30 APRIL 2026 14:30 - 16:00 LIFE SCIENCES GRADE 12

Hinzugefügt: 2026-05-08

[00:09:58]Mhm.

[00:17:53]>> [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] >> Ooh.

[00:19:58]>> [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] [music] >> Good day, grade 12 learners. Welcome to today's life sciences lesson. My name is Vedika Maharaj. This lesson is brought to you by the Gauteng Department of Education in collaboration with Sci-Bono Discovery Centre.

[00:28:05]Today, we are going to be doing response to the environment with the subtopic the human eye. So, if you remember last week, we did um human response to environment, but we did the nervous system. Um we covered the different the central nervous system, the um peripheral nervous system, the autonomic nervous system. We did the neurons. We also did the reflex arc, the reflex action, and we did disorders of the nervous system. Now, we're moving on to a subtopic, one of the organs involved with the nervous system, and that is the human eye.

[00:28:40]I'm I'm presenting alone today, so I'll be doing the entire lesson.

[00:28:45]So, I'm just reminding you of the examination guideline, the one from 2021 is still valid. So, this is the examination guideline you're going to be using in order to check that you have studied all the sub the bullets involved in this topic. So, the human eye, it looks like a very short one. However, the human eye um it it's there are quite a few concepts here, and I'm quite a few processes that you need to know, and um it's actually very commonly asked.

[00:29:18]I just want to also take this moment to um explain to you that the human nervous system human response to the environment is part of paper one.

[00:29:27]Okay? And it carries about um 54 marks.

[00:29:32]However, they um it the the the nervous system the the human response to environment is divided into those three parts, the nervous system part, the human eye, and the ear.

[00:29:43]But, there is no distinction within the amount of the mark allocation for how many marks is allocated to the eye, how many marks allocated to the ear, and how many marks allocated to the nervous system. So, it can be any amount of marks. So, I actually think it's the nervous system is 48 marks, okay?

[00:30:04]But anyway, the point is that you cannot just say the eye is definitely going to be there. It might not be there. 90% of the time it is there, but what I just means that we cannot forget about the other parts of the human response environment and only think that nervous system is the human response is about the eye and the ear.

[00:30:24]It isn't, okay? So, all of those marks are going to be in paper one, but we don't know how it's going to be distributed, okay? So, human eye, what do you need to know? You definitely need to know the structure and functions of the parts of the human eye using a diagram. So, that's the external the external part of the eye and the internal part of the eye. You need to know what is binocular vision and its importance.

[00:30:49]The changes that occur in the human eye for each of the following using diagrams. So, there are different changes that happen in the eye. These are called accommodation and pupillary mechanism. You need to know these. You need to know the parts that are involved in these changes and you must be able to use a diagram to explain them. You need to be able to interpret diagrams about them so that you can actually answer questions.

[00:31:16]Then you need to know the nature and treatment of the following visual defects using diagrams as well.

[00:31:22]Short-sightedness, long-sightedness, astigmatism, and cataracts. So, what do I mean by using diagrams? If they show you a diagram and it has this visual defect, you must be able to identify the visual defect, you must be able to explain what's happening, you must be able to talk about what is the nature of the treatment that will be used for it. And and sometimes if they ask you an application question, they can even ask you how is this corrective measure going to solve the problem?

[00:31:54]Okay. So, definitely a very interesting topic, one of my favorites. So, let's get right into it.

[00:32:02]Right. So, before we do that, let's talk about our pre-knowledge. You know, it's very important that we still link this to what we learned in the nervous system. So, let's just remind ourselves, right? What is a stimulus?

[00:32:14]Can you remember?

[00:32:16]What we said last week is a stimulus, okay? A stimulus is a change in the environment, okay? So, for the eye to also be able to carry out its accommodation or pupillary mechanism, there is this change in the environment which would be a stimulus, okay?

[00:32:36]And then it's going to be received by a receptor. We spoke about what the receptor is. A receptor identifies or receives a stimulus and converts it into a nerve impulse.

[00:32:48]So, even with the eye, there has to be a stimulus, right? What do you think the stimulus would be in the eye?

[00:32:54]Let's think about it. What what happens?

[00:32:56]Or maybe you we don't know it yet, but let's think about it for later. We won't answer it now, but let's think about it.

[00:33:01]What do you think the stimulus would be that would allow us to see?

[00:33:06]The receptor will then pick up that stimulus, it will convert it into an impulse, and that's where we get the word impulse from, and then it will take the information to the cerebrum where the interpretation will happen. Okay.

[00:33:18]So, the eye is an organ, yes, but it's going to receive a stimulus. That stimulus is then going to be received by the receptor. The receptors are in the eye. It's then going to convert it to an impulse and the impulse is then going to be taken to the cerebrum. Okay. So, let's just go into a little bit more detail. Before we go into that, the terminology, very important that you know the term different terms in all the life science topics, okay? Be familiar with the terms, make sure that you understand them because then when you read text or you are listening in class and teacher's trying to explain to you something, at least if you know you're not bogged down by thinking, "Okay, what does this word mean now?

[00:33:59]What are the rods? What are the cones?

[00:34:01]Um you if you know what they are when when the teacher's explaining to you in class, you will better understand.

[00:34:08]Right. So, your rods and your cones are the receptors, these receptors that I was talking about, okay?

[00:34:14]And they they are cells that are found on the retina of the eye and they are sensitive to dim light and they help to distinguish it between black and white.

[00:34:25]The cones are receptor cells as well, also found on the retina, but they are sensitive to bright light and they help to distinguish between different colors, okay? So, you need rods and cones in order to see bright different colors.

[00:34:42]If you only have cones only rods, you're only be able to see in black and white.

[00:34:47]Pupillary mechanism, um the regulation of the pupil size to control the amount of light entering the eye. Yes, so the pupil is basically the hole in your eye, okay? And it controls it so it it changes um the the pupil size can become wider and it can become narrower. It dilates and it um constricts, okay? And this mechanism is called the pupillary mechanism. It's the it's regulation of that pupil size. We'll go into more detail later about how that pupil size, remember I told you it's just a hole.

[00:35:26]So, how does it actually dilate and constrict?

[00:35:29]And then accommodation is the ability of the lens of the eye to alter its shape for clear vision when viewing both near and distant objects. So, accommodation has got to do with seeing things that are near or far. Whereas pupillary mechanism is has got to do with how you can see in dim or bright light.

