Anderson Video - Nearsighted vs. Farsighted

Professor Anderson
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<font color="#ffffff">Okay, so let's say you are nearsighted. Okay?</font> <font color="#ffffff">If you are nearsighted, then you can see things that are near.</font> <font color="#ffffff">Which means if this is my eyeball, I can see things that are near.</font> <font color="#ffffff">And if this object is near, then the rays coming from the object are diverging.</font> <font color="#ffffff">And the power of my lens means I can focus those </font> <font color="#ffffff">back to a point on the back of my retina with no problem.</font> <font color="#ffffff">Okay, this is nearsighted. I can see things close.</font> <font color="#ffffff">The rays from that are diverging.</font> <font color="#ffffff">But you want to be able to see things at distance.</font> <font color="#ffffff">Okay, and so at distance, the rays that are coming in are very close to parallel.</font> <font color="#ffffff">And you can't focus those to the right spot.</font> <font color="#ffffff">Either because your eyeball is a different size than normal or the focusing power of your cornea is different.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So all we have to do is take parallel rays from this object and make them diverge.</font> <font color="#ffffff">And we know how to do that, right?</font> <font color="#ffffff">We use a diverging lens.</font> <font color="#ffffff">And when those parallel rays come into this diverging lens, they're going to diverge.</font> <font color="#ffffff">If they diverge the same amount as we had up there, everything's going to come back together.</font> <font color="#ffffff">At the retina, it'll be in focus.</font> <font color="#ffffff">So for nearsighted, you add a diverging or negative lens.</font> <font color="#ffffff">Okay, that's how you fix nearsightedness.</font> <font color="#ffffff">But what about farsighted?</font> <font color="#ffffff">If you are farsighted, then you're looking at parallel rays coming in from a distant object.</font> <font color="#ffffff">Here is your eyeball.</font> <font color="#ffffff">And if you're farsighted, then they don't quite get to a focus right where they should be.</font> <font color="#ffffff">Okay? So we need to get that focus in a little closer.</font> <font color="#ffffff">And the way we get it in closer is we add a converging lens.</font> <font color="#ffffff">And remember, converging is positive.</font> <font color="#ffffff">Okay, and so parallel rays that come in, we want them to focus on the retina.</font> <font color="#ffffff">And so we add a positive lens right there and that starts to get these rays to bend in a little bit.</font> <font color="#ffffff">Now they can come to a focus right on the retina.</font> <font color="#ffffff">So, you automatically know if you are nearsighted or farsighted just by looking at your lenses.</font> <font color="#ffffff">Okay, so here --</font> <font color="#ffffff">these are my reading glasses.</font> <font color="#ffffff">Okay, when you get old like me, you get reading glasses.</font> <font color="#ffffff">And these have a positive value.</font> <font color="#ffffff">They're converging, which means I am farsighted.</font> <font color="#ffffff">I can see things that far just fine.</font> <font color="#ffffff">But when I try to read things close, without glasses, then it's hard to read.</font> <font color="#ffffff">Okay, and so I need a converging lens.</font> <font color="#ffffff">Now how do you know this is converging versus diverging?</font> <font color="#ffffff">Well, first off, if you look through them at some point </font> <font color="#ffffff">the outside world should get very big as I move this away from my eyeballs.</font> <font color="#ffffff">And if you look at that camera,</font> <font color="#ffffff">if I'm looking at the camera, my eyeball should look a little big to you.</font> <font color="#ffffff">Do the eyeballs look a little big to you?</font> <font color="#ffffff">And especially if I pull them out a little bit. Let's try this.</font> <font color="#ffffff">Trying to do without getting shadow all over the place. Okay.</font> <font color="#ffffff">This eye should look big to you guys on the computer monitor.</font> <font color="#ffffff">Okay, that's how you know it's a positive lens.</font> <font color="#ffffff">If its a negative lens, the outside world will always look smaller.</font> <font color="#ffffff">Okay, when you look at a friend and you do this, </font> <font color="#ffffff">your eyeball will always look smaller if it's a negative lens.</font> <font color="#ffffff">Okay, so this is a converging lens, the other would be a diverging lens.</font> <font color="#ffffff">Now there's other things that you can figure out here.</font> <font color="#ffffff">One is there is a little bit of astigmatism.</font> <font color="#ffffff">Which means it's not a spherical lens. </font> <font color="#ffffff">There's actually a little bit cylindrical shape to it.</font> <font color="#ffffff">And the way you can tell that with some of these glasses, is if you look through it and you rotate it.</font> <font color="#ffffff">And the image goes from squished this way to squished that way.</font> <font color="#ffffff">That's astigmatism.</font> <font color="#ffffff">And if you look at my glasses, my left eye is more astigmatic than my right eye.</font> <font color="#ffffff">So when I rotate this, the image in the right one </font> <font color="#ffffff">doesn't get as distorted as the one in the left one.</font> <font color="#ffffff">And when I look at it you guys, you become very distorted in that left lens.</font> <font color="#ffffff">The other thing to know about nearsighted versus -- or nearsighted versus farsighted is,</font> <font color="#ffffff">A positive lens means you can focus light down to a point. Okay?</font> <font color="#ffffff">So if you are out in the sunshine and you're trying to focus light to a point on a piece of paper.</font> <font color="#ffffff">Because you're trying to start a fire.</font> <font color="#ffffff">Because you're on a desert island. </font> <font color="#ffffff">You had better hope that the person's glasses you're using, they're farsighted.</font> <font color="#ffffff">Positive lens, so you can focus that sunlight to a point.</font> <font color="#ffffff">If they're nearsighted, it's never gonna help you start a fire.</font> <font color="#ffffff">Sunlight that comes through there diverges everywhere.</font>
