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Anderson Video - Microscope

Professor Anderson
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<font color="#ffffff">Okay, so you're all familiar with this idea of a microscope, you've I'm sure</font> <font color="#ffffff">look through a microscope at some point in your life.</font> <font color="#ffffff">Right, you put the bug on the table and you take your microscope and you zoom in</font> <font color="#ffffff">and you can see the bug in great detail. So, a microscope in fact has two lenses in it.</font> <font color="#ffffff">Okay, it is a two lens system and there is an objective</font> <font color="#ffffff">and there is an eyepiece.</font> <font color="#ffffff">Okay, and we're drawing it vertically because these things are usually mounted</font> <font color="#ffffff">vertically on the table, and you put your sample right here.</font> <font color="#ffffff">And then you of course look down through this eyepiece.</font> <font color="#ffffff">Now there is some sort of housing between</font> <font color="#ffffff">this stuff and that housing has a particular length associated with it,</font> <font color="#ffffff">and that length is given by L. And then there is a focal length for the objective and</font> <font color="#ffffff">a focal length for the eyepiece, and we would like to know what is the</font> <font color="#ffffff">magnification of the whole system.</font> <font color="#ffffff">Okay, and this is a little bit tough, the</font> <font color="#ffffff">derivation, so we're gonna jump to the answer and if you really want to see the</font> <font color="#ffffff">full derivation you can come visit me in my office,</font> <font color="#ffffff">but the magnification M of a microscope is the following:</font> <font color="#ffffff">negative L minus fe times N divided by fo fe.</font> <font color="#ffffff">What are these different things? L is that length that we just talked about,</font> <font color="#ffffff">it's the length of this tube. Okay, and that's usually about</font> <font color="#ffffff">16 centimeters for a microscope, it's purely governed by how high is the table</font> <font color="#ffffff">when you're sitting and looking down, what's a comfortable point for you to</font> <font color="#ffffff">view it. That determines the length L of that tube.</font> <font color="#ffffff">Fe is the eyepiece focal length.</font> <font color="#ffffff">Okay, so that eyepiece up there, you can actually pull it out on the microscope</font> <font color="#ffffff">and look through it, it's a very short focal length eyepiece.</font> <font color="#ffffff">Fo is the focal length of the objective.</font> <font color="#ffffff">Okay, so that's the thing on the bottom and usually you can rotate different</font> <font color="#ffffff">ones and they'll say like 16x, 20x, 100x, right? Those are also very short focal</font> <font color="#ffffff">length lenses and they sit very close to the sample.</font> <font color="#ffffff">N is the near point of your eye</font> <font color="#ffffff">and that is usually written as 25 centimeters.</font> <font color="#ffffff">Okay, like the tube is usually about 16 centimeters.</font> <font color="#ffffff">So, you want good magnification in your microscope</font> <font color="#ffffff">and if you look at this equation right here, that means you want</font> <font color="#ffffff">this number down here to be small,</font> <font color="#ffffff">and so a microscope, in fact, has a short</font> <font color="#ffffff">focal length fo and it has a short focal length fe. Both of those lenses have</font> <font color="#ffffff">very short focal lengths, and like I said you can take them out and you can look</font> <font color="#ffffff">at them individually. Okay.</font> <font color="#ffffff">When you go to a jeweler and they pull out their loupe</font> <font color="#ffffff">to check out your ring and see if it's real or not,</font> <font color="#ffffff">okay? They are using one of these things. Either one, it's just a short focal</font> <font color="#ffffff">length lens and that allows them to look at it very closely, just like we did with</font> <font color="#ffffff">the magnifying glass, okay.</font> <font color="#ffffff">But put them together in the right orientation and</font> <font color="#ffffff">now you get dramatic increase in the magnification because those are</font> <font color="#ffffff">multiplying each other down here. So if both those things are small, M can be</font> <font color="#ffffff">very big, and you've looked through microscopes that are 100x, 200x, 500x,</font> <font color="#ffffff">okay, you can get incredible amounts of magnification.</font>