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Current time:0:00Total duration:8:20

Video transcript

in this video I want to clarify the difference between absolute refractive index and relative refractive index okay so let's start with just refractive index I'd like to think of refractive index as a measure of how slow light travels in one medium compared to another medium and we'll treat that second medium has a reference medium okay so we'll take some examples and then this statement will be super clear to us so let's say we have been given that the refractive index of glass with respect to vacuum that's how we usually write refractive index n is the letter we used for refractive index it's given as 1 point 5 now what does this mean well if you go back to the definition this number is telling us it's a measure of how slow light travels in glass because glass is our medium compared to vacuum this time in this example wacom is our reference medium so in other words this number this this thing is telling us that the speed of light in glass is 1.5 times slower compared to speed of light in vacuum and I think this will make more sense if I write it down mathematically this means speed of light in glass equals speed of light in vacuum say 1.5 times slower compared to vacuum so it's speed of light in vacuum which will usually write as C don't worry too much about that that's speed of light in vacuum divided by 1.5 why are we dividing it because this it's that much slower you get that and by the way we know speed of light in vacuum that's a very famous number isn't it so let me just substitute if we substitute we get 3 times 10 to the power 8 that's the speed of light in vacuum meters per second divided by 1.5 refractive index by the way has no units and if you divide it see what you get you get three divided by 1.5 so we get 2 times 3 by 1.5 is 2 2 times 10 to the power 8 meters per second so you immediately see what refractive index is used for it's used to calculate the speed of light in different media all right so hopefully this statement is making more sense now let's take another example and I want you to try this one let's say it's given to us that the refractive index of water with respect to oil is 2 can you think of what this means I want you to try and pause the video and see if you can think about what is the meaning of this statement and see whether you can write an equation like this over here go ahead pause the video and try all right if you've tried let's see well this time this is this is saying that the speed of light and water because our medium now is water is 2 times slower compared to speed of light in oil this time our reference is oil so again if you write it down mathematically this tells us speed of light in water is 2 times slower compared to oil that means it's speed of light in oil divided by 2 hopefully you got this you see we did the same thing that we did over here we we took the speed of light in our reference medium and we divided by the refractive index that's what refractive index is all about now since I don't know what the speed of light in oil is I don't even know which all you're talking about since I don't know that I can't substitute if I knew it I could substitute just like what I did over here and then we could get the answer but I don't know it so I'm not going to substitute and if you if you ask me that's pretty much it that's what refractive index is all about now you can't see one thing from this by the way notice that when we define it with respect to vacuum since we know what the speed of light in vacuum is it's a pretty famous number it's very easy to calculate the velocity we can immediately calculate the speed of light in glass or in that medium but when we are defining the refractive index with some other medium and if we don't know what the speed of light in that medium is then that's a pretty useless number I can't do anything with it unless I know the speed of light in other medium and it's for that reason most of the times in almost all the cases we are going to define refractive index with respect to vacuum because we know what the speed of light in vacuum is so that number becomes super useful to us okay and since it is so common to use vacuum we just thought let's drop that letter let's not even specify it right we're gonna use we're gonna make vacuum as a standard which means just to keep it short because we're gonna use it over and over again we're not going to we're not even going to mention that it's vacuum but it's there all right so whenever we don't mention the second medium it's understood its vacuum so in your textbooks you will just see refractive index of glass is 1.5 and you may be wondering well what are we comparing with its vacuum if it's not mentioned its vacuum and if it's some other medium it will be mentioned all right so just to give you another example so that we are super clear on this if it's mentioned somewhere let me write it down over here that refractive index of water is 1.33 that's it just this much what does this mean well since the second medium is not mentioned it means this is the refractive index of water with respect to vacuum is 1.33 that means speed of light in water is 1.33 smaller or 1.3 times slower compared to the speed of light in vacuum and using this number I can now figure out what the speed of light in water is so that's pretty much what refractive index is all about then you may be wondering what's this absolute and relative refractive index actually it's a very very small thing that's why I kept it till the end so you see when we are measuring the refractive index with respect to vacuum when we choose vacuum as the reference medium we just call it as absolute refractive index so this is absolute refractive index similarly this is also absolute refractive index and when we measure the refractive index with respect to some other medium we call it as relative refractive index that's it it's just names that we have coined nothing else okay and that's important because I used to think that I should genuinely think that these are two different kinds of refractive indices that we have but now they're just names that we give depending on what we are using as a reference if you're using the standard reference as vacuum and by the way some people instead of saying vacuum they will say air that's also fine because the speed of light in air is pretty close to 3 times 10 power 8 meters per second so in most of the times unless and until specified most of the times vacuum and air mean the same thing for us I know in reality it's not but from the refraction point of view vacuum and air is pretty much the same so to quickly summarize what we learned in this video refractive index is a number that tells us how slow light travels in a in a medium compared to some other medium and when we are comparing with vacuum or air we usually don't write that because it's a standard it's like it's understood and we call that as absolute refractive index and when we are comparing with some other medium then we'll call it as a relative refractive index and to be honest most of the times you won't even use these words alright so over here we will just say refractoriness of glass is 1.5 that's it and over here we will say refractoriness of water with respect to oil is 2 that's it all right we won't explicitly mention absolute or relative but now you know the meanings of those words