[00:35:50]Then we've got these two words, convex and concave. Convex is a shape. It's a shape which curves outwards, thicker in the middle than the edges. And concave is a shape which curves inwards, thinner in the middle than the edges. So, if we have to look at this, okay, this is for example now, it is it is straight.

[00:36:14]If I have to curve this, as it says it curves outwards. If I have to curve it, can you see there it becomes thicker in the middle, but thinner on the ends.

[00:36:24]Thinner on the ends, thicker in the middle, okay? That is convex. And then concave is this side. So, it's this cave this curvature inwards where you're going to still have um we're looking at it at a different angle now. It shapes inwards, thinner in the middle, but thicker on the edge edges.

[00:36:45]So, um that is convex and concave.

[00:36:49]Right. So, I'm going to explain these in more detail as we go through the presentation, but just so that you can understand or you've heard of these terms before we go into it. Wow, have a look at this.

[00:37:00]So, as you can see there's something going on here. That pupil size is changing. The pupil is that black area in the center. Can you see that it's changing?

[00:37:09]The size is changing. So, is this pupillary mechanism or accommodation?

[00:37:13]What did we just learn?

[00:37:15]Pupillary mechanism. Yes, because the pupil size is changing.

[00:37:22]Okay, external structure of the eye. So, you all seen this, right? You've seen people's eyes, you've seen your own eyes, but have you ever stopped to think, "What are all these different parts of the eye?" So, hopefully in primary school you learned where your eyelid is.

[00:37:37]You learned what your eyelashes were, you learned about your eyebrows, and you probably even learned about the fact that we have different colored eyes, but did you know what those parts are called? Maybe not. So, that colored part of our eye is called the iris. In this one it's blue. Um people have green, some people have like a hazel color, some people have tones of gray and green mixed, beautiful colors that you can get. People have light brown, people have dark brown, blue. There are different colored eyes, right?

[00:38:11]And then we have the pupil in the center. Like I said, it's the black area. In this picture it's basically a hole. And then you have the sclera which is the white outer layer of the eye, okay?

[00:38:23]And let's think about why we have these eyelids and these eyelashes. Why do you think we have them for?

[00:38:31]It's to protect the front of the eye, to stop foreign particles from entering the eye, okay? You know, that's why you always have that reflex to like to blink. If something's going to come towards you, you're going to blink. Why?

[00:38:42]Because it's it's a protective mechanism for your eye to close so that it can protect any foreign particles from going inside.

[00:38:50]We spoke about the colored part of the eye and the opening in the center is called the pupil and the sclera is the white outer covering layer, okay? So, we spoke about that. External structure of the eye. You must be able to know how to draw this, okay? They can ask you how to draw and label this or they can even ask you just to label.

[00:39:07]Now we look at the internal structure of the eye, okay? Sorry, this is going to be the different parts, okay? Let's just have a look at it. I'm going to use the other diagram to go through the different parts, but I just want you to look at this diagram to say to see what are we looking at here.

[00:39:24]So, this is a side view of an eyeball, okay? And it is cut through the center, right down the middle. So, now you are looking inside to see what is happening inside. So, the the part that we saw now, we only this is the outside.

[00:39:40]We were looking at it this way in the previous picture. But can you see you can't really see the same things that you saw. It's looking different now, okay? Cuz now it's cut out. You were able to see the iris, you were able to see the pupil, you're able to see the sclera. Um that is the main things here.

[00:39:57]There's no eyelid in this diagram and there is no um eyelashes.

[00:40:02]Okay? So, I just want you to to bear that in mind that this is now cut in half. So, the iris that you saw when you looked straight ahead is not going to look the same as in this picture. Okay?

[00:40:12]It's very important that you try that we visualize this. It helps us to remember when we have to label this. Look how many Look how many labels are in this diagram. Okay?

[00:40:22]So, um and you need to know how You need to know every single one of them. You need to label be able to label diagrams of the eye. If they're going to give you an eye, they're almost always going to ask you to label it or provide a function.

[00:40:34]Or even compare the structural adaptations or the structural suitability of the different parts.

[00:40:41]So, let's get right into it. So, we first have the sclera.

[00:40:47]Tough, white, inelastic layer that covers the eye towards the posterior.

[00:40:53]Okay? So, that just means that it is covering the entire eyeball.

[00:40:57]And um it's tough, it's white, it is inelastic. Okay? It protects the inner part of the eye and it maintains the shape. So, it maintains that roundish shape of the eyeball.

[00:41:10]And it's tough and inelastic so that it can hold that shape and protect the contents of the eye.

[00:41:17]Or the inner parts.

[00:41:18]Then we've got the choroid. That's the middle layer. Choroid.

[00:41:23]It It's continuous with the ciliary body and the iris. It has dark pigment to absorb light within the eye and it has blood vessels to provide oxygen. Okay?

[00:41:35]So, that's why it's the darker one because it's got that pigment to absorb the light within the eye. Cuz remember there's going to be a lot of light that's going to be coming into the eye.

[00:41:44]Okay? So, light is going to come into the eye. So, now that we're talking about this, you can already start to think to yourself what the stimulus is, right?

[00:41:52]Yes, it would be light.

[00:41:55]So, um then we move on to the inner layer. This is the retina. It's the inner layer of the eye. It has the photoreceptors which are called rods and cones and they are the site for where the image is formed.

[00:42:08]It helps to convert light stimuli into an impulse. Very important. Okay, class, I don't know why, but learners seem to forget that when we come to the eye, that the eye also has a stimulus, that eye also has a receptor, it's also going to have nerve impulses and those impulses are going to be um they're also going to be conducted or transmitted via something, right? We're going to get to that.

[00:42:33]But what I want you to understand here is that the retina has the rods and cones. It's It's going to be the site where the image is formed. It's going to receive the stimulus and these photoreceptors are going to convert the light stimulus into an impulse.

[00:42:48]Then we move on to the vitreous humor.

[00:42:51]It's a jelly-like structure. It helps to maintain the shape of the eyeball and plays a minor role in the refracting light rays. We will talk about that.

[00:43:00]Then we have the aqueous humor. It contains clear fluid that gives shape to the cornea. It also supplies nutrients and oxygen.

[00:43:09]And then we have the cornea. It's transparent, it's curved, the front of the eye, allows light to enter the eye and refracts the light rays which enter the eye.

