 Okay, please give your attendance in the chat box. I'll start the class in its two minutes. Please. Okay, great. Okay, so let me start the class. So today's topic is optical instruments and it is one of the most important topics of grade 10 board examination. In this particular topic, we are going to discuss a lot of things which includes optical instruments and human eye. So I have included human eye first. What I'm going to cover in human eye is, I'm going to discuss about different parts of human eye and their functions, how the image is formed inside the human eye, how that image is transmitted from human eye to the brain. Now, what are the functions of different parts of human eye? Apart from this, I'm also going to discuss different defects which occur in human eye and what are the correction measures which are taken to overcome those eye defects. After discussing this part, I will move to optical instruments and I'll be discussing majorly three different instruments. So that would be simple microscope, compound microscope and astronomical telescope. So these three particular instruments I have selected as for the syllabus of class 10th board. After discussing astronomical telescope, I'll be just wrapping up the session. I'll see if I get time to discuss a little bit of diffraction and all those things, but having said that, primarily my focus would be to discuss with you human eye and the three kind of optical instruments, two microscopes and one telescope with you. So now people are joining up now. So let me start the class with discussing human eye Okay, so now look at here. So to discuss parts of human eye, I have taken this diagram. What I'm doing is once again, what I'm doing is I'm taking this example of human eye. This is our diagram, a rough diagram of human eye. So what happens in case of human eye is eye is nearly a spherical ball. So you can see apart from this bulge here and this one second and this inner arrangement over here, where different knobs are there, different arteries are there, which provide sensation to the brain, visual sensation to the brain for image formation, apart from this area and this slight bulge in the beginning, the human eye is spherical in nature. It's a spherical ball. So the first thing that you should know that this is a spherical ball. So if it is fair, some radius would be associated to it. That is two and a half centimeter. So a spherical ball is of two and a half centimeter with a slight bulge in the front portion. Now this part, this particular part, so let me select a different color which gives you this particular part. This is called posterior part of eye, posterior part. And this particular part consists of a fluid. So in this part, fluid is there. This fluid is known as vitreous humor. Vitreous humor. And what is the role of this fluid? So this fluid which is called vitreous humor, which is in the posterior part of the eye, it avoids collapse of the eyeball. So this complete ball, this particular portion doesn't go back and collide with the rear part or the front part of the eye doesn't collide with the rear part of the eye. The vitreous humor helps us in that particular process. It avoids the collision of front part of the eye with the rear part of the eye. So the second phenomenon, the second function I have discussed is vitreous humor. And it has been written over here, vitreous humus, what it does is avoids collision of eyeball. Now look at here. This particular part, sorry, this particular part is known as, I'll write it here, is known as sclerotic. This particular part is known as sclerotic. And this is the exterior of human eye or I'll say eyeball. So this is exterior part of eyeball. And this is covered by thick fibrous tissues. So here I have thick fibrous tissues. And what is the role of this thick fibrous tissue? So if this sclerotic is not there, what is it? Sclerotic is exterior part of human eye. So first thing I know about sclerotic is, exterior part of human eye. It is made of thick fibrous tissues. What does it does? What is the function of sclerotic? It gives definite shape to the eyeball. So the second particular part of eyeball that I discussed is sclerotic. And the role of sclerotic is to give different shape to the eyeball, where it is located. So this is exterior of part of the eyeball. And this exterior part of the eyeball is made of thick fibrous tissues. So if you write this much about sclerotic, this is, that would be a good enough answer. Now, I also know that this, any exterior part of any particular object also protects the interior part. So sclerotic also protects the interior part of human eyeball. So the basic function of sclerotic is to protect interior part of human eyeball and to give this eyeball a definite shape. So two things that I have discussed till now is the fluid present over here, which is called vitreous humor. And vitreous humor is for, to avoid collision of front part with rear part of the eyeball. And the second part is sclerotic. So two parts I have already discussed. Now let me move to the third part. One second. Let me make some space. The third part is your lens of the eye. So this is called crystalline lens and where it is located. So it is located on the left of this vitreous humor. It means that I can say this is located in front part of human eye. So crystalline lens is located in front part of human eye. Now there is, look at here, this particular part is known as sclery body or sclery muscle. It is also called sclery muscle. So this is also called sclery muscle. What is the role of sclery muscle? The role of sclery muscle is to control the RR to vary the focal length of this lens. What happens? As I discuss human eye with you, the distance of different objects in front of the human eye keep on changing. Now what is the lens formula? The lens formula is one by V minus one by U is equal to one by F. Now suppose an object is here and the ray is going like this. So you, what, instead of sign convention, we all know that if I'm going in this direction, U would be negative. So I can write this as one by F minus one by U because if this is negative, this becomes positive and on the other side, it becomes negative. Now try to understand. V is always constant. This is my V. So V is always constant. What happens? It is always constant. So if V is always constant and my U is continuously changing for V to be constant, F has to be changing. So what I'm trying to say is that this U gets varied because the distance between the object and the eye lens keep on changing. And if I want every image to be formed on retina, it means that this V is always same. Hence F has to be changing as per the requirement of the eye lens. So this F is changed by this celery muscle. So the role of celery muscle is to change the focal length of the eye lens so that the image is always formed at retina. So retina as I'm talking about here, the retina is here after vitreous humor. So retina is here and this retina acts as screen because image is formed over here, this acts as screen. So retina acts as screen and the image is formed over here. Now there is one very important thing which you should know is this is called accommodation and what is accommodation? So accommodation is this continuous change in focal length of the eye lens is known as accommodation of eyes. So this process of changing, so you can write the definition of accommodation of eye as process of changing of focal length of the eye lens is known as accommodation of the eye. So that's how it has to be defined. So till now I have defined this vitreous humor, I have defined this eye lens, I have defined a sclerotic, I have also defined your celery muscle. Now let me move to the other parts of human eye. The other parts of human eye is first one is, I mean not the first one, the other part here what I'm going to discuss is this particular part which is known as coroid. What is coroid? Coroid is between a sclerotic and retina. And what is it? You look at here, this is somewhere here. What is the role of