 Let's explore the various defects in our eyes. Now before we start, let's quickly recap the features of the normal eye. So on the top, we're looking at things that are far away. So the rays of light come in parallel and to focus them on the retina, we need the minimum power. This is when your eyes are the most relaxed. The ciliary muscles are relaxed. You require the most minimum power in your eyes. On the other hand, when you're looking at things that are very close to you, the ciliary muscles contract. They push on the lens and they increase the power. And when the object is at the near point, which is roughly around at 25 centimeters, then your eyes will be using the maximum power. So this is when you're looking at things close by. Your eyes are at the maximum power. They are stressed. The ciliary muscles are contracted, okay? So this is the situation for a normal eye. And for a normal eye, you can see anything between the near point all the way up to infinity. Okay? So that's the normal eye. Now, let's look at some of the defects. The first defect we'll see is what happens if the eyeball shrinks. So let me redraw the same thing with the eyeball shrunk. And let's see what it looks like. Here it goes. So look, I've drawn the same thing and the eyeball has shrunk. Now because the eyeball has shrunk, well clearly the rays of light will be focusing behind the retina. And the question now is, can our eyes adjust for it? So let's look at this. So if you look at the top situation, where you're looking at things far away, can your eyes adjust to bring these rays, focus them back on the retina? And the answer is, yes. Because you see, if you want to focus them back on the retina, you just need to increase the power a little bit. And so you know what your brain will tell? Your brain will tell you the ciliary muscles to push a little bit, increase the power a little bit, and it'll be able to do it. So here's what it would look like. So no problem. So all you have to do is you have to increase the power of your eye a little bit so your eyes will not be completely relaxed, a little bit of power will be there. And so even if your eyeballs are shrunk, you can see things far away very nicely. No problem. What about over here? Remember, here your eyeballs already have the maximum power. Your ciliary muscles are already contracted to its maximum. Can it push even further? No, because you're at the maximum power already. Oh, which means here it will not be possible to increase the power more and bring these rays back onto the retina. And so right in front of your eyes, you can see if your eyeballs are shrunk, you can see things far away, no problem. But you will be having a problem looking at things nearby. Therefore, this condition is what we call farsightedness because you can see things far away. This is also called hypermetrophia. And what's the cause for it? Well, the cause for it is that your eyeball could be shrunk. Or in more general, we could say that your eyes do not have enough power. So the lens of your eyes don't have enough power to bring it back to be able to see things that are close by. So how do we correct for it? Well, to correct it, you need to just help your eyes get more converging power. How do you do that? By adding a converging lens in front of it. And therefore by using a converging lens, it will provide more converging power to your eyes and that's how you correct farsightedness. Okay, now let's consider the opposite condition. What if your eyeballs are elongated than the normal? This is what it would look like. Now, because it's elongated compared to over here, the normal eye, the rays of light will be focused in front of the retina. Now the question is again the same. Can we adjust our eyes to bring the rays of light back on the retina? Focus them back on the retina. I want you to pause the video and think about it in a similar manner that we did over here. All right, let's see. Now if I look over here where your eyes already have the maximum power, how do I bring this back over here? Well, you need to reduce the converging power of the eye. And you can easily do that. Your brain will tell the ciliary muscles, hey, ciliary muscles, relax a little bit. So it'll relax a little bit and it can easily focus. So you see if your eyeballs are elongated, there is no problem focusing on things that are close by. However, what about focusing on things that are far away? Well, again, your brain will tell the ciliary muscles to relax but the ciliary muscles say, hey, we're already in the most relaxed. Remember, we are in the minimum power position over here. I can't relax anymore. Ah, which means your eyes will not be able to relax anymore and therefore you will not be able to reduce the power anymore and therefore you will not be able to see things far away. This will not be able to focus it back on the retina. This condition where you can see things clearly close by but you can't see things far away, we call it nearsightedness because you can see things. You're sighted, you're nearsighted. You can see things which are close by. The biological name is also myopia. How do you correct it? Well, you want to somehow reduce the converging power, right? So that you can bring this back over here. Well, to reduce the converging power, we're gonna use a diverging lens. So it diverges the rays of light a little bit and then when it converges, it converges back onto the retina. So by using a diverging lens, you can correct myopia. And then when you become very old, it's possible that the ciliary muscles will lose their accommodation power. In that case, it'll be hard for you to see things far away and it'll be very hard for you to see things which are very close as well. That condition is what we call press-biopia. Press-biopia is where your ciliary muscles have worn out due to age usually and you lose the power of accommodation. And remember, you don't need to mug it up or memorize this, you can always use logic. For example, far sighted, what does that mean? You can see things far away, which means you can't see things close by. Why can't you see things close by? Ooh, that's because your eyes do not have enough power, converging power. That means you need to add more converging power so you add a converging lens for correction. Nearsightedness, what's that? You can see things close by so you have no problems with high power. But you can't see things far away. Oh, that means you can't reduce the power. How do you, that means you need help in reducing the converging power. How do you do that? You add a diverging lens. And that's basically how you can logically understand how to correct these defects.