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Current time:0:00Total duration:10:25

Video transcript

check out this cool experiment I'm shining my laser light through two glasses of distilled water to this glass I've already added some sugar and stirred it very nicely so it's dissolved to this glass I have not added anything notice that we don't see the beam or the path of light in either of the glasses but now we'll go ahead and add some milk to the second glass and nobody is what happens as we stir that milk we can now see the path of light so the question is why do we see the path of light when we add milk but we don't see it when we add sugar to it so the reason we get to see the path of light in this milk water mixture is due to a phenomenon called scattering of light so scattering of light so let's see what happens when we add milk over here the milk particles get completely dispersed throughout the medium they distribute themselves throughout water and now when light enters into this mixture it hits one of these particles so let's draw that over here so here's all light let's say this is the light from the laser it enters into this mixture and hits one of these milk particles so here is one of that milk particles what happens after this is the milk particle reflects this light in all the directions so it reflects light in all the directions and the important thing to note over here is earlier we have seen that big objects like mirrors or or or eye or a ball or any any any big object if you take they will reflect light in a specific direction but turns out that when you get to these really tiny particles they will always reflect light in all the directions and if you're wondering why this happens then it turns out we need to really dig into this deeper and figure out how light interacts with matter and everything which will not do which will not do so we'll just accept that tiny this is this is a property of tiny particles they will always reflect light in all directions it's just that when these tiny particles get together to bomb forth to form a large object like the mirror or any other day to day life object that we encounter then they only tend to reflect light in a specific direction okay so this phenomenon of reflecting light in all directions is what we call the scattering of light and because of this some of the light is also reflected towards this we are somewhere over here that I can't draw that over here we are somewhere over here right that's where the camera is so some of the light gets reflected towards us or into the camera and as a result that particle we see that particle glowing red in color it glows red because it's reflecting red and therefore that particle which it hit over here that particular particle glows red and then of course not all the light gets reflected some of the light goes through as well actually motion light goes through let's draw that so most of the light goes through this is the light that didn't get scattered you got that this is the scattered light and this is the light that didn't get scattered so most of a light goes through and then it hits another particle and then again scatters off from that so another particle over here will glow and the same process continues and as a result and as a result all the particles which are in the path of that light end up glowing and since these particles are really really tiny of course i've drawn with it i've drawn them to be pretty big over here but they are really really tiny and there are so many of them that when we look at it we don't get to see the individual particles but all we see is a straight line so we see a line that is glowing and that line is what we perceive as the beam of light that's that's that beam that we're seeing so what we are actually seeing are the milk particles who are scattering light and as a result they're growing and that's what that's what shows the path of light but why don't we see the path of light in a sugar solution don't they scatter light well they're to scatter light in fact all particles tend to scatter light but here's the thing it turns out that if we if we do the analysis it turns out that the amount of light that they scatter depends on the size of the particle as the particle size become bigger and bigger they tend to scatter more and more light if the particle size becomes too small then they do scatter light but that scatter light will be so insignificant that we won't be able to see it that so considerable amount of light will not reach our camera and that's what's happening over here the particles in the sugar solution are so tiny that the scattered light is negligible and that's why we can't see it and in fact analysis shows that if the particle size is roughly if the size of the particle is roughly smaller than one nanometer all right this is not an exact value it's a rough value if particle size get any smaller than a nanometer then their scattering effect is so tiny that when you shine light through it you won't be able to see anything and that's what happens in any solution so you take sugar solution or you take soil solution whenever something dissolves those particles usually tend to be smaller than a nanometer and that's why we don't see the path of light but milk particles are way bigger than a nanometer they could be about I don't know maybe I don't know the exact value but maybe around 60 70 to 100 nanometers in size on the other hand if the particles become too large so let's say we have particles which are too large and again a rough value would be let's say they are let's say they're about larger than about a thousand nanometers if they get too big then also we have a problem their problem is not with scattering these particles can scatter light even more strongly because they're big particles and by the way an example of this would be dust particles and you may have seen that sometimes when it's dusty and you're outside it's nighttime and there are vehicles with their headlights you can see the beam of the headlight right that's an example of scattering of light by big particles like dust particles so they don't have a problem with scattering light but their problem is since they are too big they tend to usually settle down due to gravity so if you had to add dust to water which we will do in a minute we'll look at that in a minute then even if you stir it very nicely those dust particles will not stay here long in the water in through the bulk of the water they will usually tend to settle down and once they settle down then most of the particles won't be in the path of light and that's the reason they won't scatter light so the ideal case for us to see a path of light would be to have particles whose size is somewhere in between this so if we have particles which are not too small their size is larger than a nanometer but there are too big so that they would settle down it's smaller than let's say about a thousand nanometer and by the way remember these are rough values okay then we will get to see the path of light and such cases such mixtures are called codes so okay it's a little hard to see so let me write that down over here so it's called colloids and you may have already studied about colloids in chemistry the same colloids we are talking about Sokol eyes are the best example to see the path of light these are called as solutions solutions are one of the worst to see the path of light because the particles are very tiny sugar solution salt solution are examples of this and these are called suspensions they to scatter light very strongly but the problem is they tend to settle down and usually not be found in the path of light so if we go back to our experiment where the sugar solution is not showing us the path of light if we look at it carefully you can sort of see the beam isn't it now this is not due to the sugar solution don't worry if we zoom in carefully we can actually find that find the particles you just scattering light you can see there are some dust particles that got also added when I added the sugar and some impurities these are the particles which are larger than the thousand nanometer size so that is causing esses mention the scattering that we are seeing over here is due to these suspended impurities and by the way look at this dust particle which you can see over here that is so large we can even see it with our eyes directly and as it swirls and comes into the path of light notice how strongly it scatters light okay just just concentrate on this particle let's go back a little bit just just concert it on that particle here it is look at that look at that do you see it see how strongly it scatters light so big particles can scatter light no problem but their problem is that they tend to settle down so most of those particles will not be found in the path of the law of the flight and as a result their scattering or their path is not so strong it's nothing compared to what we see in quois so coal eyes are the ideal place to see this effect so that's pretty much it one last detail is whenever we have scattering of light due to colloidal particles due to quartz we often call that as the Tyndall effect so scattering of light due to cords is called Tyndall scattering or Tyndall effect because this guy John Tyndall did a lot of work on this subject and this also has a great application tomorrow if we have any mixture and you won't know whether it's a colloid or not we will just shine light through it if you can see the path of light that's Tyndall effect then it's a colloid