[00:43:20]Okay?

[00:43:27]Sorry about that. Right. Now we're going to move on to the iris. The iris is the pigmented muscular structure.

[00:43:34]Okay? It's pigmented which means it provides the color, okay? And it consists it's muscular in nature. So, that's where it comes from where I told you Remember I told you we'll talk about how the pupil size is going to change.

[00:43:45]What causes it to change? And that's because this iris has muscles. Which ones do they have? They have inner rings which are called circular muscles and they have outer layer of radial muscles and they regulate the amount of light entering the eye and that's how they basically control the size of the pupil.

[00:44:06]The ciliary body, it contains the ciliary muscle. So, the ciliary body is this triangular part here and within that we have a ciliary muscle and this enables the lens to change its shape.

[00:44:18]And why? Because when this lens the the the ciliary body is going to um uh uh uh contract or it's going to um relax the ciliary muscle. When the ciliary muscle is going to contract or relax, it is going to cause the suspensory ligaments to either become to either slacken or become taut. Taut is basically to become tight. So, these are suspensory ligaments which are attached to the lens and attached attached to the um the ciliary body.

[00:44:50]And um it helps to alter the shape of the lens, right? So, the ciliary body has a function to to alter the shape of the lens. So does the suspensory ligament.

[00:45:00]The pupil is the opening in the iris allowing the light to enter the eye.

[00:45:04]Then we've got the lens. It's elastic, it's transparent, it's biconvex structure and it refracts light and focus it onto the retina.

[00:45:14]Okay?

[00:45:15]So, biconvex. Now we already learned about convex. Remember convex is this one. So, biconvex means bi two on both sides. So, the lens is the structure.

[00:45:25]Okay? This is the kind of structure it it is. Okay? So, it's elastic. It's able to become less convex, more convex. Less convex, more convex. Okay? And then this is controlled by the suspensory ligaments and the ciliary body.

[00:45:41]And it also has a role in refraction. I will talk about that.

[00:45:45]Blind spot. This is where there are no photoreceptions. So, there are no vision can take place here. No vision occurs at the blind spot. The blind spot is this point over here which leads out from leads into the optic nerve. The optic nerve leads from this point called the blind spot. It carries the impulse from the retina to the cerebrum. So, remember I told you that the um the photoreceptors is going you're going to have the impulses, right? The the photoreceptors are here along the retina. They are going to convert the impulse the uh the the stimulus into an impulse and that impulse is going to be taken by the optic nerve to the cerebrum, right? Where the interpretation is going to take place. And then the last point we have over here is called the fovea fovea centralis better know or or or rather known as the yellow spot which is easier for you easier known.

[00:46:39]It has only cones and this is the site for clear vision. So, this is the place that it has cones, it has um many many many many cones and this is where you have the clearest vision that takes place.

[00:46:52]Okay?

[00:46:55]Right. So, before we go into binocular vision, I just want to quickly go back here.

[00:47:01]So, as much as you don't need to understand um how we see, you don't really need to know the whole mechanism behind how the image is created, etc. But what I do need you to understand is that light is going to enter the eye, okay?

[00:47:17]And then you've heard about this word I've mentioned it a few times now when I'm talking about how the light is refracted. What do I mean by refracted?

[00:47:25]So, when I talk about um refract, I basically mean bend, okay? So, the light is going to bend.

[00:47:37]So, remember what I just said. I needed to um to explain all of these terms if I can explain it to you. The light is going to enter the eye. It's going to be bent at different angles to focus onto the retina. Cuz remember now, this light is going to enter the eye. If we didn't have the cornea, the humor, the lens, the light would just all go into the eye and it wouldn't create a clear image because it would just be focused it wouldn't be focused anywhere.

[00:48:08]So, this light is going to refract is going to refract is going to bend in order to hit the retina to create a clear image.

[00:48:17]So, when this light enters the eye and it's going to be refracted and it's going to be bent, the aim is to hit the retina. And when it hits the yellow spot, that is the point of the clearest vision. Okay? So, that's in simple terms how it works. A little bit more complicated than that, but that's the part that I need you to understand in order to understand what you need to for grade 12 life sciences.

[00:48:40]Now what is this binocular vision?

[00:48:43]Binocular vision refers to the ability to focus on an object with both eyes creating a single image. Okay? So, we have we can focus on something with both our eyes, but we see one image. Okay, what do I mean by that? So, if I have an apple in front of me or let's look at this pen. I'm focusing on this pen. But I've got two eyes. It doesn't mean I see two pens. I'm seeing one pen.

[00:49:08]So, that is actually something amazing that we can do and not all animals can do it. Okay? The fact that we are having two eyes that are picking up our light coming into our eye, but the image that is being created or interpreted by our brain is the fact that we are looking at this one image. Okay?

[00:49:26]Our eyes are placed some distance apart creating the ability to observe two separate images. Since each eye um Since each eye forms an image, the two images are combined to form a three-dimensional presentation of the object. Okay. So, as it is saying over there, right? This is the region. Let me show you what's happening. So, here's the original image. Okay? This is the original image.

[00:49:51]Both views are merged in the brain to form a single image. So, the left eye is looking at it, and the right eye is looking at it. So, both of them both eyes are looking at this object. But then what is happening is our brain is going to merge these images.

[00:50:07]And it's going to form a single image.

[00:50:08]How amazing is that?

[00:50:10]Okay? What is the significance of of this? What is the significance of being able to do this? This helps us it with depth of perception, enables us to walk up and downstairs. It gives us a wider field of view. It gives us depth perception.

[00:50:26]Um it and binocular vision helps with performance skills such as catching, grasping, and locomotion.

[00:50:35]Okay. Now, we're going to move on to accommodation. Now, let me remind you what we spoke about. Okay? So, we spoke about the external structure of the eye.

[00:50:42]We spoke about the internal structure of the eye. I did explain to you a little bit about accommodation to say that accommodation and pupillary mechanism are the changes that occur in the eye.

[00:50:51]Okay? To accommodate certain certain things that are happening. So, one of them is accommodation. And accommodation is when the ability to see near and distant vision.

[00:51:02]When we refer to near vision, we we refer to to less than 6 m away from you.

[00:51:07]And distant vision is greater than 6 m.

[00:51:11]Okay?

[00:51:12]Mammals focus by changing the shape of the lens.