coroid? The role of coroid is try to understand, I'm writing here it is a membrane. Coroid is a, I'm writing here, coroid is a thin membrane. Its color is nearly dull gray. And with the help of its particular, this particular property, it prevents any internal reflection. What will happen if internal reflection of the light keep on happening, multiple images will start getting formed which is not ideal for a normal human eye. So if this internal reflection is prevented, hence the interior of the eyeball is completely dark. And this forms, if the interior of the eyeball is completely dark, this forms ideal condition for image making on this part which is retina. So if this is some light over here and internal reflection is keep on happening, the proper image formation will not be possible on the retina. And you will have multiple images on the screen. I told that retina acts as a screen for eyeball. And when multiple images will be there or blurred image will be there. So the functioning of the eyeball will get deteriorated. And what will happen is in that particular condition a proper image will not be formed by the eye lens for the object. So this particular part which is coroid and which is between your sclerotic and retina due to its characteristic of preventing internal reflections, it keeps the inside this particular part of eyeball dark which is ideal for creation of image on the retina. So after coroid, what I want to discuss is why this bulge is here. So we need to define why this bulge is here. So this bulge is due to this aqueous humor which is present over here. So there are two kinds of humors in eye available. One is vitreous humor which I have already mentioned and the other one is vitreous humor. One is vitreous humor which I have already told that this is my vitreous humor which prevents collapsing of eyeball. And the second one is this aqueous humor. So aqueous humor provides bulge to human eye or eyeball. So this bulge is there. And what happens this particular eyeball, sorry, aqueous humor acts as primary lens. And this lens is there proper on this lens what happens the image formation happens but this particular bulge which is there due to aqueous humors, this aqueous humor also acts as primary lens of fixed focal length and what happens it provides a certain space between the light entering and the lens. Hence it prevents collapsing of the eyeball. So this particular lens is at certain distance from this particular entry of the light. So role of this aqueous humors is to prevent this collapse and to act as a primary lens of fixed focal length. Now light as this is the front part of the eye, the light will always enter through this part. So see the once again, let me make some space for you. Light will always enter through this aqueous humor. Why because this is in the front of the eye and light will enter like this and then it will form on this part on this eye lens. So after this, let me move to cornea and what is cornea? So look at here, the cornea has been made over here. So what is the role of cornea? So light enters through cornea and most of the refractions which happens. So you look at here this cornea. So let me write over here. Most of the light enters through cornea and due to this, most of the reflection because the medium gets changed, it is coming from air, it is entering into a different medium eyeball altogether and as it is this cornea acts as a separating part of two mediums, it acts as a joining or separating whatever whichever part you see, it acts as a medium separation kind of plane. What happens is most of the reflection will happen on the cornea. Now this is the role of cornea. So I have given you a role of different things. I've done lenses, aqueous humor, cornea is done, salary muscle is done, sclerotic is done, chloride is done, retina is done, vitreous humor is done. Now let me go to pupil which is left out. So this pupil is you can see this is behind this cornea. So pupil is behind cornea and what happens is how much light the eye has to intake. So pupil is behind cornea and what is the role? The role is that it changes, I'm going into optics terms, it changes the aperture of the length, aperture of. So what happened? It has very small aperture and it decides how much light should fall on this particular lens. So this controls the light entering the retina and how does it happen? We all know that the muscle, the pupil muscles can contract, it can expand. So size of the pupil can increase and decrease and due to increase and decrease in the size of the pupil what will happen is it can control the light entering. So if it expands more light enters inside the eyeball if it contracts lesser light would be entering the eyeball. So due to this particular function, pupil becomes very, very important in functioning of the eyeball because it decides the amount of light falling on the lens. Now what happened? I have discussed most of the parts of human eye. I'll come to the sensory part which goes to the brain later. What happens is as soon as the light passing through pupil fall on the eye lens which is held in position by cellar muscle this rage pass through eye lens and image is formed at retina. Now what kind of image is formed at retina? So image formed at retina is always remember this. If you don't write this marks would be deducted. So image formed at retina is real and inverted. So if something is like this, erect like this on retina the image would be formed like this. So how is it getting converted? If it is there, how it is getting converted? How this particular real and inverted image is visualized by me exactly in this condition because what happens is any image formed on this retina would be converted into optical impulses. So you look at here what happens is on retina we will have light sensitive cells and the cells are of two types. So if I discuss the composition of retina with you it is made of two cells, two kind different, I mean different cells are here at retina. The first type of cells is known as rods and the second type of cells is known as cones. The rod cell decides the brightness of it. Please write it in your exam. You will get brownie points for this and the cone cells will be deciding the color. So this identify the cone cells on the retina will identify the color and the rod cells will identify the brightness. So retina has several light sensitive cells and as soon as there is some illumination it gets activated, all these cells will get activated. And what will happen because of this activation is that these central arteries which is connected from the brain will get activated and due to activation of this central artery what will happen is these signals will be sent to the brain by optical nerves. So you see here this part is connected with the help of optical nerve with the brain. Now brain interprets this optical nerves and finally process the information and that's how we perceive any particular object. So I have discussed different parts of human eye with you. Let me go through it one by one once more. First I decided, I started with vitreous humor. I told that vitreous humor is between the lens and the retina. What is the role of vitreous humor? The role of vitreous humor is to avoid any collapse of this eyeball. Hence the front part is not in touch with this rear part and image formation happens properly over here. After I did this vitreous humor, the second thing that I did was your sclerotic and I told that this part sclerotic is made up of, sclerotic is made up of fiber tissues and it protects the inner part of eyeball and it also gives definite shape to the eyeball. So the two function of sclerotic is that it gives shape to the eyeball and also provides protection to interior part of the eyeball. After that I went to lens and told that this is your crystalline lens and this is possible, this is responsible for formation of image on retina. And I told that the focal length of the lens, eye lens has to be changed because this lens, the distance between the lens and the