 Let's summarize this entire chapter on the right side. You can see the different subtopics and The times at which they begin if you want to jump to a specific subtopic. All right, let's start with the mind map So human eye and colorful world consists of two topics the human eye and the colorful world in human Eye we first study the structure of the eye This is where we learn about the different parts and their functions and one major function is Accommodation very frequently asked question. That's why I'm highlighting that the second major topic in human eye is Learning about the different defects how to correct them and this is where you can expect your numeric calls to come from Now the colorful world can be divided into three parts. The first one is atmospheric refraction basically seeing how the atmosphere Refracts light famous example for that is how stars twinkle the second topic is dispersion This is the colorful part of the colorful world and this is basically white light splitting into seven colors And the example of this would be rainbows and finally we learn about scattering, which is not the same thing as dispersion So, you know, I used to get confused over here a lot But this is the one of the one of the reasons why sky appears blue to us So with this broad overview we can now start with the first topic. Let's start the structure of the eye The outermost layer of the eye the bulged part is called the cornea inside which there is a liquid called the aqueous Humor the word aqueous means water. It's kind of like water together these are responsible for Most of the refraction that happens in our eye most of the bending of light in our eye happens over here the next part is this disc strafe structure, which is called the iris and at the center of the iris There is a hole, which is called the pupil the main job of the iris is to control the size of the pupil When there is a lot of light the iris makes the pupil smaller and when there is less light the iris makes the pupil larger Then comes the crystalline lens. It's called crystalline because the lens is made of proteins called crystalline That's the reason it's called crystalline lens. What does it do? Well, the remaining refraction is done by the lens Basically, its job is to focus the rays of light on to the retina This brings us to the retina the retina is the back part of the eye think of it as the screen It contains all the light sensitive cells. So when light falls on it these cells convert light Into electricity and then that electricity is sent through a bunch of wire like like Nerves called optic nerves and these optic nerves send that electricity to the brain and finally your brain receives these signals and says ah, I see something Two more important parts one is the liquid that fills up our entire eye It's called the vitreous humor the word vitreous means glass basically it's saying that the liquid is transparent like glass and its main job is to maintain the Spherical shape of our eye without this liquid our eye can easily get squashed by the weight of all the stuff that comes on top of it Now over here. I had a confusion of which humor goes where how do I remember that? So the way I started remembering it is I imagine that my entire eye is made of glass This way the liquid that fills up my eye is the not aqueous humor because aqueous means water ah The other one vitreous humor. That's how I remember that this is vitreous humor And so the other one has to be aqueous humor that that basically helps me and finally the the muscles that hold our Lens our crystalline lens in place are called the ciliary muscles and their job is Accommodation Since this function is a little detail will talk about it separately and so this now brings us to the next topic Accommodation Feel free to pause and take notes if you need and of course these notes quick notes are provided on a summary article on our website Khan Academy website So it's clear all those things out and keep the important ones. So what exactly is accommodation? Accommodation is the eyes ability to adjust the eye lens Refracting power What does that mean? Imagine you're looking at something far away then the rays of light will be parallel and now to see clearly The lens has to focus those rays onto our retina. I'm ignoring the refraction done by the cornea just to keep the diagram very simple This means that the incoming light has to be bent by this much amount That's basically what refraction is all about right now Let's see what happens when that object comes closer as the object comes closer Notice that the incoming rays become more divergent and now to again bend it focus it onto the retina The bending required is more That means now the refracting power the bending power required by our lens is more So again, if the object come object goes far away Look the bending power required is less if the object comes closer the bending power required is more So our eyes adjust this refracting power bending power depending upon how far the object is and that's what we call accommodation But how does it happen? It happens due to the ciliary muscles when the object comes closer the ciliary muscles kind of push on the lens Making it bulged more curved more curved meaning more refracting power This is how the power is increased and of course when the object goes far away The eye lens relaxes the ciliary muscles relax decreasing the refracting power and the object comes closer It bulges increases increasing the refracting power. This is accommodation brought to you by ciliary muscles But there's a limit to this you see as the object comes closer the ciliary muscles keep pushing on our lens, right? But after a point, they just can't squash it anymore So that means let's say this is the limit So that means if the object comes any closer and the lens will not become any more bulged And so the rays of light will no longer get focused onto our retina That means this Represents the nearest point to which an object can come and we can still focus it onto our eye So this point is called the near point of our eye So if the object comes any closer than the near point, we will not be able to see it