[00:51:17]The lens is less convex for distant objects and is more convex for near objects.

[00:51:22]Convex, more convex, less convex. More convex, less convex. So, less convex for distant vision, uh more convex for near vision. Okay?

[00:51:34]So, um you can see the lens is changing here. Look how cute it is. Okay? So, you can see what is happening here. Can you see the arrows? The arrows are showing you that something's happening in the ciliary body. Then the suspensory ligaments are doing something, and the lens is changing its shape because it's elastic. It can do that. Okay?

[00:51:53]So, um accommodation is basically focusing of light on the retina. So, like I said, accommodation is now the lens is going to change shape in order to accommodate either near or distant vision. And it's going to do that by changing the shape of the lens.

[00:52:08]Right. So, let's first look at far vision. Far far vision, less convex.

[00:52:14]So, what is going to happen is the ciliary muscle is going to relax.

[00:52:21]The ciliary muscle is within the ciliary This is the ciliary body.

[00:52:25]But the ciliary muscle is there within it.

[00:52:28]That ciliary muscle is going to relax.

[00:52:31]Muscles relax and contract. They relax and they contract. You should know this from grade 10. Okay?

[00:52:38]So, in this instance when it's far vision, we're trying to see something that's more than 6 m away, we are going to have the ciliary muscle relax.

[00:52:46]And this is going to become cause the suspensory ligaments to become taut.

[00:52:51]Okay? So, it's going to relax. The muscle is going to relax, and it's going to cause the suspensory ligament to become taut or to or to become tight.

[00:53:02]Why do we not say contract or relax?

[00:53:06]Because ligaments don't contract.

[00:53:09]Okay? So, that is the reason why we don't use that word. So, you need to say suspensory ligaments become taut or they become tight.

[00:53:17]The tension on the lens increases. Now, remember you are going to be pulling.

[00:53:21]It's tight. And what did I say about the lens? It's elastic. So, now these suspensory ligaments are going to become tight, and they're going to pull this lens.

[00:53:30]And the tension is going to increase in the lens. But the lens is going to become and the lens is going Lens is now going to be stretched. It's going to become less convex. Not thin, not flat, less convex.

[00:53:42]And the refractive power of the lens decreases because now remember you're making it um you are making it less convex. So, it's going to bend the light less.

[00:53:54]Um the refractive power decreases.

[00:53:56]A clear image is formed on the retina.

[00:54:00]Okay? A clear image is now formed on the retina.

[00:54:05]So, that is for far vision.

[00:54:07]Now, for distant vision. This is when something is now 6 m away.

[00:54:12]So, what's happening here is that um the the ciliary muscle is going to contract. So, now the ciliary muscle is contracting. The sclera is now going to be pulled forward.

[00:54:25]Okay. So, remember that the the the the ciliary body is attached to the sclera.

[00:54:31]So, if the ciliary muscle is going to contract, it's going to now call cause this whole sclera to move forward.

[00:54:39]And when it does that, the suspensory ligaments are going to slacken.

[00:54:44]And when the suspensory ligaments slacken, it's going to become it's going to put less tension on the lens. And then it's going to become more convex. So, remember it was now it's like this, right? Suspensory ligaments are keeping it in place. But when the suspensory ligaments slacken, then it's going to become more convex.

[00:55:03]Ciliary muscle contracts, sclera moves forward, suspensory ligaments slacken, the lens becomes more convex, and then we are also going to have less tension.

[00:55:16]Um the tension on the lens I'm sorry, the tension on the lens is going to decrease now. The refractive power is going to increase. The lens becomes more convex, and the clear image is focused on the retina. So, the lens is going to change its shape, and that is going to cause the diffraction to change.

[00:55:35]Okay? Cuz remember I told you the the bending the bending of the light is going to change because now the lens has changed its shape. Becomes less convex or more convex, and that's going to affect the bending of the light.

[00:55:46]On so that it can be focused onto the retina.

[00:55:51]Okay. So, I hope I explained that well. Um the nice thing about this is that they work opposite to each other. So, if I was you, I would study one of them, either far vision or near vision, and then you know that the other one is the opposite. Okay? Because you actually see over there um the words that are are different. The ciliary muscles relax in the one, contract in the other one.

[00:56:15]Suspensory ligaments become taut or slacken.

[00:56:19]The tension on the lens increases or decreases. The lens either becomes less convex or more convex. And then the refractive power either decreases or increases. So, that's how what I mean by opposites.

[00:56:32]So, you must be able to draw this part of the lens of the the eye. You must be able to interpret diagrams. I can give you this diagram, and I can ask you which type of vision it is. I can cover near vision, and I can ask you what type of vision it is. You need to see those signs of the the suspensory ligaments slackening and the lens becoming less uh more con- vex.

[00:56:54]Right. This is another picture of accommodation. Now, this is showing you the refracting of the light.

[00:57:00]Okay?

[00:57:01]I like this diagram because it can help you to see what I'm talking about here.

[00:57:06]So, here they are telling showing us the ligaments are tight and they tell us the less convex lens. Okay? Like cuz it's pulling it. Remember I said it's elastic. And then here the ligaments are relaxed, so it becomes more convex.

[00:57:17]Now, let's look at this one. Rays from a distant object are focused on the retina. So, this one is the distant object, right?

[00:57:26]Distant object is focused on the retina by a less convex rays, right? And then this one a nearby object, more convex.

[00:57:33]Now, this is what happens with the light rays. This this arrow the um this red line here shows you the refraction of light.

[00:57:41]So, what's happening here? It's going to enter. It's going to be causes some refraction. Remember this is here's the aim. The aim is to hit the retina. So, what's happening is light is now entering the eye in this uh uh in this angle because the lens is less convex. It's going to bend a little bit at the cornea. It's going to bend at the lens, and then it's going to focus onto the retina. It's going to bend at at put at an angle to be able to focus on the retina. But when it's a nearby object, it's going to um the lens the rays are going to be coming this way like this.

[00:58:16]Right? And it's going to then refract again and refract again and to focus onto the retina at this point.

[00:58:24]Okay?

[00:58:27]Right. Now, we go to the pupillary mechanism.

[00:58:31]This is the one. I love this. Look at this diagram. There, pupil's going bigger. Well, not bigger. It's dilating.

[00:58:36]It's constricting. It's dilating. It's constricting. Right?

