today matrix we are going to look at accommodation and if you are following in your textbook we are on page 160 to 161. so effectively accommodation is the way in which the eye changes the thickness of the lens in order for you to see objects close up the natural state of the eye is actually to look at distant objects so in other words when you look far away your eyes actually more in its natural relaxed state whereas when you're looking up at close up objects things that are less than six meters away from you your eye needs to focus more light now in order for us to explain how accommodation works we need to be very familiar with the anatomy of the eye in particular three different structures and that would be if i were to do a cross-section through the front of the eye you would need to be familiar with the lens and the lens is elastic which means that it can change its shape it is convex which means it bulges out slightly and then attached to that are what we call suspensory ligaments and they're attached all around the edge of our lens and my lens here i've just cut in half i'm only going to draw the one side then attached to these suspensory ligaments is what we call a ciliary muscle and it is in a circular shape and it sits around the edge of our lens there is a very similar diagram that i've drawn now on page 161 and it's just so that you orientate yourself with these three structures because when you explain how accommodation works you're going to have to explain what's happening to each one of these structures now in particular things like the suspensory ligament and the ciliary muscle they are antagonistic in that when one is contracting the other one is relaxing so let's have a look first of all at what it means to say that you are looking at something near which means that you are looking at an object that is less than six meters away from you now if we look at the diagram alongside here you will notice that the suspensory ligaments have slackened and the reason for that is because the ciliary muscle which sits along the outside of the suspensory ligaments has contracted so when we explain this process we are going to go as follows the ciliary muscles contract then the suspensory ligaments slacken as the suspensory ligaments slacken this will then cause the lens to become more convex or to bulge more and if you notice here in the diagram you will see that the lens has changed slightly particularly if you compare it to what the next diagram will look like on the next slide because the lens has become thicker and fatter you can refract more light which means that you can see in more detail especially if you're looking close up you want to see more detail and finally you want to mention that a clear image is formed on the retina remember whenever you do explanations like this you always want to round your answer off by getting to the ultimate goal here which is to create a clear image now this was looking at objects that are less than six meters but we are also going to have to compare it to when you are looking at distant objects as well and if you flick between the two uh diagrams you will see that the lens has changed shape in between the two and you will notice obviously the ligaments have also um constricted and you can see that they're no longer slackened so now with distance vision the ciliary muscles now relax so everything happens in the opposite now and that will then cause the suspensory ligaments to contract and we can see the suspensory ligaments pulling on the lens in this little part of the diagram over here and you can see that the lens has gone from being a lot more bulge like to a lot flatter and thinner and so when the lens becomes flatter refraction decreases and yet again we always need to finish off our answer by simply saying that a clear image is formed and it is formed on the retina now just so that you can see them compared side by side in some diagrams we can have a quick look at this diagram here where we're looking at distant vision which remember that's more than six meters and near vision is close up and you will notice some differences the first thing is if we look at neovision you will notice that the ciliary muscles are contracted and that means that these suspensory ligaments have moved inwards because they have slackened and in doing so that means then that the lens is becoming more bulgy and bigger which increases the refractive power but if you look at distant vision you will notice that these suspensory ligaments are pulling up and outwards which has an antagonistic relationship with the ciliary muscles at the top here and these have relaxed while the suspensory ligaments at the top have contracted and we are now pulling the lens flatter so that we can see further again this is a really important topic you need to explain yourself very clearly with very specific wording i have uploaded on today's lesson a picture of what a memo would look like for how you would explain this and i suggest that you keep it you write it down and you use that as your guide to write an explanation for accommodation