retina, the retina which acts as a screen for eyeball, the distance between lens and retina is same. Hence, if V is same one by F minus one by U, the lens formula, if U is changing, F has to change accordingly. And I told that we have a pupil here which decides how many, sorry, how much amount of light needs to be falling on this particular lens. And hence it keeps on contracting or it keeps on expanding itself. And due to expansion or contracting of this pupil, what happens is focal length gets adjusted. Now, after this what I discussed was coroid. And coroid, I said that it is membrane, dull gray membrane. And what it does is it prevents the internal reflection inside this eyeball between lens and retina. Why it is important, why there should not be any particular internal reflection because if internal reflection will happen, then multiple images will get formed over here on retina and the image will be blurred or we will not be able to view that object properly. And due to this particular issue, what happens is this coroid helps us in preventing any internal reflection. As there is no internal reflection, this particular part of eye is completely dark which helps us in finding a very clear image on the retina. Now, after I discussed coroid, I went to your discuss aqueous humor and I told that this bulge is due to aqueous humor and light enters through this bulge. This aqueous humor acts at primary lens of fixed focus lens. Then I discussed ciliary body and I told that ciliary muscle holds this particular lens and one more thing that I discussed was, apart from this was pupil I have discussed. I have discussed, so I discussed cornea and I told that cornea acts as a separating body between air and the eyeball. And as this is separating body, a plane which separates the eyeball and the outer edge mass field most of the refraction happens over here. So this is what the parts of eyeball and I hope you understood the discussion about different parts of eyeball. And after this, what I do is I'll move to discuss different terminologies related to human eye. So what I'm doing is I'm going to next slide and here I'm telling you the walking of the eye. So as I already discussed, so here is my object and what happens, light falling from this particular object on the eyeball through pupil, I mean pupil helps in converging this particular falling on pupil converges with the help of eye lens. So through pupil help in deciding the amount of light going inside the lens and through this lens, the conversion of this light happens on the retina. So on this part of retina, this convergence happened and I told that on retina real and inverted image will be found. Again, I repeat about retina. I don't want to miss this point. That is why I'm writing again. There are two kinds of cells on retina, the rod cell and the cone cell. Rod cell is for brightness and cone cell is for identifying colors. Please write this in your exam if human eye is coming and what happens? So from here, different optical knobs are here and due to this different optical knobs, the brain senses when this cells gets activated, they send signals to the brain with the help of optical knobs and brain deciphers the image formed over here on retina and then rereads it and reforms it in its own cells. And that's how we are able to perceive the image formed in front of us. So this is what working of the eye is. Now let me move to another topic. Which is power of accommodation. So I discussed that accommodation means process of altering focal length. So look at here, this ciliary muscle which is here, I hope you can see the picture here. I have taken this particular, because this ciliary muscle holds the lens at its position. And due to which with the help of people, what happens is depending on the amount of light coming inside, people will decide amount of light coming inside and due to its contraction or expansion, there would be change in the position. So this will keep on moving. And due to this movement, what happens is the focal length of the lens changes. So this ciliary muscle helps in changing the focal length. The process of altering of this focal length is known as accommodation. So you look at here, I have taken two particular figures over here. So here light ray is coming from very distance object. When they are coming from very distance object, these light rays would be parallel to the principal axis and there would be no refraction and they'll go and it will fall on retina. So what happens? This particular length V has to be equal to F. So that is how the focal length of this particular lens has to be changed. Now if some image is, some light ray is coming from closer object. So what happens? This refraction happen over here and due to refraction, there is divergent, divergent of the light rays and due to divergent, what happens? This particular lens over here, it needs to converge this divergent rays over here. So that's how in this case, the focal length has to be adjusted so that the convergence of the light coming from the distance, sorry, nearer object has to happen at this particular point retina. So after this, let me move to different important terminologies. So what is it? It is called near point and what is near point? So near point is the nearest point to the eye at which an object is visually distinct. What do I mean by this particular statement? So what happens is there is a limit to the power of accommodation of eye. So if something comes very, very near to a normal eye, the eye will not be able to make proper image of that particular object. So what happens is if this is my eye lens and I keep something very, very near to my eye, you can do that experiment now also. If you start pushing, you take your index finger and you keep it at certain distance and start bringing it towards your eye, you will see that a blurred image or you will see that you don't have one image of your finger. You find many images of your finger or at least two images of your finger as soon as it starts coming nearer to the eye lens. So this is what is concept of near point that what happens if something is kept very, very close to the eye lens, the proper imaging of that object will not be possible. So what happens? Suppose it starts from infinity, the distance starts from infinity and the near point. This near point is also known as least distance of distinct vision. So I'm writing here least distance of distinct vision. Least distance of distinct vision for a normal eye is, so I'm writing here for a normal eye, this is 25 centimeter, nearly 25 centimeter. Now if the distance between the object and the eye lens is less than 25 centimeter, in that particular scenario, the lens, the eyeball lens will not be able to focus or converse the light rays on the retina properly due to that blurred image would be formed and hence the, because there is no proper imaging of the object, it will not be visible to us distinctly. So that is why I have written over here the nearest point to the eye at which an object is visible, visually distinct. So that distinction will not be there, it will not be visible distinct below 25 centimeter, that is what near point is. What is far point? The maximum distance up to which normal eye can see the things clearly. So this is not a problem for a normal eye, eye without any defect, this particular distance, the far point is infinite for any normal eye. Now what is persistence of vision? This particular concept is also very important. So what happens is generally whenever the sensation would be produced when an image falls on retina, it does not disappear immediately after the light source is cut off or the image is removed. So what happens is I have an image and I have a light source and my eye is able to see it, the image sensation is produced properly on the lens and through lens on the retina and due to which I'm able to see it, suddenly if the light source is switched off or the image is removed, will it happen automatically that we will not, or the image will disappear from the