Isaac Newton – “A Letter… Containing His New Theory about Light and Colours” (fragments)

This one’s gonna be really short, but I could not get any more physics into my head yesterday. Newton concludes that colour is not some kind of intrinsic quality of substances but is caused by light reflection. Red objects reflect more red light than other in the spectrum, blue objects reflect more blue etc. He proved it by putting unadulterated light of one colour on some objects and he found that red objects remain red, regardless of the light thrown at them, they only appear redder in red light. This brings him to the conclusion that light in fact may be a kind of substance as well, and colours are a kind of embodiment of this substance. But as always in science, one answer produces even more questions. What exactly is light? How does it produce the effect of colour in our minds? Newton refuses to answer these questions, writing “I shall not mingle conjectures with certainties.”

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Isaac Newton – “A Letter… Containing His New Theory about Light and Colours” (fragments)

Isaac Newton explains point by point the conclusions his experiments brought him to. The colour of light is its innate feature, correlated with its refrangibility and there was nothing he could do during his experiments to change the colour of a ray of pure light. Of course there are also light rays which are combined of many different colours and so they may seem to be green, while in fact they are a mixture of blue and yellow. But white light is the combination of all of these, Whenever one colour in the spectrum predominates, it’s going to tinge the resulting colour, that’s why for instance the candle burns yellow and stars have different colours. Finally he writes something about the colours of the rainbow, which I am very sorry I could not understand. I told you I was a physics blockhead.

Isaac Newton – “A Letter… Containing His New Theory about Light and Colours” (fragments)

I am not so sure about this one.  I understand that the editors of the NAEL wanted to give the readers some background in the nascent Enlightenment science and make us acquainted with Newton’s “crisp and vigorous English” (even though he wrote most of his text in Latin, the then common language of international science), but I still think it’s ovestretching the definition of literature. The fragments from Newton’s letter to the Royal Society describes his famous experiments with the prism, about which there are umpteenth excellent videos on the web (Khan Academy and others) so I am not quite sure if it makes sense for me to summarize it, especially since Newton’s crisp English is not clear enough for me, a physics blockhead. But I’ll try.

Newton buys a prism, whose ability to produce rainbows was known earlier, but he tries to get past the entertainment value and figure out how it works. He wonders why the bars of colour are oblong, if basing on what was believed then about the nature of light, they should be round, and why the produced rectangle is five times as long as it is broad. He tries to put the prism in different positions, or varying the size of the holes through which the ray of light is let in, or putting the prism behind the hole instead in front of it, but the results are always the same. He then tries two prisms, thinking that if the first one has some irregularities in it, the second one should amplify it. But all the second prism does is to make the ray of light circular again. He then takes two boards with small holes in them and puts two prisms in front of these holes at a distance of about 12 feet. When he turns the first prism around so that only a part of the spectrum reaches the second hole and what comes out from the other hole is a uniform ray of light in the colour near the end of the spectrum caught by the first prism. I am not sure if I describe it well – I could not myself quite get it until I saw this image I’m posting below. Anyway, this leads Newton to the conclusion that the ray of white light consists of all colours, just “differently refrangible”.

Newton's "crucial experiment"
Image credit: Helen Klus/CC-NC-SA.