LI=true) – M. Farooq May 7 '19 at 21:02
• In a developed form, Newton's theory was not corpuscular, it was a hybrid of corpuscular and wave optics. His corpuscles had vibration modes ("fits"), upon which the colors and refraction coefficients depended, and interacted with the surrounding ether. This is how he explained the interference rings as well. – Conifold May 7 '19 at 21:27
• See Isaac Newton, A new theory of light and colours (1671) : "THE THEORY [...] (2) A given colour always has the same degree of refractability, and a given degree of refractability always goes with the same colour. [...] (5) So there are two sorts of colours: original and simple colours and colours made by compounding these. The original or primary colours are red, yellow, green, blue, and a violet-purple, together with orange, indigo, and an indefinite variety of intermediate shades." – Mauro ALLEGRANZA May 8 '19 at 9:43

## 1 Answer

Consider one description of Newton's theories: (reference: Newton, I., Philosophical Transactions of the Royal Society 1672, 80, 3075–3087.)

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 because of the Ancient Greek belief that seven is a mystical number.[5]

Newton showed that every colour has a unique angle of refraction that 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

BTW, you can find excellent transcriptions of the original Opticks at many places online, including gutenberg.org

• Could you point to the exact passage of the Opticks where the ancient Greek belief is mentioned? – kimchi lover May 7 '19 at 15:45
• Could you point to the exact passage in the Phil Trans letter you cite, where the ancient Greek belief is mentioned? – kimchi lover May 7 '19 at 17:32
• @kimchilover I'm quoting a third-party article. But download the epub or pdf and do a quick search. – Carl Witthoft May 7 '19 at 17:32
• Well, I did (for both) and no such text appears. Instead of parroting 3d party internet effusions, why don't you do the checking yourself? – kimchi lover May 7 '19 at 17:33