retina? No, it doesn't happen like that. So instead of it, what happens is the image lasts on the retina for a fraction of second and what is that amount? That amount is 1 16th of a second. So the image will last on retina even after the light source is cut off or the object is removed for certain amount of time after this event and that is a fraction of second, 1 16th of the second. So I can tell you this time varies from individual to individual, even for normal eyes. So this particular process of effect of vision due to which impression of an image last on the retina even after the removal of the object or removal of light source is known as persistence of vision. So if somebody is asking you for one marker, what is persistence of vision? You just go through this particular slide or this particular topic that you should explain that if light source is cut off or object is removed, the image does not disappears immediately and it can be observed for a fraction of second after removal of light source or object which is nearly 1 16th of a second and you should also write that it changes from individual to individual. So if you discuss these many things in persistence of vision, you will find out that you get full marks in this case. Now let me move to the one thing I would like to discuss here, guys. This particular thing is important. I am teaching you a topic which is called perception of colors. So I stole that perception of color is with the cone shaped cells. Brightness is through rod cells and there are three type of cones. So in cone shaped cells also there are three types and these three types are for three primary colors. So what are three primary colors? Red, green and blue. So with the help of these three primary color cone shaped cells, the eye is able to identify either these three particular colors or amalgamation of these three particular colors and hence it will be able to distinguish between different colors. Now there are several people who can't distinguish between different colors. They are called color blind people. So what is the reason behind color blindness? The reason behind color blindness is absence of any one of these cells, cone shaped cells or absence of all of the cells. So if any of these cone shaped cells are absent in any particular human being, he will not be able to properly distinguish between different colors and due to that what will happen is he'll suffer from color blindness. So why I'm teaching you this topic that if you get a one mark question or a two mark question about color blindness, you should write about perception of color by human eye. You should say that on retina we have cone shaped cells. The cone shaped cells also can be categorized into three parts which is red cell, green cell and blue cell. These are for three primary colors and with the help of these three cells, the eye human eye is able to distinguish between different colors. Absence of any one of these cells or all of the cells will lead to absence of distinguish, absence of eyeball distinguishing between different colors and due to which color blindness would be observed in any particular person. So this is how you should write your answer when the color blindness is being asked to you in a one marker or a two marker question. Now let me move to the other part of human eye, the other part of human eye is, so here you see there are four defects of vision I have taken here, cataract I have left out because I don't think it is there in your syllabus. So I have left cataract here. I have taken four different defects of vision. One is, first one is myopia which is also called short sightedness. The second one is hypermetropia which is your long sightedness. The third one is presbioopia which is hypermetropia due to easing and the fourth one is astigmatism which is also called which is due to cornea not being spherical. So these are the this is called stigmatism is also called cylindrical lens, cylindrical cell problem and we use cylindrical lenses for that. So I will discuss these defects one by one. The first one which I have taken over here is myopia. So myopia is short sightedness. How do I define myopia? So short sightedness means ability to see near object and inability to see distance object. First of all, name itself confuse many people. So always remember that short sightedness means at short distance you can see and at long distance you cannot see. So short sightedness means ability to see near objects and inability to see distance objects. Now why does it happen? So what happens if the distance between this lens and the retina increases, so what happens even if there is alteration of focal length, the process of accommodation, the image will not formed on the retina. It will fall in front of the retina and what will happen due to that reason, proper imaging will not be possible. We are unable to see something at very distant object. Now why this problem? Because you should write here, I'm making a line just look at the line you should write here elongation of eye volume. Due to elongation of eyeball, this image is getting formed here not on retina. Now how can I overcome this problem? So try to understand for myopia and hypermetropia, we use different kind of lenses. For myopia, we use concave lens. Why do we use concave lens? So we need to know that this particular, see the correction that has to be done is this image should get formed over here. That's the correction needed. Now how concave lens will help me in overcoming that particular defect. So what happens? It is not able to see distant object, but it is able to see nearer objects. So what I do is if I make distant objects as nearer objects, if I am able to convert distant object as nearer object, what happens is this particular lens will be able to make image directly on the retina. So what happens? I have a concave lens here. And what concave lens is diverging lens, we all know. So the distant from distant object parallel lines will come and it will diverge like this. So due to the divergent, what will happen is a virtual image would be formed here. And as this virtual image is formed here, this particular image will act as imaginary object for the eye lens. And as now the distance of the object and the eye lens has been reduced due to the use of this concave lens, this eye lens will read this object as nearer object and will be able to form the image on the retina properly. So what this concave lens is doing is this concave lens is decreasing the distance between the object and the eye lens. So I will help it, whatever I'm going to explain is not there in most of the books. And as it is not there in most of the books, you should be very careful, you will not find it out anywhere. You need to understand what happens is in normal eye, try to understand if this is my normal eye and this is my lens over here and this is the retina and distance between lens and retina is V1. And in this particular scenario, distance between lens and retina is V2. So I know that due to elongation of the eyeball, V2 would be greater than V1. Now my objective is to correct this V2. This elongation has to be corrected. It means that I have to take this V2 to the normal position V1. So I write this lens, one by, I write the lens formula, one by V minus one by U is equal to one by F. Now U would be negative because the light ray is traveling in this direction and U I'm taking in opposite directions. I write this as one by V plus one by U is equal to one by F. Now one by V is equal to one by F minus one by U. Now what happens is if V has to be decreased, so if V has to be decreased, if this decreases, V decreases, one by V increases. So it means that your right hand side is increasing. Why? Because I am decreasing V. If I decrease the denominator, hence the fraction will increase. It means that one by V increase. One by to increase one by V, something which is getting subtracted here has to increase. So it means that if I increase one by U, one by F is constant over here. So if I increase one by U, I