[00:58:40]And um what does this? Let's talk about what does this. The iris controls the amount of light entering the eye by controlling the size of the pupil. So, as you can see here, in bright light the pupil is smallest. Have a look at this.

[00:58:53]In bright light, the first picture shows us that the pupil is constricted. In average light, you can see that it's not uh it's a little bit bigger. And then in the dim light, it's quite wide. Okay? Um so, we have complete more dilation there.

[00:59:09]So, okay, let's think about why.

[00:59:12]So, when we have bright light when you're in bright light, the reason why the pupil constricts is because uh it allows less light to enter the eye. Because when a lot of light is going to enter the eye, it can damage the retina. But when it's dim light when it is a dim area, you need to allow more light into the eye. So, the fact the the fact that the pupil constricts is to allow less light. And when it dilates is to allow more light into the eye.

[00:59:40]Because now it's a dim space, so you need as much light as you can in order to see.

[00:59:45]So, now that is the mechanism that's the reason behind it. So, now what actually happens here in bright light? Now, let's have a look at the diagram first, okay?

[00:59:53]So, that you can picture what is going on.

[00:59:56]So, in this instance here, we've got the sclera. Let's look at this this this diagram at the top. The white part is the sclera. Let's familiarize ourselves again. The white part is the sclera, the the colored part is the iris, and this black area here is the opening, which is the pupil, okay?

[01:00:12]Now, look at this diagram. We've got the sclera once again. We've got the pupil, and we've got the iris. But this iris looks a little bit different, right?

[01:00:20]Why? It's got radial muscles, and it's got circular muscles of the iris, okay?

[01:00:26]And it's got the pupil.

[01:00:28]So, the ones that are going like this, they are the circular muscles, okay?

[01:00:37]And then the ones that are going like this are the radial muscles. So, if you think about it, you can take the word If this is a circle, I know it's a very lopsided circle, but let's picture that it's a circle. If you have to draw a line from the one edge right across to the other edge, in maths we would say that is the diameter of the circle, right? And if we have to dissect that line in half, we would say that the one portion from the middle to the edge is the radius, right? You should know that from maths.

[01:01:05]So, that is the way way the word radial comes from. So, this is the radial muscle.

[01:01:13]And these ones are the circular muscles.

[01:01:16]Circular circular because they are circle. Very important to mention that that they are of the iris, though. The circular and radial muscles of the iris.

[01:01:25]Now, let's go back to bright light. So, in bright light, the circular muscles of the iris are going to contract.

[01:01:30]Contract. Remember the main aim is for constriction.

[01:01:34]So, the circular muscle is going to contract, the radial muscles are going to relax, and the pupil is going to constrict, allowing less light to enter the eye.

[01:01:44]Now, before we get to the mnemonic here, I like to think of it this way. This is my eye, as we're saying the same picture over here.

[01:01:53]And yet, this is my pupil. I want this to become smaller.

[01:01:57]So, in order for this to come smaller, that would mean that my circular muscles must contract, right? Because when the circular muscle contracts, then this muscle is going to become shorter. And this muscle is going to become shorter. And the outer muscle is going to become shorter. And when all of those are going to contract and become shorter, they're going to cause the the pupil to constrict.

[01:02:23]And the the the radial muscles and the circular muscles work antagonistically.

[01:02:28]They work opposite to each other. So, when the circular muscle is going to be constricting, the radial muscle is going to relax.

[01:02:36]Right. And then, that's for people that are visual, okay? And then, we have mnemonics over here. We have the the bold letters in my paragraph here are being are being um basically put here into this mnemonic. Circular muscles contract. Radial muscles relax.

[01:02:55]Pupil constricts. Less light entering the eye. Isn't that cute, right? C C R R P C L L. My problem is, I'm the kind of learner that will forget the letters in the order the letters come, and then I'm then I'm then I don't know what's going on. Okay? I'm a visual learner, though.

[01:03:14]Every learner is different. I'm a visual and auditory learner. You may be different, okay? You will learn these mnemonics, and then you'll be sorted. If I have to wake you up in the middle of the night and I tell you bright light, you'll tell me C C R R P C L L. No problem. Go for it, okay? That's why we have these steps to help you.

[01:03:31]Then, [snorts] in dim light, this is now when I'm saying our pupil needs to become dilate so that it allows more light to enter the eye, okay? So, now I'm going to draw these again to show you what will happen.

[01:03:51]Radial circular muscles. Now, um the circular muscles of the iris are going to now relax. The radial muscles are going to contract. The pupil is going to dilate, allowing more light to enter the eye. How do I remember this?

[01:04:06]Dim light, I want more light to enter the eye. I need the pupil to dilate.

[01:04:10]The pupil is controlled by these muscles that work opposite to each other. Which muscle is going to allow my pupil to become to dilate? Which one is it? It's the radial muscle. The radial muscle is going to contract. Because when all of these radial muscles contract, remember when the muscle contracts, it's going to become shorter. So, when this radial muscle becomes shorter, it's going to pull this pupil bigger.

[01:04:35]All these radial muscles are going to contract and become shorter, and pull open the pupil so that the pupil dilates. And then, because the circular muscle works opposite, it's going to relax. The circular muscle relaxes, the radial muscle contracts, okay? And once again, we've got the C C R Now, this one is different. C R R C P D M L. This is the circular muscle relaxes, the radial muscle contracts, the pupil dilates, more light enters the eye. Excellent.

[01:05:10]Okay. So, that is basically what is happening in the pupillary mechanism.

[01:05:16]Now, let's go quickly between the differences between accommodation and the pupillary mechanism.

[01:05:23]Okay.

[01:05:24]So, um before we do that, let's just quickly look back at this one. I want you to show you and let's go to the pupillary mechanism. Let's just recap a little bit of what we were doing, okay?

[01:05:36]So, we spoke about the outer the outer layers of the eye. We spoke about the inner layers. We spoke about how light is a stimulus. It enters the eye. How the light gets bent or refracted because of the cornea, the lens, the aqueous humor, so that it can be focused onto the retina. And the retina is the point of uh where the clear image is is going to be formed. And that retina has photoreceptors, which are called rods and cones. They are going to receive the stimulus. They going to convert it to the nerve impulse. And then, it's going to be taken via the optic nerve to the cerebrum, where it's going to be interpreted, okay? To and then the image is going to be interpreted.