clearly and The average near point of human eye is about 25 centimeters. It's good to remember this number So a near point is about 25 centimeters. What about the far point? Well as the object goes farther away, then we can easily focus it on our retina All we have to do is just relax our eyes and the farthest we can see we can see the sun moon or you all the way up to Stars so the farthest things we can see is virtually infinity meaning our far point is infinity Makes sense, right now. This is true for a normal Undefected eye, but what if our eyes are defected so that now brings us to the next topic defects in our eye So there are three major defects that we need to study the first one is called myopia meaning near sightedness This means that a person can see things which are near to him or her for example Let's say a person can see things closer than 50 centimeters Meaning any object that comes over here that person can see it. Let's say I'm gonna put it as green But let's say that person cannot see any object clearly, which is farther than 50 centimeters So anywhere over here. I'm putting red that person cannot see it clearly Then this person is myopic because he can see things close to him So this means for this person 50 centimeter is the farthest that person can see So you see that person's far point is not infinity that person's far point is 50 centimeters So you see for a myopic eye the far point is not infinity the far point gets shifted The second defect is the exact opposite of this we call hypermetropia or far sightedness Meaning that person can see things which are far away For example, let's say there's a girl who can see things farther away than 50 centimeters So any object is farther away than 50 centimeter. She can see it properly But let's say that person cannot see any object closer than 50 centimeters. So that's right for her Let's say then she is hypermetropic. She's far sighted now because 50 centimeter is the closest that she can see This means her near point has been shifted her near point is no longer 25 centimeters. It's 50 centimeters So for a hypermetropic person, then near point gets shifted. The far point is fine and If a person has problem with looking both the far away objects and closer objects Basically both their far point and near point have been shifted. Then we will say they have press biopia. This usually happens with old age So what do we do when we have these defects? Well, we have to correct them and this brings us to the next topic correction of the defects To correct these defects we need to first know what causes them. So let's start with the first two Turns out one of these are caused by the eyeball elongation The other one is caused by the eyeball shrinkage, but which one causes which This could be confusing to memorize. So let's think of this logically So what I like to do first is think about start with your normal eye Remember if you have a normal eye, then when you're looking at things far away Your eyeballs are very relaxed and they are the extremely low power And when you're looking at something very close to you your eyeballs are extremely stressed And because they are the highest power Okay, now let's take the same eyes and let's elongate those eyeballs and see what happens If we elongate them, you can see the rays of light tend to get focused in front of the retina to see the object clearly We need to refocus it back on the retina. How do we do that to refocus it back on the retina? We need to reduce the amount of bending. Oh, that means we need to reduce the bending power of our lens And that can be done very easily over here. You see here my lens is already at super high power They're extremely stressed. So if I just relax a little bit I can reduce that bending power and refocus it back on the retina And so I can see this object very clearly. No problem But over here I cannot relax anymore because my eyeball is already relaxed most relaxed position lowest power It's minimum power. I cannot relax any further Oh, that means over here. I will not be able to refocus it back on the retina So if my eyeballs are elongated, I can see things close to me. Ah, that means I am near sighted So eyeball elongation causes near sightedness And because of this when I'm looking at things far away, the rays of light get focused in front of the retina All right. So how do I how do I correct this? So to reduce the bending power? That's basically what we want to do. I have to use an external lens Now if I use a convex lens, it adds more converging power So I'm going to use a concave lens that reduces the bending power reduces the converging power And if I use a suitable focal length, then I can refocus it back on the retina Okay, so next up what happens if my eyeball shrinks Well now what happens is well now there is a flight tend to get focused behind the retina Now again to see it properly. I need to refocus it back on the retina To refocus it back on the retina. I need to increase the bending power. Does that make sense? I need to bend more to refocus it. In which case can I do this? Look over here, my eyeballs are super relaxed I can easily just stress a little bit and increase that bending power and I'm fine. I can see it clearly But over here, I can't stress it more because I'm already stressed out. I am at the highest power Oh, that means I can't see this. So you see if my eyeball shrinks, I can see things far away. Oh, that means I am farsighted This is how we can logically see in which case you get nearsighted or farsighted So over here because I'm unable to increase my bending power naturally All I have to do is add an external glass which glass will I add a convex or concave? I need to add more bending power more converging power, right? So I'm going to add a convex lens Another reason for this could be your eyeballs might be fine But your lens itself might have unusually high power and unusually low power Again, don't memorize think about this over here because the rays of light are being focused in front of the retina We can guess that if the eyeballs Shape is fine. Then maybe the lens has an unusually high power That's why it's bending too much