<font color="#ffffff">Okay, so let's say you are nearsighted. Okay?</font> <font color="#ffffff">If you are nearsighted, then you can see things that are near.</font> <font color="#ffffff">Which means if this is my eyeball, I can see things that are near.</font> <font color="#ffffff">And if this object is near, then the rays coming from the object are diverging.</font> <font color="#ffffff">And the power of my lens means I can focus those </font> <font color="#ffffff">back to a point on the back of my retina with no problem.</font> <font color="#ffffff">Okay, this is nearsighted. I can see things close.</font> <font color="#ffffff">The rays from that are diverging.</font> <font color="#ffffff">But you want to be able to see things at distance.</font> <font color="#ffffff">Okay, and so at distance, the rays that are coming in are very close to parallel.</font> <font color="#ffffff">And you can't focus those to the right spot.</font> <font color="#ffffff">Either because your eyeball is a different size than normal or the focusing power of your cornea is different.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So all we have to do is take parallel rays from this object and make them diverge.</font> <font color="#ffffff">And we know how to do that, right?</font> <font color="#ffffff">We use a diverging lens.</font> <font color="#ffffff">And when those parallel rays come into this diverging lens, they're going to diverge.</font> <font color="#ffffff">If they diverge the same amount as we had up there, everything's going to come back together.</font> <font color="#ffffff">At the retina, it'll be in focus.</font> <font color="#ffffff">So for nearsighted, you add a diverging or negative lens.</font> <font color="#ffffff">Okay, that's how you fix nearsightedness.</font> <font color="#ffffff">But what about farsighted?</font> <font color="#ffffff">If you are farsighted, then you're looking at parallel rays coming in from a distant object.</font> <font color="#ffffff">Here is your eyeball.</font> <font color="#ffffff">And if you're farsighted, then they don't quite get to a focus right where they should be.</font> <font color="#ffffff">Okay? So we need to get that focus in a little closer.</font> <font color="#ffffff">And the way we get it in closer is we add a converging lens.</font> <font color="#ffffff">And remember, converging is positive.</font> <font color="#ffffff">Okay, and so parallel rays that come in, we want them to focus on the retina.</font> <font color="#ffffff">And so we add a positive lens right there and that starts to get these rays to bend in a little bit.</font> <font color="#ffffff">Now they can come to a focus right on the retina.</font> <font color="#ffffff">So, you automatically know if you are nearsighted or farsighted just by looking at your lenses.</font> <font color="#ffffff">Okay, so here --</font> <font color="#ffffff">these are my reading glasses.</font> <font color="#ffffff">Okay, when you get old like me, you get reading glasses.</font> <font color="#ffffff">And these have a positive value.</font> <font color="#ffffff">They're converging, which means I am farsighted.</font> <font color="#ffffff">I can see things that far just fine.</font> <font color="#ffffff">But when I try to read things close, without glasses, then it's hard to read.</font> <font color="#ffffff">Okay, and so I need a converging lens.</font> <font color="#ffffff">Now how do you know this is converging versus diverging?</font> <font color="#ffffff">Well, first off, if you look through them at some point </font> <font color="#ffffff">the outside world should get very big as I move this away from my eyeballs.</font> <font color="#ffffff">And if you look at that camera,</font> <font color="#ffffff">if I'm looking at the camera, my eyeball should look a little big to you.</font> <font color="#ffffff">Do the eyeballs look a little big to you?</font> <font color="#ffffff">And especially if I pull them out a little bit. Let's try this.</font> <font color="#ffffff">Trying to do without getting shadow all over the place. Okay.</font> <font color="#ffffff">This eye should look big to you guys on the computer monitor.</font> <font color="#ffffff">Okay, that's how you know it's a positive lens.</font> <font color="#ffffff">If its a negative lens, the outside world will always look smaller.</font> <font color="#ffffff">Okay, when you look at a friend and you do this, </font> <font color="#ffffff">your eyeball will always look smaller if it's a negative lens.</font> <font color="#ffffff">Okay, so this is a converging lens, the other would be a diverging lens.</font> <font color="#ffffff">Now there's other things that you can figure out here.</font> <font color="#ffffff">One is there is a little bit of astigmatism.</font> <font color="#ffffff">Which means it's not a spherical lens. </font> <font color="#ffffff">There's actually a little bit cylindrical shape to it.</font> <font color="#ffffff">And the way you can tell that with some of these glasses, is if you look through it and you rotate it.</font> <font color="#ffffff">And the image goes from squished this way to squished that way.</font> <font color="#ffffff">That's astigmatism.</font> <font color="#ffffff">And if you look at my glasses, my left eye is more astigmatic than my right eye.</font> <font color="#ffffff">So when I rotate this, the image in the right one </font> <font color="#ffffff">doesn't get as distorted as the one in the left one.</font> <font color="#ffffff">And when I look at it you guys, you become very distorted in that left lens.</font> <font color="#ffffff">The other thing to know about nearsighted versus -- or nearsighted versus farsighted is,</font> <font color="#ffffff">A positive lens means you can focus light down to a point. Okay?</font> <font color="#ffffff">So if you are out in the sunshine and you're trying to focus light to a point on a piece of paper.</font> <font color="#ffffff">Because you're trying to start a fire.</font> <font color="#ffffff">Because you're on a desert island. </font> <font color="#ffffff">You had better hope that the person's glasses you're using, they're farsighted.</font> <font color="#ffffff">Positive lens, so you can focus that sunlight to a point.</font> <font color="#ffffff">If they're nearsighted, it's never gonna help you start a fire.</font> <font color="#ffffff">Sunlight that comes through there diverges everywhere.</font>