[01:06:19]So, what happens then? That's about light in general, okay? So, then you've got two processes that we need you to know, accommodation and pupillary mechanism.

[01:06:30]Accommodation is the change in the lens of the eye in order to see distant or or near vision.

[01:06:36]Okay, near vision is less than 6 m, and distant vision is greater than 6 m. Then we've got and and that and the structures and or parts involved there are the lens, the suspensory ligaments, and the ciliary body with the ciliary muscle, okay? And then, what are the parts that are involved with the pupillary mechanism? The pupillary mechanism is the one that regulates the amount of light entering the eye, and it it is the pupil is the one that is going to be changing its shape, and it's controlled by the muscles in the iris, the circular and the radial muscles of the iris, okay? And now, you also going to need to know how to do um application questions of this. So, they might not just ask you um explain dim vision. Explain what would happen to the eye in dim vision.

[01:07:28]Name the process and explain what would happen in dim vision. Yes, they could ask you that. So, you're going to say pupillary mechanism. You're going to say that the um radial muscles contract, the circular muscles relax, more light in the pupil dilates, more light enters the eye um to be focused onto the retina.

[01:07:46]But, they could also ask you um a scenario. They can say, "Okay, you were in a >> [clears throat] >> a dim room, um and then you step out into the bright sunlight, okay? How is your eye going to adapt?" So, now you must know that you're talking about pupillary mechanism and not accommodation. Accommodation is about near or far vision.

[01:08:07]Distant or near vision. Pupillary mechanism, the amount of light entering the eye. So, now you're going to say pupillary mechanism, and you're going to talk about what happens in the bright light.

[01:08:17]In life sciences, when you are talking about accommodation and pupillary mechanism, and they give you a scenario of "I was in a dim room, and now I'm waiting to a bright room," you are going to explain what happened what hap what's happening to your eye in the bright room. You're not going to say, "This is what happened when I was in the dim room, and now it changes to what's going to happen in the bright room." No, you will get zero. You will explain what happens in the bright room. If it was accommodation, they were saying that you were reading a book. I'm now reading a book, less than 6 m, near vision. I look up to the TV, more than 6 m, distant vision. You're going to talk about the distant vision.

[01:08:54]So, let's look at this other question that they can ask you.

[01:08:58]Why I like this table is because it helps you to distinguish or differentiate between accommodation and pupillary mechanism.

[01:09:07]So, what is the stimulus with accommodation? It is the change in the distance of the object from the eye, and the pupillary mechanism is the change in the light intensity. So, the stimulus here for accommodation is the distance of the object from the eye. So, this is the eye if it is less than 6 m or greater than 6 m. That is the change the distance that they are talking about.

[01:09:30]When they say change in light intensity, they're talking about the amount of light that's coming in.

[01:09:35]The parts involved um accommodation, we spoke about it, lens suspensory ligaments, the muscles, the pupillary mechanism, it's the pupil and the radial and circular muscles of the iris.

[01:09:46]What are the main changes in accommodation? It's the shape of the lens. In pupillary mechanism, it's the diameter of the pupil.

[01:09:54]What brings about the change? In accommodation, it's the suspensory ligaments and the ciliary muscles. And in pupillary mechanism, it's the radial and circular muscles.

[01:10:04]Okay, now we're going to do a question, an application question.

[01:10:10]The diagrams below represent the pupillary mechanism in the human eye.

[01:10:14]So, they are telling you now that it's the pupillary mechanism and they are telling you that it is the eye because there are other structures that could look a little bit like this, right? In life sciences. I mean, we have a lot of structures that could look a circular in nature. Because we are doing this topic, you will you will know that it's the eye. But if the the question will also give you information when it comes to these types of diagrams.

[01:10:35]So, now we need to identify the part X and Y. So, X it is what you think it is.

[01:10:42]It's the pupil and Y is the iris.

[01:10:47]Now, before we go into detail, let's see now what's happening here. They're showing us an arrow. And they're also showing us diagram A and diagram B.

[01:10:54]Let's quickly look at what's difference between diagram A and diagram B. Diagram diagram A, the pupil is constricted.

[01:11:00]Diagram B, the pupil is dilated. Diagram A, the iris, you can see it's a larger area of the iris there, which means that the radial muscles are relaxed but the circular muscles are contracted. And then we've got the um diagram B, the iris, you can see it's a thinner layer.

[01:11:17]The radial muscles here are contracted and the circular muscles are relaxed.

[01:11:21]Explain why the pupillary mechanism is considered to be a reflex action. Oh, this is a nice question. So, why is it a reflex? And it's because it's rapid.

[01:11:31]It's involuntary and it's a response, it's an involuntary response to light.

[01:11:35]Okay.

[01:11:37]Name the two effector muscles that are involved in the pupillary mechanism. So, these are the radial and the circular muscles. Yes, they are the effector muscles because they are effecting the change of the pupil size.

[01:11:49]Explain the significance of the change in diameter of X from diagram A to diagram B. So, this is one of the things that I'm talking about now. They want us to do explain the significance of the change from a diagram A to diagram B. So, you talk about what's happening in diagram B. The pupil dilate is dilated. More light is going to enter the eye to improve and what what what is the significance of this?

[01:12:15]To improve vision in dim light. So, can you see the kitchen the question is twofold. They're asking you to explain the significance of the change. So, first you need to identify the change.

[01:12:26]So, this point over here identifies the change. The pupil is dilated so that more light enters the eye. And the explain part tells us it must be cause and effect. So, what is the cause? The pupil is dilated. What is the effect of the cause? Light will in more light will enter the eye. And then what is the significance of this? More vision especially in dim light.

[01:12:51]Then we move on to another activity. The diagram below represents the structure of a human eye once again.

[01:12:58]Identify parts A, C, and D. Okay, so let's look at this diagram. Let's identify more than just those parts, okay? Why do we have to identify only those parts? Now, they're showing us the iris, okay? That was nice of them. Then we've got C. What do you think C is?

[01:13:14]That outer layer so this is is the cornea.

[01:13:19]Um they did not label this part over here, but we can label it. It's the pupil.

[01:13:25]They also did not label this part over here. It's the lens.

[01:13:29]This one is the yellow spot.

[01:13:36]And then D is the sclera. It's the outer layer. Remember the sclera inelastic tough white.