focusing the rays of light in front of the retina That could be another cause for nearsightedness and again to reduce that power We're using a diverging lens a concave lens Similarly over here if your eyeball shape was fine Then another reason could be maybe your eyes don't have enough bending power And that's why they're not able to bend it and focus it on the retina It's getting focused behind and so to add more bending power more converging power We are adding a convex lens So if you practice learning it this way logically, you don't have to memorize anything and less chances of confusion Finally, what causes press biopia that is caused due to the loss of accommodation Basically your ceiling muscles wear out and your lens cannot accommodate any further. This happens due to old age It can happen to anyone And so how do you correct this now since our eyes are unable to see far away objects and very close objects as well We need a combination of these two lenses And that is done by building something called a bifocal or at least that what that's what we used to build before So bifocal means combination of two lenses So when you want to look far away, usually look straight ahead And this is why the top part of the lens corrects for far away objects And then you want to look something close like you know, usually reading a book usually look down And so this so the down part contains the correction for close objects Okay, now that we have corrected the defects the next question could be What should be the power of the lens that we should recommend for our patients? And that now brings us to the numericals of this chapter Numericals are usually on the correction of near sightedness and far sightedness So for example, let's say the question says the far point of a person is 80 centimeter Find the nature and the power of the lens required to correct it. This is actually your ncrt problem How do we solve this? So first let's draw a quick diagram This means that the person cannot see anything farther than 80 centimeters because it's far point But he can see closer than 80 centimeters So how do I figure out what is the nature and the power of the glass required? Again, we don't have to remember anything. We just think of it logically The way I had to think about it is Our glass should be in such a way that if an object is kept at his actual far point Okay, because we are correcting for his far point remember for at his actual far point What is the actual far point infinity? Then its image should be formed at his far point. Does that make sense? Then we will be correcting his far point. What is his far point? 80 centimeters So now that I know what the object distance and the image distance is I can just go ahead and use the lens formula figure out what the focal length is or what the power is And then the sign I have to use this with the sign So the sign itself will tell me whether it's a converging lens or a diverging lens So this is the way I like to solve this problem Similarly, if you had another problem, let's say this point this time It said that there's a person which is near point of 80 centimeters same question What is the nature of the lens and the uh power of the correcting lens? now That person can see Anything farther than 80 centimeters So his far point is fine, but his near point has been messed up So we need to correct for his near point now. We do the same logic Now we say look if I kept an object That is actual near point because we are correcting for the near point this time, which is 25 centimeters Then the glass that I'm using should be in such a way the lens I'm using should be such a way that the image Should now be formed at his near point. That is 80 centimeters So again, we know the object distance imaginations use the lens formula Figure out what the focal length is the power is and then the sign will tell us what is the nature of that lens This wraps up our human eye and we now go to the colorful world. This brings us to the next subtopic atmospheric refraction The first consequence of atmosphere refracting light is twinkling of stars. Why do stars twinkle? Light from the star bends multiple times in our atmosphere before reaching our eyes and because because the atmospheric conditions are continuously changing This part taken keeps on changing slightly with time. What does this mean? This means that the amount of rays of light that enter our eyes that also keeps changing with time And as a result The brightness of the star that we keep seeing that changes with time causing it to twinkle The second consequence we have to study is that we can see the sun even when it is below the horizon How this works is consider a couple of rays of light from the sun Which hits our atmosphere these rays of light would have missed our eyes But because it enters into our atmosphere a denser medium There is bend towards us and now we can see them and our brain thinks that these rays of light comes from somewhere over here And therefore we see the sun Above the horizon and this is why even before the sun has risen or even after the sun sets We can still see the sun above the horizon Next step dispersion Dispersion refers to the splitting of white light into its constituent colors when it passes through a prism And to understand how this happens. We need to first look at how a prism works Let's ignore colors for a minute and shoot a ray of light onto this prism Then it enters into the denser medium and so the ray of light will bend towards this normal And then it exits the prism and enters into the rarer medium and therefore this time the Ray will bend away from this normal And so the ray bends twice towards the base of the prism And so the important thing to note is that the emergent ray has changed its direction compared to the incident ray. It's not parallel to it Okay, so how does this give us colors? First people thought that the prism was just magically creating colors But then it was newton who argued that uh-uh the white light already had the colors in it The prism was just splitting them I've only shown three colors here just to keep the diagram simple But what causes this newton argued is