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Physics33. Geometric OpticsRay Diagrams For Lenses

So to understand the telescope we first need to go back to the single lens discussion. Okay, and let's say that we do the following, let's say that we have a single lens, and now let's put an object right there. I'm gonna change colors because that pink is really squeaky today, you notice that? All right, where's the focal length of this lens? Let's put the focal length right there. If the object is there, where is the image? We just did this one a second ago but let's redo it. Parallel rays go through the focus, rays through the center don't bend. The image clearly can't be located over here, right, these rays never meet. But if I trace these rays back. We did this one a second ago, it looks like the image is right there. This is where our image is, but let's add a second lens now. We know that a telescope has two lenses in it, and keeping this in mind for a second, let's go back to the two lens system. So a telescope has an objective and an eyepiece and we want to see a distant object. The distant object means that the light rays coming in from it are nearly parallel, and so it's going to form an image right in here. And now you're going to image that image with the eyepiece, to send it to your eyeball. So, there is a another derivation that goes along with this problem that is a little bit tricky but the idea is when you are looking at this image, it can in fact look a lot bigger than it would look, if you're looking straight at the object. And there is a magnification for the telescope, which is the following: M is again given by theta prime over theta -- what is the angular magnification with the telescope versus without the telescope? And in the telescope case, it becomes this: negative fo over fe. The negative sign just means it's upside down don't worry about that. This is still the objective focal length, fe is the eyepiece focal length. So in your telescope, if you want a large magnification, then you want the number up top to be big -- you want a long focal length objective and you want the number in the bottom to be short. And if you think about a telescope, and you've all seen telescopes, okay, that first lens that it hits is very far away from your eyeball and it's not very curved, okay, it's fairly flat, that gives you a long focal length objective. But the one at the other end of the telescope, the one right next to your eyeball, that has a very short focal length lens. Okay, it looks just like the eyepiece of a microscope. So a telescope you want long objective, short eyepiece. Microscope, you want short objective and short eyepiece, so they're a little bit different design but it's basically the same idea. You're trying to take small objects and make them look bigger. The only difference is are those objects far away and you need a telescope, if they're close, then you need a microscope. Okay, any questions about that stuff so far? Everybody feeling okay?