Huygen's theory of light & wavefronts

newton proposed that light is made up of particles so when you're looking at a bulb for example he would imagine i mean there were no bulbs back then but if there was then he would imagine bullets coming out of these bulbs and that's what light was particles he called them corpuscles on the other hand of a dutch physicist named christian higgins and that's not how you pronounce his name turns out that his name is pronounced somewhat like organs or something like that but i'm just going to go ahead and call him higgins he proposed that maybe light is not particles but instead light is made of waves very similar to the waves in water and i want to explore i mean the goal of this video is to explore this wave nature of light so one question we could be having is when it comes to water particles or waves in water the water particles are the ones that are doing the vibrations right when you disturb um you know when you disturb still water then the disturbance is carried away by the vibrating water molecules but in light what particles are vibrating that's a question that you can immediately ask huygens because he's claiming light to be a wave well back then huygens and a bunch of other folks assumed that there is there are there are particles everywhere in the universe and this this medium which consists of these particles they called it the ether medium this was a very popular notion back then aether medium they assumed that this medium was everywhere sort of like air you can't see it but you can't even feel it but it's there that's what they said okay and hygiene says that light is vibrations light is you know light waves are carried by the vibrations of the ether medium so light is propagated by the vibrations of the ether medium now if you want to visualize something like that here here's our animation here's a light bulb and these particles are the particles of the ether medium that's present everywhere in the universe and this is how huygens imagined it light is basically disturbance propagated by the vibrations of the ether particle ripples light is ripples in the ether medium beautiful right well now there is one difference big difference between the ripples of light and the ripples of water water these waves are on the surface which is 2 2 2 2 dimensions and so these ripples are indeed circular and you may have seen them but light is a wave in three dimensions so although you can't see this in this diagram it's in three dimensions so these ripples are not circles but these are spherical ripples that's a major difference that you're finding so this is a 3d wave but now we can ask a question we can ask what exactly are these ripples whether you consider light or whether you consider water what exactly are those ripples so let me get rid of this and talk about that so what exactly are these ripples well in physics these ripples have a very technical and fancy name we call them wavefronts and you'll keep hearing this word over and over again in the future videos so let's clarify what these wavefronts are well wavefronts let me just define them first and i'll show you wavefronts are basically set of particles set of particles which are in sync with each other which are vibrating in sync and again there is a technical term for particles vibrating in sync with each other we call them in phase with each other we just need to familiarize ourselves with these technical terms so what what do i mean by this well if i come over here and look at one of these ripples or wave fronts and concentrate just on them what i will find now is that every particle on that ripple or on that wavefront is at the topmost position and that's why that ripple is a crest a little later in time you'll find all those particles will be at the bottommost position form forming a trough let me show you one more which is already a trough over here we usually did not buy dotted lines a trough if i look at that particular ripple and you'll find all the particles on that are at their bottommost position the important thing to note is that all the particles on these circles they are always vibrating in sync with each other they are vibrating together in phase with each other such a set of particles is what we call a wavefront and the same would be true for our light as well again let me bring back that visualization if i were to concentrate on one of those spheres and just look at those spheres what you are finding now is look all the particles on that spheres are vibrating in sync with each other you see that look here one more time all right there you go look at that they are vibrating in sync with each other such a set of particles which are vibrating in sync with each other are what we call wavefronts the last question i want to explore is what in general would be the shape of these wavefronts well think about water first if you were to disturb it at only one place at just one tiny spot then you'll get these nice circular ripples or circular wavefronts but if you want to disturb this all around with your with your hand everywhere then the wavefronts are going to be all messed up and will have weird shapes right same would be the case with light as well again looking at that animation if the light source that we're dealing with is pretty tiny or we're looking a little bit far away so that the bulb appears tiny then the wavefronts are going to be spherical in nature just like those ripples but if you were to have a big source or if you were to go very close to that light bulb where you can no longer neglect the size now very close to that light bulb the wavefronts won't be spherical they might have different shapes depending upon the shape or the size of this light source but if you go far away a little bit far away where it appears to be a point then the shape is going to become spherical but i want you now to pause this video and think a little bit about what would be the shape of the wavefront if you were to go really really far away from this light bulb think about that that spear is going to become really really big what now would be the shape of that wavefront can you pause the video and think a little bit about that all right if you were to go really far away that sphere would become so big that the sphere would appear flat to us just like how earth appears flat to us when we look at it from a small distance right so let me show you that if you were to go really far away from this and we'll do that in a second now there we go if we go really far away i go really really far away then look at this this is a sphere of course but what we are seeing now is not a line which remembers in three dimension is flat surfaces so now we are getting flat ripples or flat or plane wavefronts that's what we call them plane wavefronts okay so usually what would be the shape of these wavefronts well if you're dealing with say point sources point sources then the shape is going to be spherical as we saw in 2d they are going to be circular but if you are dealing with point sources or sources which are very far away you can think of them as infinitely far away so sources far away sources far away then the wavefronts the shape are going to be the shape is going to be plain they're going to be flat flat wavefronts so the light that we're getting from the sun for example on the earth the light is so far away from the sun that we can assume that the wavefronts or the ripples of light coming from the sun are plain wavefronts now before i end the video remember that there is no ether medium it has been proved and in a beautiful experiment that that doesn't exist and light is a much more complicated wave you may have heard of it as an electromagnetic wave but when we start learning about light as a wave we like to start with heightens theory because it's simple and it can make some really really good predictions