am subtracting more, sorry, if I decrease this one by U, I am subtracting less from here and hence one by U will increase. So what happens is one by U has to be up, so U has to be less. So this is why if I reduce this U, so how do I am reducing the U? U is initially infinite. If I use a concave lens, U comes out to be infinite distance from here. As U comes out to be infinite distance from here, so the process is that U from infinite changes to this particular distance. So it can be taught in opposite manner also that if we are not able to overcome this V1, so V1 has to be taken to V2. So initially I told that V2 has to be taken to V1. It's actually opposite. V1 has to be taken to V2 because you cannot change the length of eyeball. It is already defected. So this V2 will not change. Hence this original V1 has to be taken to V2. It means that you have to increase this, not decrease this. This was one mistake over here. So I'm correcting it. So this V has to be increased. It means that one by V has to reduce. One by V has to be reduced means you have to subtract more here. It means that one by U has to be more. It means that U has to be less. And I make this U less with the help of concave lens. I hope you understood it. I will explain this once again to you guys so that this is always there in your mind. This is very, very important concept. What happens is in your normal eye, this is your lens. I'm finishing it in next 30 seconds. This is V1 and in your defected eye, this is V2. I know that due to elongation, V2 will be equal to V1. Now this distance cannot be changed. Hence I want my V1 to become V2. It means that I want to increase this distance. If I want to increase this distance, I write one by V is equal to one by F minus one by U. If I have to increase V, then I have to decrease one by V. So if V is increasing, denominator is increasing, this fraction one by V is decreasing. For decreasing this, this particular portion has to be more. More number has to be subtracted from here. A minus B, if C is equal to A minus B, if I have to decrease C, I have to increase B. If A is constant, more number is getting decreased. So one by U has to increase. If one by U is increasing, it means that U is decreasing. And how U decreases? So length in finite decreases to some finite length with the help of concave lens over here. So this is what is meant to be explained in this particular concept. So I have explained myopia to you. Now let me move to hypermetropia. And hypermetropia is exactly the opposite thing. So what do I mean by exactly opposite thing? It means that ability to see distance, distance objects and inability to see nearer objects. And why it happens? Because the distance between, the distance between, so I'm again taking this as V2. The distance in defected I, it is V2. In normal I, it is V1. So I'm writing here in normal I, distance between lens and retina is V1. And in defected I, it is V2. Now because this has contracted V2 would be less than V1. Now I can't do anything about V2. So what I will want is the V1 decreases to V2. And how will it decrease to V2? So what happens is one by V is equal to one by F minus one by U. Now this V2 has to decrease. If V, this V2 has to decrease, it means that one by V2 will increase. This complete thing will increase. To increase this, I have to reduce this subtraction part. To reduce this subtraction part, I have to increase U. So what I'm trying to say over here is, if the eye is able to see distant object, anything which is nearer to it has to be taken at distance, at certain distance so that the eye can view it properly. So your reading lens and all those things fall into this particular category. And due to that, what happens is a person who is not able to read newspaper properly is able to see it. Why it happens? Because we use a concave lens. So what concave lens does is try to understand what concave lens does is. So suppose something is kept at focal length of concave lens. After reflection here, this will be acting as or this will become parallel to the principal axis. So I keep this particular convex lens such that the object is near around this focal length of this particular convex lens which I'm using. And due to this, what happens is the light rays nearly becomes parallel to the principal axis of my eye lens and due to which the image is formed at retina. So what I'm doing, this U was finite in case of eye lens due to usage of convex lens. I'm able to make the U infinite or nearly infinite for my eye lens and due to which the image falls exactly on my retina and the defects gets corrected. And how it gets corrected, I have explained it with the help of lens formula here. I hope you understood that. So now let me move to press biopia. Now what is press biopia? So press biopia is, see what happens? You see that when people start using automatically they start using reading lens and all those things. Why? Because the eye automatically starts getting contacted and due to which the image formation is not proper. So I will put it in this format that inability to see the objects clearly which are near due to aging of eye and which part of the eye ages. So ciliary muscle, you should write this very clearly in your exam that aging happens for ciliary muscle. Ciliary muscles ages and due to which there is a change in distance between lens and retinite. It decreases and due to decrease in it and this is natural in process due to decrease in its in it the image formation does not happen properly. So we are unable to see something which is nearer to us and this particular process is press biopia. So what do we use? We use for to overcome the defects caused by press biopia. We use convex lens of suitable focal length and depending on how much contraction has come in the human eye. And what will happen is that press biopia can be overcome. We call it, if it is press biopia we call the lens as reading lens or reading glasses. Now let me move to astigmatism. Now what is astigmatism? So when the cornea is not spherical and if it is curved differently in different direction then what will happen? Simultaneous focusing on horizontal and vertical lines becomes very, very difficult. So that condition is known as astigmatism and this can be overcome by using cylindrical lenses as simple as that. So with this I come to, let me check if there is anything that I need to explain in human eye. I don't think so. So with this I come to the end of topic human eye. I have explained different parts of human eye and after explaining different parts of human eye what happens is I shifted to different terminologies about human eye and then I shifted to defects of human eye. Now I'll move to the optical instruments which are simple microscope compound microscope and terrestrial, sorry astronomical telescope. So what simple microscope? So I've written a few things which says that observed used for observing magnified images of tiny objects and we use converging lenses of a small focal length and virtual erect and magnified images found. But what's the process of this happening? How does it happen? So the simple process of this is that or a simple one of the simple concept is that the size of image of an object found are on retina. It will increase if the distance of the object from our eyes decreases. So what I'm trying to say is that if I compare the size of image on retina with the distance, what happens is if any particular objects comes closer to our eyes, the angle substituted by the object on our eye will increase and hence the size of the image on the retina will also increase. So the closer the object to our eyes the bigger the picture we can see. So I'll make a diagram over here. This is my eye, I have my eye here and this is the object which has been placed at a distance D from my eye. So this is alpha. Now what happens is if the object comes closer to our eyes, the object comes closer to our eyes, the angle substituted by the object would be more. So if I keep it here, the angle substituted is this theta and you can see