 Let us see the construction of compound microscope and will quickly derive the formula for that as well. So, compound microscope has two lenses. One is objective, another is eyepiece. Objective is kept near the object. Let us say this is objective. Suppose this is your object, so the image of this object gets formed somewhere here like this. Is this ray diagram clear? Draw this ray diagram and here you have another lens. This is called eyepiece. This is FO and that is FE focal length. This point must be FO and let us say this is FE. This is FE. Now if you try to form the image of this particular object, see this is the image of this which is the object for eyepiece. Are you getting it? This is intermediate image. Your observer is here, so observer won't be able to see this. Observer will see the image of this through eyepiece. So, let us try to find out final image location. So, one ray goes like this, passes through the focus, another goes like this. So, these two rays they appear here somewhere, still it is curved, leave it. This is the final image. So, observer will be able to see this. One more thing is usually given in the compound microscope is distance between the two focal lengths. This is suppose L. Assume that this is HO and this height is HI and this is intermediate image. Let us say this is height H. I need to find the magnification of this, total magnification. Total magnification formula is what complete magnification will be equal to HI divided by HO. Is this thing clear? Now, I can rearrange that like this. This is equal to H by HO multiplied by HI by H. Tell me what is H by HO? Magnification of objective HI by H is magnification of eyepiece. So, this can be written as MO multiplied by ME. Here, we have an approximation or assumption. The assumption is write down the intermediate image gets formed at FE, I mean near FE, near FE. If that is assumption, I can say L is the distance between the image and FO. Now, tell me what is H by HO? Capital H by HO. One hint is that this distance is also HO. Tell me. Tell me what you are saying? No, not that. Try something else. You have to find H by HO, capital H by HO in terms of what is given. It comes from geometry. How will you find out? Can I say that this triangle and this triangle, they are similar? I can say that, right? So H by HO, capital H by HO is what? L divided by FO, right? So MO is L by FO, okay? Now tell me what is ME? How much it is? ME is what? The image gets formed at D from here. That is the hint. This is D. This is case number one. We are talking about case where the final image at D from IPs. Tell me. Is it? I just forget about this. Forget about what is here. Then this becomes simple microscope. An image is at D. So remember case number one, 1 plus D by FE, 1 plus D by FE. So this is 1 plus D by FE. So total magnification becomes what? L by F naught into 1 plus D by FE. This is compound microscope magnification. So if L by FO is 25 and 1 plus D by FE is 30, so it becomes 25 into 30. So magnifications are multiplied. Any doubt? See, forget about that there is an objective. Then this behaves like a simple microscope whose image gets formed at D. So that case we already did. Magnification in that case was 1 plus D by FE, fine? Any other doubt? This is case number one. Write down case number two. This final image is at infinity. Then what will be the magnification? Tell me. What will be the magnification then? Will MO change? MO will be same? We are anyway assuming that this image gets formed near FE. Now it will be formed exactly at FE. So what it is now, magnification? ME will become what? You remember case number two? Yeah. ME is? D by FE. So total magnification now will be equal to L by FO into D by FE, fine? So this is a derivation for compound microscope, two cases, fine? There can be infinitely many cases, but we have learned only about two cases because they are the extreme cases, either at least distance or maximum distance, okay? Any doubt on this? Now we will discuss about telescope, magnification due to telescope. Write down telescope. Telescope will also have two lenses, but this time you have this objective and you have the eyepace. We have drawn objective smaller than eyepace earlier or larger than eyepace, okay? So there is a small correction in compound microscope. Objective is smaller, okay? And eyepace is larger. And in telescope, objective is larger and eyepace is smaller. This is eyepace. Why objective is larger in telescope? There is a reason for it because you are looking at very far away object. Light is anyway coming out which is very less. So you want to capture maximum amount of light, fine? So you take your aperture very, very large, fine? And in microscope, anyway you are looking at object which is very, very small and from very close. So why you want a huge objective, fine? So here, since objective is bigger, the reason for that is you are looking at object with a very far light is anyway very less, you want to capture maximum amount of light. And another reason you learn in ray optics. Larger is the aperture, better is the resolution, okay? You learn it about it in ray optics, okay? One more thing is about telescope, that focal length of eyepace and objective, they both coincide, they both coincide, okay? So length of the telescope will be always equal to sum of the focal length. So this is f o, this is f e. So f o plus f e is length of the telescope, okay? Now how the rays will come towards the telescope, there will be parallel rays because they are coming from infinity. Now will these parallel ray need to be parallel to principal axis also? They need not be parallel, right? To the principal axis, but they should be parallel to each other. So suppose rays are coming like this and there is this alpha angle. Now where they will converge at a distance of focal length, right? They will converge suppose here like this. This is what intermediate image, the observer is here. What will happen for final image? Final image will be formed again at infinity only because the first image gets formed at infinity. Sorry, first image gets formed at focal length of eyepace itself, are you getting it? So this is alpha, let us say that is beta, okay? Here also we will try to define angular magnification, fine? Can you tell me how will you define angular magnification here? Magnification will be equal to what? What is the angle for image, beta and object, alpha? So we will define magnification as tan of beta divided by tan of alpha, fine? Suppose this distance, this one is h, then tan of beta is what? h by fe and tan of alpha is h by fo because this is also alpha. This angle is also alpha, clear? So h and h get cancelled, so magnification is fo by fe, fine? So this is how you define magnification for the telescope also and it out, great? So the problem with the telescope is that the objective becomes so big, you know, because you are looking at very, very far away objects, fine? So the size of the entire telescope will become like 50 meters, 100 meters like that, it is huge, it will be on top of some mountain, huge telescope, fine? And it becomes very difficult to handle the lenses, lenses are much heavier compared to mirrors. If there is a way to form the telescope using mirrors, it will be lot more welcome because maintaining or constructing such huge lenses becomes very difficult. It may chip off from the edge and it can break also and then it is extremely heavy, fine? So that is why you have this telescope, the name of this telescope, every time you know, it is difficult for me to pronounce, it is casegrain, write down C A double S, casegrain telescope that is made from the mirror, fine? Just construction is there in your syllabus, no, you know, formula as such are there. So it looks something like this, light comes like this, it reflects off and then here you see the image. This is casegrain, fine? So we have finished the entire theory part of this chapter, fine? After the break, we will take a few numericals. You have any doubts? You can ask now because the telescope with lenses becomes very heavy and becomes very difficult to deal with such huge objective lenses. It may chip off from the corners, fine? Let us take a small break and then we will meet.