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Human eye: accommodation and near point

Why do distant things get blurred when we concentrate on things close to us? Why can't we see things which are very close to us? Let's explore how our eyes work and get answers to questions like these by exploiting the power of ray diagrams . Created by Mahesh Shenoy.

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Video transcript

when you're focusing on something very close to us like this Batman think which are far away from us like this tree gets blurred similarly if you were to focus on the tree then Batman gets blurred why does this happen well the short answer is because our eye keeps changing its focal length well in this video we're gonna explore exactly what it means why and how it does that to understand this logically we will first take a look at a human eye now an eye is a really complicated thing with a lot of parts in it but we're going to ignore almost all of them and only consider it on the ones which are relevant to us and one of them is this lens it's a convex lens that converges the incoming beam of light and the second important part is the retina the retina is like a screen which has all light-sensitive cells and here's the key thing if you want to see something clearly if you want to focus on something very clearly then the rays of light from that object have to be focused on the retina so I repeat to see something very clearly the rays of light have to be focused on the retina so let's go ahead and draw a principal axis and let's see what happens when you're looking at that tree which is far away suppose we took a point on this tree and drew a couple of rays of light coming towards our eye since this tree is really far away the rays of light that are coming towards our eye are almost parallel to the principal axis here they are let's see what happens to them after refraction well if you look at this way it is passing through the optic Center and the ray of light passing through the optic Center goes undeviated means this way volcko and hit our retina at this point what happens with this ray of light well this ray of light will Bend downwards because it's a convex lens it'll converge the beam of light and in order for us to see this tree clearly if you're focusing on this tree then this ray of light must meet up with this ray at the retina they have to get focused at the retina so our eyes will Bend these Ray's somewhat like this all right this means that the principal focus of our eyes must be at this point does that make sense because any parallel ray of light after refraction through it convex lenss passes through the principal focus and so this has to be the principal focus of our eye so right now that is the principal focus of our eye and this is the focal length this distance is the focal length now let's see what happens to Batman all right remember Batman is really close to us this distance is not to scale that tree is far away and also remember that our eyes are right now still focusing on that tree so the principal focus is right over here so let's draw a couple of rays from Batman and see where those rays of light get focused again one ray of light passing through the optic Center goes undeviated and the ray of light which is parallel to the principal axis well that will just Bend like before passing through the principal focus and here is our principal focus so this ray of light will hit the retina over here and as a result can you see the two rays of light are not being focused on our retina they might get focused somewhere behind the retina and that's the reason and as a result Batman will not get focused right now and that's exactly what's happening right now the rays of light from this tree which are almost parallel to each other the green rays of light are being focused onto our retina but the rays of light from that Batman is not being focused a Batman is blur so this is the case when we are looking at things which are far away when we are focusing on things which are at large distances okay now let's say you want to focus on Batman what should your eyes do again if you just look at the ray diagrams we can figure this out now if you want to focus on Batman that means these rays of light must meet on the retina well this ray of light will always go undeviated so nothing can be done over here which means that our eyes must Bend this ray this ray all the way to meet here and that can only happen if this were the principal focus so in other words when you are focusing on Batman or something which is very close to you the principal focus comes closer the focal length of your eye gets shorter so this is now the new focal length and now what happens is the rays of light from tree well I'm pretty sure you can guess now the rays of light from the tree are parallel again this ray goes undeviated it doesn't matter what the focal length is but now this ray of light well it has to pass through the focal length and so it'll also bend the same way and as a result now the rays of light from the tree are no longer focused on the retina they are now being focused over here and that's why when you focus on Batman the tree gets blurred and this is the situation when you're focusing on things close to us at short distances so the key takeaway is that when your eyes are focusing on something at short distances very close to you they require short focal length and while focusing on something at large distances they require a large focal length and it is for this reason when you're focusing on one object the other one gets blurred so we are pretty much answered our question but let's go a little bit deeper well first of all this ability that our eyes possess to change its focal length it's given a name it's called accomodation eco-mode Asian so accommodation is the ability that our eyes have to change its focal length depending upon what it's focusing on well the big question is how does our eye decrease its focal length what does it do well think about it if you want to decrease the focal length then the rays of light was been more does that make sense and what is this bending depend on well the bending really depends on the curvature of Orleans more the curvature of the lens more will be the bending so in order to decrease the focal length from here to here the lens must be more curved and how does that happen well this is not a you know this is not a glass lens obviously we don't have a piece of glass in our eyes this is a lens which has some fluid inside it and so its shape can be easily changed so our eyes do that it's just you know they make this lens more curved by this pushing on it from the top so when it gets pushed it gets squashed a little bit and as a result it ends up getting bulged so something like this happens here so you'll get both a little bit and as a result the lens is now more curved than before of course I've exaggerated up over here and that's how the focal length decreases and similarly if you want to increase back the focal line bring it back to what we had before well then you let go of this pushing force it'll snap back over here so this is how accomodation happens but then we can further go here and ask well what's making that what's pushing this lens well there are a couple of muscles over here which haven't shown let me show that it's these muscles called the ciliary muscles the name is not so important but they are the ones which are responsible for pushing on this lens and decreasing its focal length and again if they release that pushing force the lens snaps back to its normal position and now the focal and increases of course in reality it's a little bit more complicated than that we don't have to worry too much about this so as the objects come closer and closer the ciliary muscles start pushing onto it to make that lens more curved so that the focal and decreases in order to see the object all right one last thing is that if our Batman were to come even closer then if you drew the same ray diagram again the focal length should get even shorter closer the distance shorter the distance of the object shorter is the focal length and that means our lens must get even more curved but there is a limit to how much your lens can curve and therefore there's a limit to how short the focal length can be suppose now we have reached the limit then this is the nearest distance at which you can keep an object and you can still focus it and that point we usually represent as D and we call that as the near point so near point represents the shortest distance at which you can keep your object and still focus it and now if you keep the object if Batman were to come even closer well now the ciliary muscles can't push anymore we've reached our limit and so we cannot shorten the focal length anymore and as a result this ray of light cannot be focused on the retina anymore it can't because it needs to have a smaller focal length and so as a result the focal line will not get shortened anymore and so the rays of light will no longer get focused and you can test this just take your finger and bring it very close to your eye and you will see you'll not be able to focus it the near point distance really depends on a lot of factors but the average distance we usually take that as 25 centimeters this means any object kept closer that 25 centimeters cannot be focused on our retina remember this is an average distance it doesn't mean every eye has a near point of 25 centimeters some eyes will have smaller than that some eyes will also have larger than that interestingly our eyes don't have a far point there is no limit to how far an object should be to bring it to focus even if the Jake was at infinity for example stars at large distances you can still bring them to focus this is the maximum focal in that our eyes have and so those rays can be easily focused over here it's only the minimum distance that matters to us that's the near point that matters to us so to quickly summarize everything our eyes adjust its focal length depending upon how close or how far the objects are to bring them to focus and this phenomenon is called as accommodation but there's a limit to the accommodation if the objects are too close then your eyes cannot decrease the focal length anymore and so it will just be blurred