in the diagrams theta is greater than alpha and because the angle substituted is more, the size of the image will also increase and this is called angular magnification. If the angle is increased, the size of the image of the object increases and how can we increase the angle? So angle can be increased by placing a convex lens of sort focal length between the eye and the object such that the distance between the lens and the object decreases and due to this decrease, what happens is we can adjust the strain of the eye and proper magnification of the object would be possible due to that proper magnification of the object, we will be able to see the tiny objects. So this is the principle of the compound microscope. So what happens, let me define it, let me define compound microscope to you. Sorry, once again, I've changed the slide. I should be here at simple microscope. So let me define the simple microscope to you once again. So everything is good. You look at here, the object is here. So what I do is this object is at very tiny object and if I'm using this particular convex lens, what this convex lens is doing? So this convex lens is what is the distance between I want you to check because all of you have done this in the class, I want you to check if we vary the distance between the object and the convex lens, what happens? So if you keep the object between the focal length and the optical, between the focal length and the optical center, and you can see that the focal length of the compound, sorry, this convex lens is here, I am keeping the object between this focal length and the optical center. So U is less than F over here. What happens in case of convex lens when U is less than F? So when U is less than F, I get virtual erect and magnified image. So this is how the simple microscope works. So what I do is I put a convex lens, increase the angle and due to increase in angle, what happens greater size of image is available to me. So when you have to write down the principle of working of compound image, you have to write this down, the complete angular magnification thing, which I discussed now, the changes in angle, this particular diagram, if you make, it will be very, very good for you. And due to that, what happens is I need to change this angle, to change this angle, what I need to do is I need to put this convex lens. If I put this convex lens and U is adjusted such that U is less than F, at particular condition, I find virtual erect and magnified image of the object and I will be able to see it. So what is the magnification? Now try to understand the magnification over here due to magnification is the ratio of angle substituted by object at the eye in normal adjust and divided by angle substituted by eye when it was at the least distance of distinct vision. So this is H by D is least distant of, let me write the formula for you. Please write this formula. This is very important for your exam. I'm writing the formula, magnification in case of simple microscope is defined as ratio of angle substituted by the object at eye in normal adjustment to angle substituted by E when it is, when object is at, is at least distance of distinct vision D. So it means that you see here it is ratio of two angles. So M is equal to alpha divided by alpha naught and I draw two figures. I told that this is D, this is H, D I'm assuming here to be least distance of distinct vision. So the alpha naught is because this is very small. So tan alpha can be written as tan alpha naught can be written as alpha naught and alpha naught is nothing but H by D. And when we do adjustment, adjustment means putting a convex lens over here. And suppose it makes an angle alpha. So suppose it is here, this is making an angle alpha over here. So this alpha is equal to H by F naught. So nearly H by F or F naught whatever you take. So this is H by F divided by H by D and this comes out to be D by F. So this is your formula of magnification in case of, so if you are keeping it at focal length, if this distance is at focal length, if distance is less than focal length, I need to find out the, I'll give you that formula also. If the distance is changed, if I keep it at focal length, this comes out to be D by F. If it is kept at, which I discussed here in this case, if it is kept over here, so in this particular case, what would be the, this thing? So you see here, this is D and this is what? This is my U and angles would be, suppose, so suppose this angle is beta formed by this. So formed by this particular line. So M would be beta divided by alpha naught. Alpha naught, I know alpha naught is H divided by D and beta would be equal to H divided by U. Where it has been kept over here, so H divided by U. So I keep over here, H divided by U, H divided by D, so I find out D divided by U naught. Now I have to find out this U naught in form of this focal length F. So how will you find out? We have to use the lens formula. So one by F is equal to one by V minus U. So one by F is equal to V is formed at D, so minus D because the image is virtual and U will be also negative. So this becomes, suppose I assume U to be U naught here, just to not confuse you. So one by F is equal to one by F, sorry. So this is one by F is equal to minus one by D plus one by U naught. So one by U naught is equal to one by F plus one by D. So what I get over here is U naught is equal to FD divided by F plus D. So I can replace this U naught by FD divided by F plus D. So what happens is D divided by FD and F plus D goes above. So this D and D gone, I get M is equal to F and F1, plus D by F. So if this has been kept as focal length, this object has been at the focal length of this convex lens, this comes out to be D by F. If it is between somewhere focal length and optical center, the magnification is one plus D divided by F. So this is the working of simple microscope. Now let me go to compound microscope. Now what is compound microscope? So look at here, in compound microscope, you can see that there are two lenses. So before that, let me discuss the scenario in which compound microscope is needed. Because simple microscope, also I told that that was for greater magnification. But what happens is simple microscope can produce images having magnification up to, I mean, there is limitation in case of simple magnification. And because of that limitation, we need a microscope of greater power and due to that, we use compound microscope. So compound microscope, the construction is, it is a combination of two different lenses, objective, the lens which is towards the object is known as objective lens and the lens which is towards the eye is known as eye lens or eyepiece. So in compound microscope, the objective lens is kept of lesser focal length. It is of, I'm writing here, of lens, objective lens is of lesser, and why this is of lesser focal length so that I'm able to form the image in front of this eye lens. If this would be of greater focal length, the image can be formed somewhere either this side or that side of the lens and proper magnification will not be possible. So hence the smaller focal length for this objective lens is required. And what happens is you can see if I have an object here which has been kept here in front of this objective lens, it forms a real and inverted image of the object in front of the eye lens. Now what happens is as the focal length of this is very, focal length of this particular eye lens is more than this, the image formation would be happening inside the focal length area of this eye lens. And due to that what will happen is as the U is less than F for this eye lens, I'll get virtual image from this particular eye lens. So suppose this object is AB which is placed beyond the principal focus of the objective, its image would be formed here at A dash B dash between the objective and the eyepiece. This image is inverted and real and magnified as I have already told you. This first image acts as an second image of convex lens which is eyepiece. The distance between this A dash B dash from the