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How telescopes work

Let's explore how telescopes work intuitively using a demo. Then we will talk about some nuances involved in drawing ray diagrams and derive an expression for magnifying power.  Created by Mahesh Shenoy.

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Video transcript

let's explore how telescopes work by taking an example here I'm looking at the beautiful moon now you know we'll make it even more beautiful if I could zoom in and magnify this and the first thing that comes to my mind when it comes to magnifying something is a magnifying glass so let's go ahead and bring in a magnifying glass here it is it's just a convex lens difficult to show that because pretty dark around but I hope you can see that's just a UH yeah there is you can see that that's a magnifying glass a convex lens let's keep it close to our eyes and see what happens we get just a blurred image we don't see anything just blurred moon why is that happening we've seen in a previous video how a magnifying glass works in great detail but in short in order for the magnifying glass to work the objects have to be within its focal length if there's any object which is outside the focal length of your convex lens it's just gonna look blurred and we've talked a lot about this in a previous video so if you need more clarity on this or if you need more details about that it'll be a great idea to go back watch those videos and then come back over here the focal length of the glass that I'm using is maybe 20 25 centimeters and the moon is what thousands of kilometres of a so there's no way we can magnify the moon this way you will just see blurred images so in order to use our magnifying glass somehow we need to bring that moon closer to us and that's what a telescope does it brings a faraway object closer to you and then uses a magnifying glass to magnify the image that's how a telescope that's the principle of a telescope so all we need to figure out now is how to bring that moon closer to us this can be done just by using a convex lens let's look at the theory first so here's our convex lens let's say it has a focal length of F these are the principal force I and far away here is our moon the object of our interest so here is our giant moon and we've kept our moon right on top of the principal axis all right now to figure out what's going to happen we have to draw some rays of light and we'll do pretty much the same thing what wish to all these two we're going to dror rays of light from the top of the moon one ray of light we will shoot from here all the way to the optic centre of our convex lens now remember we have to at least draw two rays of light from a given point and usually we would draw another ray of light parallel to the principal axis isn't it but this time if we draw that that ray is going to miss our lens because our moon is so huge so we cannot draw another ray of light patter to the principal axis so let us draw another ray which is a little bit below it's going to hit our lens a little bit below the optic center so these rays of light after traveling through our distances wash distances will finally come will finally come over here and hit our convex lens so notice that the two rays of light by the time they come over here are pretty much parallel to each other that's because they are originating from a point which is extremely far away sort of like infinity and we've seen the rays of light coming from infinity are all almost parallel to each other the ray of light that's hitting our optic center goes undeviated we've seen that as well and the blue ray we are assuming let's assume that blue ray passes through the principal focus of our convex lens well we know that the ray of light that's passing through the principal focus what happens to it well it's going to go paddle to the principal axis after refraction so the top part of the Moon gets focused at this point over here the bottom part of the Moon is on the principal axis so it'll get focused over here and as a result and so an inverted image of our Moon is formed right on the principal focus over here and we've seen this before right rays of light which are paddle gets focused right on the principal focus and now we can treat this image as our new object and we can magnify it using our magnifying glass because this is close to us so notice how this convex lens has brought the moon closer to us so this lens is usually called as the objective lens so the purpose of the objective lens so this is called as the objective lens and the purpose of the objective lens is to bring in some sense bring the object closer to us the far away object closer to us and so we'll call this as the principal focus of the objective and we call this as the focal length of the objective and since now this object is very close to us we can go ahead bring in our magnifying glass and we can magnify it so let's bring in our magnifying glass so here's our magnifying glass we usually call this as the eyepiece because it's kept very close to our eyes and let's say it's focal length is f e but I think that so that we can distinguish between these two focal lengths and so now we can magnify this object now remember to magnify any particular object using our magnifying glass all we have to do is make sure that this object is within our focal length that's the whole point so we have to bring our magnifying glass and our eye closer not enough not enough we can bring it all the way till here such that this new object lies right on the principle focus of our magnifying glass and now after refraction the rays of light become parallel to each other because the object is kept right at the principle focus and these parallel rays are incident on our eyes and our eyes can easily focus them on the retina and look at the angle subtended by this new object so we'll just have to draw a reference line from the top of this new object all the way to our optic center we are assuming that the optic center of both the lenses are pretty much the same so we're gonna assume this is the optic center and if you draw that notice this is now the new angle that is formed right at our eye and as a result a huge image will be produced in our eye and that's how this moon now will look very very big to us we'll talk more about this ray diagram it's magnificent a little bit later but first let's demo this so here's our moon one more time so the first thing we'll do is bring in a convex lens which is going to be our objective notice that's forming a real image close to us so again we're going to bring in a convex lens that's our convex lens difficult to see that in the dark but that is a real image formed very close to us now of course it's extremely difficult to convince ourselves that it's a real image formed very close to us because it pretty much looks the same first of all alone is a little bit inverted I don't know whether you can see that because the moon is round its inverted image it's close to us you all way to convince us convince ourselves this is close is now we can use our magnifying glass and bring that object within the focal length and magnify it so let's go ahead and see that so we have the image of the objective we'll bring in now the magnifying glass and preparing the magnifying glass just one sec there is you're bringing the magnifying glass we're going closer just like what we did before going closer to it we're going closer and now there it is difficult okay but there it is you can see the moon okay I'm gonna stop over here you can see the moon magnified so I took a couple of pictures so here it is without the telescope and here with the telescope you can clearly see there is it is a little bit more magnified compared to what we could before now obviously the image is blurred because of many reasons first of all the lenses that I'm using a non ideal they're bad lenses there's a lot of distortions in them and that's why it's not very clear and it's more importantly I'm holding the objective lens and the eyepiece and the camera with my hand so it's extremely difficult to you know position them but hopefully we got hopefully you know this demo could at least show you in principle how a telescope works