[01:13:44]Then A is the blind spot.

[01:13:49]This is the optic nerve.

[01:13:54]Oh, they do have B. B is the lens.

[01:13:58]Then [snorts] here we have the ciliary body. The round circle in the middle is a ciliary muscle. We have the suspensory ligaments and then we have the lens.

[01:14:07]Okay, so I think we've labeled that quite well. So, let's see. What is A? A is the blind spot. We said that. C is the cornea and D is the sclera.

[01:14:19]Describe how the muscles in the iris enable a person to see in dim light. Oh, lovely. So, this is just asking you to explain dim light, but they're asking you for the muscles, right? Need to talk about the muscles. The radial muscles contract, the circular muscles relax, more the pupil widens or dilates, and more light enters the eye. You can say widen, dilate is the proper term, but you can also say widen. You can't say bigger, you can't say larger. It's either dilate or widen.

[01:14:48]Name the process that occurs in the eye when a person focuses on objects at different distances. That's accommodation.

[01:14:55]Explain how the shape of part B enables a person to read a book. So, here we have part B, which is the lens. How the shape is going to enable a person to read a book. Lovely. So, if you're reading a book less than 6 m, which means that it is um near vision. When we have near vision, what happens to the lens again? It becomes more convex. Yes.

[01:15:19]So, if you start for me, if I for me, life science is about understanding. So, if I can understand that, I must ask myself the ques- the prompting questions that I'm asking now are the prompting questions you can ask yourself, okay?

[01:15:34]Um what happens when I'm reading a book?

[01:15:36]Is it near or distant vision? It's near vision. Right. What do I know about near vision? What happens to the the lens? It is more convex. What causes it to be more convex? The suspensory ligaments are going to slacken, right? Why do the suspensory ligaments slacken? Because the ciliary muscles are going to contract. Okay. So, that is how you can walk yourself through it. So, it's more it becomes more convex so that light rays are um refracted. Okay, now in this instance, they want to know explain the shape of the lens.

[01:16:06]Okay, so they just want to know about the shape of the lens and to explain how it's enabled you to read the book. So, you've now identified that it is convex.

[01:16:15]If they had asked becomes more convex, if they had asked you for that mechanism of accommodation, then you would explain the whole thing about the ciliary muscle contracting, suspensory ligaments become relaxed, slacken like that.

[01:16:28]But now, you just need to talk about the shape of the of B. It's more convex so that light rays are refracted more to focus on the retina to form a clear image on the retina. Okay. Or to So, it's either to focus on the retina or to form a clear image on the retina. Okay.

[01:16:49]So, it becomes so that light rays can be refracted more.

[01:16:55]Okay, now we move on to the defects of the eye. Okay, so every you can look over here. Oh, shame. This person has blurry vision. It's like seeing double, right? This is what they mean when they see say seeing double.

[01:17:09]Okay.

[01:17:11]Now, we move on to short-sightedness, which is myopia.

[01:17:16]Okay? So, um short-sightedness or myopia, it is when you have um you can see This is something that is confusing, but the name is actually self-explanatory.

[01:17:31]But people seem to think that when you are short-sighted, it means that you can't see near. But it is short-sightedness, the name says it. I can see near. I cannot see far. So, short-sightedness means that I can see from near, but I cannot see far. So, look at this picture. This phone that this man is holding, it's close to him.

[01:17:51]So, if he looks at the camera, it's close to him, he can see the picture.

[01:17:54]It's close to him. Can see near vision.

[01:17:57]But he can't see the people behind him look blurry because he can't see far.

[01:18:02]So, what is happening here? So, this is a defect. So, remember what we said was that when it is less convex, the light is going to be refracted more. Okay? So, now what's happening in this instance is that we have our lens, but the image is not falling on the retina.

[01:18:19]The light rays are going to enter.

[01:18:21]Refraction is going to take place, but because this lens cannot become more biconvex or sometimes it's the shape of the of the um It's can be two reasons why. It can either be that the lens is unable to become more convex or the eyeball is too long or the cornea is too curved for the length of the eyeball. So, that affects the angle of the refraction. And when the angle of the refraction is is um is affected, it's going to now cause the image to fall in front of the retina.

[01:18:57]Not going to form on the retina. Can you see there? The the focused image is formed here. So, the light rays are going to be scattered along the retina and you're going to see blurry.

[01:19:06]So, the the the focus the image focus falls here.

[01:19:11]Okay. So, that is the reason or that's what happens here. Now, um how do they fix it? So, that was the de- defect, okay? But now, how do we fix this problem? So, remember now, it cannot become um a convex. So, what we do is we're going to put a concave lens. Can you see it's biconcave? Because it's going in this way and in this way. It's biconcave.

[01:19:36]Okay? It's a biconcave lens. Light rays are refracted outwards on the retina.

[01:19:43]So, the light is now going to enter.

[01:19:45]It's going to add a layer of refraction.

[01:19:48]It's then going to refract This this lens, this concave lens, is going to allow the refraction, the bending, to to be at the correct angle in order to bend in the cornea, bend in the lens. So, nothing is changing here.

[01:20:05]This is being introduced to bend the light so that when the light bends at these other points that it's normally bending to, it will focus onto the retina.

[01:20:19]But, when it is long-sightedness, this is a person they can't see close objects. Okay, they can only see far.

[01:20:26]And this is because the distance between the lens and the retina is too small.

[01:20:31]The eyeball is maybe too short.

[01:20:34]Um and also the lens can lose its elasticity. So, it cannot become convex enough. Okay? So, what's happening here is that um the light rays are once again entering, but the image is going to be focused past the retina. Okay? So, how do we do this? We give a convex lens.

[01:20:56]Right, so we put another biconvex lens cuz remember this lens is not working properly or this eyeball is is now too short. So, what's happening here? The light is now going to be refracted inwards to fall on the retina. So, previously it was going outwards. Now, it's going to be going inwards and then the light will be bent and then it will focus on the retina.

[01:21:18]Right, what is astigmatism?

[01:21:21]Okay, so that is now got to do with far-sightedness and near-sightedness.

[01:21:24]Now, that is you're going to put a corrective lens. So, the lens is basically going to add a layer of refraction. The refraction is still going to happen in the cornea, the lens, and the aqueous humor. But, it's going to add an added layer of an added point of refraction. It's going to now refract at the correct angles in order for it to focus onto the retina.

