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Newton's prism experiment

Let's explore how Sir Issac Newton experimentally proved that white light is made of all the colours of the rainbow.  Created by Mahesh Shenoy.

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Video transcript

today we know white light is made of the seven colours of the rainbow but how did we figure this thing out well you may already know the answer to this prisms prisms are usually pieces of glass which have two triangular phases which are connected by three rectangular faces like this and you may already know the famous experiment the when you shine white light through a prism it splits into the seven colours of the rainbow but here's the question if we were to perform this experiment for the very first time do you think we could conclude based on this experiment that white light is made of seven colors personally I don't think so you know what I would conclude I would conclude that the prism is just creating the colours I mean think about it why would anyone think that the white light is made of seven colours and that these colors are bending differently and that's why we're getting coloration so why would anyone think like that and so the popular opinion back then was pretty much that white light is just white light I mean why would you assume it's made of seven colors it's just like any other colour white is a single color and when it when it passes through the prism the prism pains that white color into different other colors like red and green and blue and yellow and everything so the prism is creating the colors that was the popular opinion back then that's what people actually concluded that the prism creates colors so we're gonna explore in this video is how did we figure out that that prism doesn't create colors but the colors were already present in the white light and the prism is just bending them differently it takes a genius to figure out that this this theory that the prism is creating colors is actually wrong and that genius is norther than Sir Isaac Newton he believed in the scientific method which means if you want to check whether your theory is right or wrong then all you have to do is perform an experiment now this might seem super obvious now but back then people didn't do this people just went with their in tuition if an explanation or a theory fell satisfactory to them they've they believed in it and so this this theory that prism creates colors was very satisfactory right that's what it looks like even from this drawing and so people went with it but Newton asked himself could I perform an experiment to really check whether prism is really creating colors or not and one of the first experiment he did was he put another prism in front over here the logic was if prism can create colors out of white light maybe it can create more colors out of red and blue and yellow and other colors that we're getting over here so he wanted to check whether he can get even more colors out of this and what he saw was he didn't get any more colors he got the same colors again which means our prism cannot create more colors out of these seven colors so that made him a little suspicious about this theory why is it that the prism is only creating more colors out of white light why is it not creating more colors out of these other colors but this doesn't completely disprove our theory right maybe white light is special maybe only when you shine white light on the prism it starts creating these colors that's still possible right so he performs a second experiment and this experiment conforms his suspicions and the second experiment is he takes that prism and he keeps it inverted in front of the first prism and you know what he finds on the other side over here now he just sees white light by looking at this he immediately realized that the prism was not creating colors but instead those colors were already present in the white light and the prism is just bending them differently and that's how he figured out that white light is actually made of seven colors and even we can figure this out now just by continuing this ray diagram so let's see if these two prisms are identical and they're made up of the same material then these rays of light as they move from this prism to this prism they will not Bend they will not reflect because they are moving in the same medium they're not changing their medium and as a result these rays of light will go just straight continue in their path but when the rays of light emerge out notice we can treat this whole thing as a single parallel sided medium and we've already studied in previous videos that any ray of light which emerges out of a parallel sided medium will always be parallel to the incident ray and as a result we can now further draw and you'll see that the red the blue all the colors must come out parallel over here and so we can go ahead and draw this and this is what it would look like but as mentioned earlier this does not look color to us this looks white to us and here's a picture of this notice the emerging light is pretty similar scenario the emerging light is white why is it looking white well we've talked about this in previous videos the main reason for this is because the incoming light does not contain just one single ray of light it contains multiple rays of light a beam is always a collection of rays so if we were to consider let's say one more ray of light one more ray of white light then even that would split in two colors and now notice the emerging red color is overlapping with the blue color of the first ray can you see that and similarly if there was another ray of light somewhere in between then maybe it's yellow would start overlapping with the blue and so because all the colors are emerging out parallel to each other when we consider all the rays of light all the colors eventually overlap with each other and because this is looking white to us we can now conclude when the colors are overlapping or when the colors are mixing that's what appears white was so maybe that's how Newton concluded that white light is not a separate white is not a separate color the prism is not really you know creating other colors white is a com nation a mixture of all the colors of the rainbow and maybe that's how Newton guessed on written realized that the prism is what the prism is really doing it's just bending the light differently and of course he may have also performed other experiments to support his argument one of the other famous experiment is the Newton's disc so what he did is he took a disc divided into seven parts painted them with the seven colors of the rainbow and then he spun it very quickly and he supposedly saw it as white and this is something we can also try at home and you pretty much see white light again confirming that when all the colors of light fall on our eyes that's what we perceive as white so white is made of mixture of the seven colors of rainbow so what I want to highlight here is that the experiments performed by Newton were very simple but its conclusions were groundbreaking this created a scientific revolution because it changed the way we thought about light and colors