Anderson Video - Power of a Lens

Professor Anderson
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All right. So, where do you get positive lenses? Well, you go to the drugstore and you buy a pair of these. These are my reading glasses and these have a particular power associated with them. And the power of a lens is given by P. And, it is 1 over the focal length. Okay? This is 1 over meters. This is a very special unit called a diopter. So if you have glasses, your prescription will say something like plus 3. What does that mean? It means plus three diopters. So if your prescription says plus 1, that's the power. What does that mean in terms of the focal length? That means it has a focal length of 1 meter. Okay? In other words, these have a focal length of a little bit less than a meter it's around plus one. I think it's one and a half. Okay? What it means if you have a focal length of 1 meter, if you go outside and you image the Sun onto the ground, that image will be one meter away. And, if you go up or down with your glasses, then the thing will go out of focus. So where it's the smallest spot, where it's a crisp sharp image of the sun is one meter away. Okay? Likewise, you can use these for magnifying glasses Right? I use them for reading, put them on like that. But, if I start to pull them away from my eyes, you should see my eyes getting bigger and bigger and bigger. Okay? Can you see that? Right about there, they're pretty big. I'm looking pretty funny. How's that looking Sean? Okay? You can use these as magnifying glasses, obviously. If you have reading glasses and they are positive prescription, then they are converging lenses. You can use these to start a fire. But, if it's a negative lens, it's a diverging lens. You cannot use those to start a fire. There's another thing that's really cool to do with reading glasses. When you go to, you know, Walmart or, you know, CVS -- any place that sells reading glasses. The reading glasses are really very spherical. Okay? The lenses are spherical. If you cover them up with tin full -- tin foil -- and poke a small hole in that tin foil, it's just like we talked about with cutting out some of the rays. The image will still be there, but it will be less bright. And so, I have used these before to view partial solar eclipses. So when the moon goes in front of the Sun and blocks it partially, right? You still can't look up straight at the sun, but you can image it onto a piece of paper. And if it's too bright on the piece of paper, then you just put some tinfoil on your glasses to cut out part of the glass. And, that will make it dim. So, you can try that. Buy a pair of plus-one reading glasses. Put tinfoil on, poke a hole in the tinfoil and then image it onto a piece of paper one meter away. You'll see an image of the Sun, and you can see a sharp cut out where the moon is. It's kind of a fun little experiment. Okay. That's the power of a lens. Let's talk about negative lenses and image formation.
All right. So, where do you get positive lenses? Well, you go to the drugstore and you buy a pair of these. These are my reading glasses and these have a particular power associated with them. And the power of a lens is given by P. And, it is 1 over the focal length. Okay? This is 1 over meters. This is a very special unit called a diopter. So if you have glasses, your prescription will say something like plus 3. What does that mean? It means plus three diopters. So if your prescription says plus 1, that's the power. What does that mean in terms of the focal length? That means it has a focal length of 1 meter. Okay? In other words, these have a focal length of a little bit less than a meter it's around plus one. I think it's one and a half. Okay? What it means if you have a focal length of 1 meter, if you go outside and you image the Sun onto the ground, that image will be one meter away. And, if you go up or down with your glasses, then the thing will go out of focus. So where it's the smallest spot, where it's a crisp sharp image of the sun is one meter away. Okay? Likewise, you can use these for magnifying glasses Right? I use them for reading, put them on like that. But, if I start to pull them away from my eyes, you should see my eyes getting bigger and bigger and bigger. Okay? Can you see that? Right about there, they're pretty big. I'm looking pretty funny. How's that looking Sean? Okay? You can use these as magnifying glasses, obviously. If you have reading glasses and they are positive prescription, then they are converging lenses. You can use these to start a fire. But, if it's a negative lens, it's a diverging lens. You cannot use those to start a fire. There's another thing that's really cool to do with reading glasses. When you go to, you know, Walmart or, you know, CVS -- any place that sells reading glasses. The reading glasses are really very spherical. Okay? The lenses are spherical. If you cover them up with tin full -- tin foil -- and poke a small hole in that tin foil, it's just like we talked about with cutting out some of the rays. The image will still be there, but it will be less bright. And so, I have used these before to view partial solar eclipses. So when the moon goes in front of the Sun and blocks it partially, right? You still can't look up straight at the sun, but you can image it onto a piece of paper. And if it's too bright on the piece of paper, then you just put some tinfoil on your glasses to cut out part of the glass. And, that will make it dim. So, you can try that. Buy a pair of plus-one reading glasses. Put tinfoil on, poke a hole in the tinfoil and then image it onto a piece of paper one meter away. You'll see an image of the Sun, and you can see a sharp cut out where the moon is. It's kind of a fun little experiment. Okay. That's the power of a lens. Let's talk about negative lenses and image formation.