[01:21:45]What is astigmatism? This is when the cornea or the lens is not equally rounded in all directions. Okay, so now this is when we have a little few wobbles here and there.

[01:21:54]It leads leads to blurred vision, headaches, and squinting of the eyes.

[01:21:58]Treatment may include spectacles, contact lenses, or laser therapy.

[01:22:04]So, let's look at the astigmatic eye.

[01:22:06]So, what's happening here is that light is entering the eye.

[01:22:10]Um but we have multiple focal points. Can you see that?

[01:22:16]Because of the unevenness. So, it's not smooth. It is not even. So, now the light is going to be bending and it's going to add another layer in the inside of the eye. And now we're going to have one going this way, one going one one going that way. So, we have multiple focal points and therefore we can see this blurry vision.

[01:22:36]So, there's a question on astigmatism.

[01:22:39]The diagram below represents the structure of a normal eye and that of an eye with astigmatism. Okay, so here we have a normal eye. As you can see over here, we have to label the diagram.

[01:22:50]Let's look at label eye so we can see what's happening. So, we have A.

[01:22:55]Um A is the aqueous humor.

[01:23:07]B is the pupil.

[01:23:09]C is the iris. Okay?

[01:23:13]And then just for good measure, let's also label this one. It's this this is the ciliary body. This is the ciliary muscle inside.

[01:23:22]We have the suspensory ligaments. We have the lens.

[01:23:28]Now, what's happening in the astigmatic eye? Ooh, look at this cornea. Can you see this cornea is evenly curved?

[01:23:34]Curved, curved, curved, gentle curve.

[01:23:36]This one has a point.

[01:23:39]Mhm, because it has astigmatism. Right?

[01:23:43]So, now let's answer the questions. What is fluid A? It is the aqueous humor. The way I remember it is if we have to look at the the front of the eye has the A because in the alphabet A comes before V. So, the this part here is the aqueous humor and the part on the inside is the vitreous humor.

[01:24:02]So, when I'm labeling a diagram, I just put a quick A there and I put a V there so that I know. Aqueous humor, vitreous humor.

[01:24:10]Part B is the pupil and part C is the iris.

[01:24:15]Describe the appearance of the cornea of an eye with astigmatism. It is unevenly not equally curved.

[01:24:22]Explain how astigmatism affects vision.

[01:24:25]Explain is cause and effect.

[01:24:31]Okay? So, how does it affect your vision? So, as I explained now, we need to explain it. So, you need to first basically say that the light is going to be now refracted unevenly. So, remember I said the cornea is uneven. So, that now is going to cause the light to be refracted unevenly. It does not focus on the retina and it forms a blurred image.

[01:24:52]Okay? The light is refracted unevenly.

[01:24:54]It goes in different directions. It is not focused onto the retina and it forms a blurred image. So, where is the explain part here? The cause is the light is refracted unevenly.

[01:25:06]What does that cause? It causes the focus to not be on the retina. And what is the effect of that? Blurred vision.

[01:25:14]Give one treatment of astigmatism. Laser surgery or spectacles.

[01:25:20]Cataracts. Okay, this is a disease of the eye. So, the defects you need to know is um near short-sightedness, far-sightedness, and um astigmatism. But, the diseases of the eye, cataract. It's a clouding form clouding forms in the lens due to the denaturing of lens protein. Obstructs passage of light caused by age, chronic or chronic exposure to UV light, or due to trauma. And how can you treat this?

[01:25:50]During surgery, the lens is removed and a synthetic lens is inserted. Okay? So, you can see there you can see that clouding that forms, right? So, you can imagine if there's a clouding there on the lens, then that means that the pupil now is still going to be a hole in the eye, but the reason why you say that see that is because the lens is now cloudy.

[01:26:09]It's not transparent. So, the reason why if you look we only see dark. Um the pupil is the opening, but the lens is transparent. So, we cannot we do not see anything, but when there's a cloudiness there, we will see.

[01:26:24]Okay, so thank you. That is all from um us for today um with regards to the eye.

[01:26:31]We can quickly just go through a quick recap again on the eye. Let me quickly go back here. Okay, so just to recap what are we talking about here? Remember I said this is part of human response to the environment. Um so, we have the human eye is only a part of it. Last week we did the um nervous system. Like I said, now we are doing this one organ of the which is called the the human eye. You need to know the external structure, the internal structure, what is binocular vision, the importance or significance of binocular vision. You need to know accommodation and pupillary mechanism, the difference between them, the structures that are involved, how do they work, also how to answer application questions. As you could see in the ones that we did, there were some application questions over there. It talks about reading a book. Okay? You can see different ones like that. Um you can also get scientific investigations here.

[01:27:26]Um I've seen the eye being asked throughout the paper.

[01:27:31]It's either in paper in section A as a multiple choice question, or sometimes it is just like a 1.5 or 1.4 question where they give you the eye and they ask you low order questions, just labeling or provide the function. I've seen it in question two and question three where they've got more complex questions, um more longer questions on pupillary mechanism or accommodation, um comparing uh accommodation pupillary mechanism. So, there are many different places or many different ways to ask the eye. But, like I said, it's not always guaranteed that you're going to get many marks on the eye. Some learners are so disappointed at the end of the year, "Oh, we didn't even see a picture of the eye." But, the it will be there somewhere, but it's not always just like that. Sometimes it's the ear, sometimes it's the eye and the ear, sometimes it's the brain. Okay? You don't know which part you're going to get. It's very important that you don't underestimate the section and that you study all the different parts.

[01:28:27]Lately they've been they love to ask about the diseases of the eye, the defects of the the eye defects. They like to ask those questions. They like to ask you how you can correct it and explain how it's working. That's why I like to show you those pictures to show you that it's about the refracting or the bending of the light that we are using now. The either the concave lens or the convex lens. Okay? So, please make sure that you understand this. You go through it. You do a lot of questions.

[01:28:54]Um you will see that your your teachers have received the PowerPoint presentation as well as the worksheet.

[01:29:00]Please go through these questions on your own time and you can um and do as many questions and past paper questions as you can on this section.

[01:29:10]So, please enjoy the rest of your day and enjoy your long weekend. I will see you next week where I'm going to be doing the human ear. Okay, that then that will bring us to the end of the nervous system and then we move on to the next topic the following week.

[01:29:26]All the best. Keep safe.