that it's because different colors bend different amount The red color bends the least the blue or the violet you can think of bends the most And to prove his point newton kept a second similar prism but inverted and this time he saw that white light emerged out Why do we get white light now? Let's see when I keep the second prism These rays of light will bend the other way around they will bend towards the base of this prism So that will cancel out this bending and as a result We will see the rays of light over here end up traveling straight And that kind of makes sense because when you go from this prism to this prism There is no change in the medium and so the rays would just travel straight And by the way, this is where your ncrt has done a big mistake in your ncrt They have assumed that the rays of light will just combine over here to give you white light That's not what's going to happen. Like I explained the rays of light will just go straight. They're definitely not combined Anyways, now we'll see when it exits the prism These rays of light will all be parallel to each other. They will all be parallel to the incident ray Basically the deviation of this prism is cancelling out the deviation of this prism making the emergent ray parallel to the incident ray Anyways, so if the rays are not combining, why do I see white light over here? That's because in reality, you don't have just one ray of light if you consider another Similar ray of light now you will see that the colors of this ray will overlap with the colors of this ray And so if you have lots and lots of rays all the colors will overlap And as a result when you look from here, you will see white light That's the actual reason and why doesn't this happen if I just take a single prism? Well, here if I take another ray of light, sure it feels like they are combining and overlapping over here, but Because the rays are bent the different colors are bent differently You see if I go far enough the colors eventually separate out and that's why I do see colorations over here And so the important thing is over here. The rays are all parallel So they overlap and that's why we see white over here. They are not parallel and that's why they separate out giving us colors Now an application of this prism can be seen in rainbows When white light from the sun enters a raindrop, it acts like a prism and splits it into its constituent colors Disportion most of that light actually exits that raindrop, but we're not interested in that light So let's get rid of it. The remaining light gets internally Reflected and then exits it from this side And so when you look from here, you get the rainbow So in a rainbow, you have two refractions one here and one here and one reflection And again, your ncrt has done a couple of mistakes in the drawing First of all, you can see the red must be at the bottom when it exits And the violet must be or the blue must be on the top. They have reversed it over here And they have shown that after exiting the red and the violet combines to give you a rainbow That's wrong. You can see that the red and the violet actually separates out just like in the prism So then how do you see the entire rainbow? Well, a single drop doesn't give you an entire rainbow For example, if you're looking at this drop from here, then you see only the red light from this drop enters your eye So maybe this drop will only look red to you But a similar drop somewhere lower in the sky from that blue light will enter your eye And so that drop will look blue to you And so a rainbow is formed by lots and lots and lots and lots of raindrops You will always see that the red is on the top The blue or the violet will always be at the bottom And that brings us to the final topic scattering of light When light passes through a bunch of particles like say dust particles in the air or maybe water vapor or some colloidal particles These particles will reflect that light in all the direction This is what we call scattering. So you see it's very different than dispersion Even though the names are very confusing dispersion is all about splitting the white light into constituent colors Scattering is where the light gets reflected in all directions. We don't call it reflection in all directions. We call it scattering And now since these particles are scattering red light in all the directions They these particles will glow red when you look at them from any direction And this is why when I shine my laser through water which has a little bit of milk in it We can see the beam of light That's basically because the milk particles in the path of that light are scattering that light in all directions And that's why they're glowing red This phenomenon of seeing the beam of light due to scattering is called tindall effect Our sky looks blue also due to the same reason When light hits an atmospheric particle like say oxygen or nitrogen They have a tendency to scatter blue light the most and the red light gets scattered the least And of course the other colors in between And so during the daytime when the white light from the sun enters our atmosphere Almost all the particles scatter blue light everywhere and that's why our atmosphere looks blue That's why the sky looks blue This also means from the incoming white light a lot of blue is gone And so the remaining combination of the colors look yellowish to us And that's why when we look in the direction of the sunlight the sun looks yellow to us And the same thing happens during a sunrise or sunset as well Now the light passes through a much larger region in our atmosphere and therefore almost all the blue gets scattered out But not just that all the green and yellow They also do get scattered out because there are lots of atmospheric particles over here compared to over here And therefore the only light that survives in the incoming light Is red or orange and that's why now the sun looks reddish or oranges to us And so since red light gets scattered the least they can travel longer distances And this is why the stop signal or usually the danger signals are denoted by red color And that my dear friends summarizes the entire chapter. Yay