Anderson Video - Magnifying Glass

Professor Anderson
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Okay, so a magnifying glass is something you have probably used at some point in your life. And a magnifying glass is designed such that you can see small things, right? You can't read the font on the page, you use a magnifying glass. And now you can see it. So, let's think about the bare eyeball for a second. No magnifying glass, and let's take a look at an object. Okay, here's my eyeball. There's the object. I can follow the various lens rules, but really the only one that I need is ray three. And ray three, according to our lens rules, is: go through the center of the lens and it doesn't bend. So that one went through the center of the lens, it didn't bend. The image is therefore gonna be that big. Okay? But if I move the object closer, what's going to happen? Okay, here's my picture again. And now, when I move the object, and we'll try to draw it basically the same size, if I think about ray 3, that image got bigger. And it got bigger because ray three was steeper, right? Ray three came in at a steeper angle. And you know this of course, right? If I look at text this far away, it's pretty small. As I bring it in, it gets bigger and bigger and bigger. But at some point, I can't focus on it anymore. Alright, I'm gonna get to a point here where I can't focus. And so, let's put an extra lens right against our eyeball. So here's the eyeball, here's the object, same position, and now let's put an extra lens right there. That's our magnifying glass. And if it's right up against our eyeball, then the ray 3 follows the same trajectory, and it's the exact same size. The image is the same size in both of these pictures. Here's my big lens. Okay? This is a particularly powerful lens. And now if I want to look at something, the way I do it is I put the lens right up against my eyeball, and then I can bring the text in and I can see this far away. Okay, that's very close. Can you guys see my eyeball? So what's the purpose, right? What I just said was: if I bring the object in here, it's going to be big. But if I put a lens here and then bring the object in, it's going to be the exact same size. So what is the purpose of this magnifying glass? Well, it's this. If I put it here, I have a nice big image of my text. If I take it away, I have the same size image of the text. But I can't read it because I can't focus on it that close. And so, this is the whole point of the magnifying glass. When used in this kit in this manner, right up against your eyeball, it just does the following: it simply allows you to focus much closer. That's it, and it's really obvious with this sort of demonstration. Okay, so if you want to try this after class, you can come up and look at it. But if I look at a particular piece of text and I go like this, it's nice and clear, it's big. And if I take the lens away, it's still big, it's the same size. Except it's way out of focus. I just can't focus that closely.
Okay, so a magnifying glass is something you have probably used at some point in your life. And a magnifying glass is designed such that you can see small things, right? You can't read the font on the page, you use a magnifying glass. And now you can see it. So, let's think about the bare eyeball for a second. No magnifying glass, and let's take a look at an object. Okay, here's my eyeball. There's the object. I can follow the various lens rules, but really the only one that I need is ray three. And ray three, according to our lens rules, is: go through the center of the lens and it doesn't bend. So that one went through the center of the lens, it didn't bend. The image is therefore gonna be that big. Okay? But if I move the object closer, what's going to happen? Okay, here's my picture again. And now, when I move the object, and we'll try to draw it basically the same size, if I think about ray 3, that image got bigger. And it got bigger because ray three was steeper, right? Ray three came in at a steeper angle. And you know this of course, right? If I look at text this far away, it's pretty small. As I bring it in, it gets bigger and bigger and bigger. But at some point, I can't focus on it anymore. Alright, I'm gonna get to a point here where I can't focus. And so, let's put an extra lens right against our eyeball. So here's the eyeball, here's the object, same position, and now let's put an extra lens right there. That's our magnifying glass. And if it's right up against our eyeball, then the ray 3 follows the same trajectory, and it's the exact same size. The image is the same size in both of these pictures. Here's my big lens. Okay? This is a particularly powerful lens. And now if I want to look at something, the way I do it is I put the lens right up against my eyeball, and then I can bring the text in and I can see this far away. Okay, that's very close. Can you guys see my eyeball? So what's the purpose, right? What I just said was: if I bring the object in here, it's going to be big. But if I put a lens here and then bring the object in, it's going to be the exact same size. So what is the purpose of this magnifying glass? Well, it's this. If I put it here, I have a nice big image of my text. If I take it away, I have the same size image of the text. But I can't read it because I can't focus on it that close. And so, this is the whole point of the magnifying glass. When used in this kit in this manner, right up against your eyeball, it just does the following: it simply allows you to focus much closer. That's it, and it's really obvious with this sort of demonstration. Okay, so if you want to try this after class, you can come up and look at it. But if I look at a particular piece of text and I go like this, it's nice and clear, it's big. And if I take the lens away, it's still big, it's the same size. Except it's way out of focus. I just can't focus that closely.