eyepiece is less than the focal length of the eyepiece. So this gets further magnified and the final image would be formed here which I am naming as A dash B dash. So which is highly magnified in nature. So this is the working of construction and working of compound microscope. So compound microscope, if I repeat it for you, compound microscope is used with the help of, or is constructed with the help of two convex lenses. The lens towards the object is known as objective lens. The lens towards the eye is known as eyepiece or eye lens. In few books you will see eye lens written. In few books you will see eyepiece written. Don't get confused by the terminologies here. Terminologies here. You should know that the lens which is towards your eye is your eye lens or eyepiece. Now how it happens. So the first lens which is your objective lens forms an image that image is inverted and real and magnified. And this image acts as an object for the eyepiece. Because the eyepiece has greater focal length than objective lens and due to, because the distance between the objective lens and eye lens is maintained in such a way that the image formed by the objective lens falls within the focal length range. Hence the U for the eye lens would be less than its focal length and due to that we get a virtual and highly magnified image of something. So this is the construction and working of the compound microscope. Now let me discuss magnification for you. So magnification of compound microscope is given by, so if I write magnification here it would be A dash B dash divided by AB. So this can be written as A dash B dash A double dash B double dash divided by A dash B dash and I'm multiplying here A dash B dash divided by AB. So what I'm trying to do is that this is magnification by my eyepiece because object for the or object for eyepiece is not AB object for eyepiece is A dash B dash and it is converting A dash B dash to A double dash B double dash so this is I'm calling it ME and this I'm calling it as MO because this is the magnification provided by objective lens. Now ME can be written as if distance between these two lenses are L. So ME can be written as I'm not getting into derivation ME can be written as what is ME by and this distance is ME by and this distance is D. So ME can be written as D divided by FE this FE and MO can be written as L divided by FO. So this can be written as L by FO multiplied by D by FE. So this comes out to be LD divided by FO into FE. So this is how simple it is in case of compound microscope. Now let me move to you just have to write this formula guys you don't need to derive it in your exam. If you write it you get full marks. Now let me move to the last slide of this class and this is nothing but your astronomical telescope. So what is astronomical telescope? Construction wise let me discuss construction of it. Construction wise it is same as compound microscope with one different and what is that difference? Focal length of objective is greater than focal length of eyepiece. Now why it is so? So let me explain the working of it. Suppose at a very distant object what is astronomical telescope used for? For observing very distant images heavenly bodies, stars, planets. So what happens? Suppose this is an object which is very, very far from us. So lines which will come from distance object would be nearly parallel to the principal axis and they will fall on this objective and an image would be formed here which is my image A1, B1, P1 or PQ here in this particular thing PQ has been written over here. So PQ image would be formed. Now this first image is diminished, inverted and real. This is diminished, inverted and real. This particular see try to understand one thing which can act as a confusion here is why in case of compound microscope, I told that in compound microscope image formed by this objective was real, inverted and magnified. I'm saying here in case of astronomical telescope that the image found by this objective is real and inverted but it is not magnified, this is diminished. What is the reason behind these two things? Try to understand this difference. In case of compound microscope, my job is to see tiny objects in magnified form. So the object which is viewed by the eye is very, very tiny and I need to magnify it. Now the object which I'm looking over here is already very big. Why I'm not able to see it because the distance between the eye and that object is very large and due to that I'm unable to see that particular object. Now my intention is not to see that object in that shape. The, if I'm looking at Venus and Venus is called sister of Earth because similarity in the shape and size of Venus with the Earth, I don't want to see the diameter of 6,400 kilometers. I just want to see Venus rotating around me. It means that the real size of that particular object is not required. It is just that the image on my retina by that particular celestial object which is very, very small, that image I need to enlarge so that I can see that particular object properly. That is why in case of a compound microscope, I magnified it with the help of first lens and then I magnified it with the help of second lens also. So double magnification was done. Here the first object which is observed, astronomical object is very large in size. So what I'm doing that very large size object, the size is diminished and that is why here the objective has greater focal length than this eyepiece so that I can diminish the size of this, so that I can diminish the size of this particular celestial object and then I can increase the size of this particular image with the help of my eyepiece, whatever size I want to see, depending on that I keep on changing the distance. So this is how astronomical telescope works. I find a diminished image of the celestial body over here as this is diminished, real and inverted, but the U is less than FET, hence a magnified image of the celestial object would be formed in front of my eyes. This is how astronomical telescope works. I have already explained you the concept of, I have already explained you the concept of how, why objective here has larger focal length and in compound microscope it was less. The reason behind objective is having a greater focal length here because I need to diminish the actual size of the celestial body and hence I find a diminished object over here and then I magnify it as per my requirement in case of tiny objects which is seen by compound microscope. I do the double magnification. For double magnification what I need is that it is first magnified by this and then it is magnified by the second thing. So why change in focal length of objective? Why, how the construction happens? What is the working of it? I have tried to explain everything in a very concise and clear manner. I hope you understood it. So I'll just go through the revision of this particular, all the slides once so that there is nothing, I'll not write anything, I'll just look at the slide and explain you everything. Now one thing which I'm missing over here is, here the magnification, you just know the formula, it is FO by FE. So magnification is FO by FE. Depending on what size of the object you need, you can change your FO and FE. So this is how it is. Now let me go to the first slide from where I have started. One second, guys. So this is my first slide. So I told you about different parts of human eye. It started with vitreous humor. I told that vitreous humus prevents collapsing of eyeball and there is no contact between it. Make sure that there is no contact between the lens and the retina. And the second thing that I explained over here is that after vitreous humus, I explained sclerotic and I told that it provides protection, it's the exterior surface, it provides protection to interior part of the eyeball. It is made of fiber tissues and it gives definite shape to the eyeball. So four things I told about sclerotic, I then went to Coroid and I told that Coroids are membranes and due to that what happens is it prevents, this