Let's explore the common defects in the eye, myopia (shortsightedness) and hypermetropia (farsightedness), using ray diagrams. Created by Mahesh Shenoy.
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- for correcting a myopic eye concave lens are used which diverge the rays and using the same lens when we look at nearby objects (which were on a beforehand clear to us) wouldn't the rays coming from them also diverge and image wouldn't be clear enough as it was to be before using the lens but ironically in reality the nearby objects are clear to us how is this possible?(4 votes)
- Actually your definition for 'near' is wrong. The nearest visible point is even less for an myopic eye compared to a normal eye whose near point is 25 cm. Let's assume that the near point for a myopic eye is 10 cm so the eye would be able to see objects at 10 cm of distance but when he wears glasses the near point is now 25 cm again(2 votes)
my friend Marcus can see things close to him very clearly but faraway objects not so clear we say he is short-sighted you know because he can see things which are close to him short sighted on the other hand my friend Julie has the exact opposite problem she can see things far away very clearly but the close objects not so clear we say she is farsighted you know because she can see things far away clearly so let's figure out what defects in our eyes caused these behavior now before we get down to it let's first quickly recap how in normal unaffected eye works in a previous video we saw that if your eyes while looking at something very far away say this tree which is extremely far this this lines are saying that it is not to scale the trees kept very far away then the rays of light from that tree would be pretty much parallel to the principal axis and remember in order to see something very clearly the rays of light must always get focused on to the retina and as a result this now is the principal focus of our eye because a parallel rim must pass through the principal focus but as the object comes closer say looking at Batman who's standing pretty close to you then again to see him clearly the rays of light must again get focused on to the retina but to do so can you see now the new principal focus is over here over here which means the focal principal focus has come closer that means the focal length has become shorter so as the object comes closer the focal and gets shorter and this happens due to the ciliary muscle so you've seen that the ciliary muscles are the one that can push this lens and make it more curved increasing its converging power but of course we're going to ignore that to keep things simple it's happening but will not show that but there's a limit to how small the focal length can be so at a particular distance let's say at this position the focal length of our eye becomes minimum becomes minimum which means it cannot become any smaller and as a result if this object were to come even closer then you can't see it clearly and we call this point as the mere point which is represented by D so the key thing to remember is as the object distance changes our eyes will adjust its focal length it can take a range of these focal lengths you can take a range of these focal lengths this by the way will be the maximum focal length and because of this it will ensure that the rays of light will always focus onto the retina and if you need more clarity on this it would be a great idea to go back watch that video and then come back over here all right now let's come to the defective eye so what causes these defects in the eye well the short answer is the power of the lens is a little bit higher than usual or a little bit lower than usual well look at why and how that happens a little bit later but let's explore these cases and see what happens so let's consider the first case as the power of our lens is a little bit higher than usual so let's say high power a little bit high power what would this mean this means our lens has more than usual converging power in other words it will have smaller focal lengths than in normal eye in other words the range of focal length that this I can take on it can accommodate two will be smaller these all these values will be smaller than what a normal eye does all right so now let's look at what happens when you keep an object close to us and what happens when we keep an object far away consider an object which is close to us say Batman standing close to us if you draw a rays of light then I read through the optic centerville go undeviated and the Ray parallel to the principal axis well in order to see this Batman clearly this ray must meet up with the other ray and it should they should focus right at the retina and the question is is that possible and the answer is yes our eyes can easily do that because to do this the required principal focus lies within the range that it can take and so there is no problem looking at things close to us but what happens when you're looking at things very far away say we're looking at this far away tree again the rays of light are pretty parallel to the principal axis again the Ray passing through the optics and records undeviated and in order to now see it clearly this ratio again again meet up at the retina all right they should get focused at the retina but is this possible the answer is no that cannot happen because in for this to happen the principle focus of our I must lie here outside the range our eyes will not be able to accommodate to increase its focal length beyond this value and as a result what will happen our eyes will try its best and they'll just have maximum focal length over here this is what will happen and as a result can you see that the Rays are being focused in front of the retina so we say the image is formed in front of the retina and so this will look blurred to us so notice because of high power we are able to see things which are close to us not a problem but we're not able to things see things which are far away in other words this is the reason for nearsightedness so it just discussed is nearsightedness or short sightedness short sightedness the biological name for this is myopia so the next question is how do we correct this you may already know that people wear spectacles or contact lenses these are just converging or diverging lenses so I want you to pause the video and think about what lens would you put in front of the eye would you put a converging lens or a diverging lens to correct this all right if we use a converging lens then you can see it'll increase the overall converging power that means the rays of light will get converged even closer that is making situation even worse so we need to use a suitable diverging lens so that the rays of light on our eyes will be diverging and become easier to focus them back onto the retina there are a couple of other ways to think about this one way is to think that because we're adding a diverging lens we are reducing the overall converging power and as a result we are increasing the overall values of the focal length back to normal another important way to think about this especially for new miracles is that now the rays of light which are diverging or they appear to come from somewhere over here or somewhere close by and our eyes have no problem focusing them on the retina so either way you think about it a diverging lens of suitable focal length will solve this problem now let's look at the other problem what happens when our I have lower than usual power all the analysis similar now that our eyes have lower power than usual it means the rays of light will get can much farther than usual that means all these values of focusing would be higher than usual so all these values would be a little bit higher than what we have okay that would be a great idea to pause the video and try this out yourself see what happens when the rays of light are coming from far away see what happens when the rays of light are coming from close by and see which of them will get focused nicely alright let's do this if there is a flare coming from far away can our eyes focus it the answer is yes because our focal length is within that range but what if the object comes closer let's say when Batman is standing close to us can it be focused the answer is no because notice in order to focus this array now the principal focus has to be at this point which is lower than the minimum value that's not allowed our try our eyes will try its level best but it can only decrease the focal into this value and as a result notice the rays of light are being focused behind the retina and as a result you can't see things which are close to us but we can see things which are far away in other words this is the cause for farsightedness the biological term for this is hyper Metropia hyper Metropia and again pause the video and think whether we should use a converging lens or a diverging lens well this time our eyes have lower than usual power which means we need to increase its converging power we have to use a converging lens a convex lens of suitable focal length so that it will help it converge the beam of light back on to the retina and that's how we can solve the problem of farsightedness well one last question we might have is what really causes this higher than usual power or lower than usual power well turns out there are multiple reasons one of the reasons could be that the eye lens itself has more curvature or less curvature for example over here it has low power because the islands over here has less curvature I have exaggerated over here and similarly over here it will have more curvature I can and exact duration that could be one of the reason another possibility is that these lenses might be fine but the eyeball over here is longer than usual and this eyeball might be shorter than usual but whatever it is there is a very common problems and they can be easily solved by using appropriate lenses now the best part of this is we don't have to remember any of this because all of that can be worked out just by drawing ray diagrams like we did in the video so all I remember is that these defects has caused either due to the high power or low power and then we can just work it out to figure out which causes which and then logically understand which lenses to use to correct them and that's why I love ray optics