is dark gray in color and due to that it happens is it prevents internal reflection, hence clear image is found at retina. This remains dark. No blurring of image is possible because of absence of internal reflection. Then I went to sclerae body and I told that sclerae muscles are responsible for change in focal length and also they hold this islands, provide balance to this islands. Then I went to cornea and I said that cornea is a separating surface between the outer atmosphere and the eyeball and hence here most of the reflection happens. So and most of the light enters through cornea. Then I went to pupil and I told that pupil is responsible for amount of light entering inside the lens. Then I told that aqueous humor is responsible for providing this pulse to the human eye and it also acts as primary lens and which has a definite focal length. Then I went to retina, retina I told that it acts as pure screen for image formation. I told that retina is made of different cells. I discussed about rod cells and I discussed about cone cells. I told that rod cells are responsible for brightness and the cone cells are responsible for identifying light over here. Now after that I told that this is not part of human eye let me repeat it here only that the cone cells which are responsible for identifying the color of the object is again divided into three parts which is for green, red and blue colors, the three primary colors green, red and blue and I told that with the help of these three primary cone cells the eye is able to distinguish between different colors. Now what happens in case of color blindness, any one of them or all of them, all these cells are absent and due to that differentiation between the different colors are not possible and hence the person becomes color blind. I also discussed about different, so I discussed about image formation. I told that in retina real and inverted images are formed and here we have optical nerves and this optical nerve send signals to the brain, the brain then processes the form and we are able to perceive the image in the erect form, it is not inverted. Now I discussed about power of accommodation and I told that Celery muscle is responsible for changing the focal length. I explained clearly that the change in focal length is required because the distance between the object and the eye keeps on changing as you keep on changing and the distance between the lens and the retina is not changing, the V is constant. So to keep V constant, F also has to adjust itself with respect to change in you and the change in focal length is possible due to Celery muscle. I'm saying that the power of accommodation, see the change in altering of the focal length time and again depending on the change in you is known as power of accommodation. So after power of accommodation, I explained to you least distance of distant vision. It means that the distance below which the formation of image by the eye lens will not be proper and I told that for normal eyes, this is 25 centimeter and below 25 centimeter, what happens is the blurred image of the object is formed at the retina. The far point was infinite. I told that you can see till infinity. Persistence of vision, I explained as if an image is formed by an object and if light source is removed or the object is removed, the image remains on the impression of the image does not vanish immediately. It goes away after a fraction of seconds which is generally one sixteenth of a second but I also told that even for normal eyes that will keep on changing. This one sixteenth of a second will keep on changing and this particular process by which the image does not disappear immediately after removal of light source or the object is known as persistence of vision. After this topic, I went to defects of vision and I went to myopia and I told that here due to elongation of eyeball, one second, due to elongation of eyeball, this distance between the lens and the retina increases, it means that the V increases. So what happens to increase V? We saw that we have to increase U also and to increase U, what we, sorry, we have to decrease U there. To decrease U, what happens is I use a concave lens and that is a diverging lens due to the diverging property, it forms image in front of the eye. So the distant objects becomes nearer object and due to that, the image is formed on the retina. And after that, I discussed hypermetropia, which is like seeing distance object and inability to see near objects. I told that here V decreases, so we need to rectify it. I used convex lens over here and this makes objects at, I mean, the image formed in front of eye lens with the help of convex lens would be such that the distance between the object and the eye lens increases and due to that, the person is able to see things properly. We use concave lens over here. Then press biopia is said because the contraction of ciliary muscle, the eyeball contracts and due to that what happens is the image formation happens beyond retina and due to that, the person is able to read things properly and that is called hypermetropia. So we use reading glasses and all those things. Astigmatism is because the cornea is not a spherical, so we are unable to view everything properly. This horizontal and vertical focusing is not possible and due to that we use cylindrical lenses. After this, I moved to simple microscope. I told that this is for magnification of tiny objects and I use the concept of change in distance and relative change of angle and hence, you can see with the change of angle how the size of the image changes and that's what I did. I put a very convex lens of very small focal length and focal length is small so that I can put a tiny object over here and I can get a magnified image like this. Now let me move to compound microscope. So in compound microscope, I have two lenses. One lens is called objective. The second lens is called eyepiece or eye length. So what I need to do is that here, I am keeping the objective of less focal length. I'm keeping it of less focal length so that I can get a magnified image over here. Why I need double magnification because it's a tiny object, I need to magnify it properly. So that's why the focal length was adjusted in that fashion. So after this, I went to astronomical telescope and astronomical telescope, here the focal length was more so that I can diminish it and after diminishing it, then I am using eyepiece and I'm putting this particular thing in focal length. This U is less than F and I'm getting magnified image. So this is the general discussion of optical instruments. I have tried to cover everything deeply, whatever they are in your syllabus. If you need any other help on any other topic related to optical instrument, you can revert to me individually and I would love to help you out if you have any particular doubt in this topic. So with this, I come to end of the session. Wish you all the best for your examination. I have for peoples of regular CBSE board, I don't have any more physics topic to do. You guys are welcome to ask your doubts if there is any doubt, I am completely up to it. For autonomous syllabus, there is one topic which is left out, which is a photoelectric effect. I would try to take that topic and that topic is scheduled for next Thursday. So all those who whom I'm meeting, I mean the CBSE regular students, I hope that your class 10th exam would be very, very good. You all have done a lot of hard work and I hope that hard work pays off. I'm very sure that God will help you out in securing very good marks. You all are very good students. It was fun teaching you guys for one long year. Having said that, please don't stop putting hard work till the time your board exam is going on and I'm very sure that you all will pass by flying, I mean pass in flying colors and with very, very good marks. So thank you so much for joining this session. God bless you all and I hope you get very, very good result. So thank you all for joining this session. Thank you so much.