Anderson Video - Thin Lens Image Formation

Professor Anderson
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Here is our optic axis. Here is our thin lens. Okay? This is called a converging lens. It tends to converge the rays to a particular focal point. It's also called a positive lens. All right? So, biconvex, converging, positive, those are all essentially synonymous. So, let's draw the focal point f, for this lens. And now, let's do the following: let's take our object and let's put it inside the focal length. So, we brought the object all the way through the focus and now it's sitting right there. Okay? This is not hard to do. Right? If you take an object and hold it right up next to your eyeball that's essentially the same idea. How is this going to form an image, and where is that image? Let's follow our rules. It says that rays that are parallel are gonna go through the focus. That's number one. It says that rays through the focus are gonna go parallel. Well, that one's gonna be a little hard to draw because if I come from this focus, it's gonna miss the lens entirely. Okay? That would be our ray number two, but we're not going to worry about that. Remember, there's only two of the three rules that you need. The last rule, I'm gonna erase that one just for clarity. The last rule was rays through the center. Do not bend. All right. We can draw that: one rays through the center do not bend. That's number three. So, where's the image? Well, it's where those two rays meet. That looks like a problem, right? It looks like they are separating. They're not getting closer together. But, that means somewhere, back over here, it looks like they were coming from the same point. And so, all we have to do is extend this dashed line back, take this one and extend this dash line back, and they meet right there. This is the object. This is the image. Remember, dash lines mean virtual rays. It's not real light over here. It's just where you perceive it coming from. So if you're sitting over here looking at this, you see these rays 1 & 3 coming from this point over here. So, is this a real object, a real image, or a virtual image? It's obviously virtual. In other words, I can't take my piece of film or a piece of paper and form an image on it there. That doesn't make any sense. It's upright. Okay, not inverted because it's also pointing up, above the optic axis. And, it is magnified. So, this is a way to see objects very magnified, is to put the object very close to the lens. And, this is what you do with a magnifying glass. Ultimately, we see it works with a microscope. Okay? That's what the image looks like in this case.
Here is our optic axis. Here is our thin lens. Okay? This is called a converging lens. It tends to converge the rays to a particular focal point. It's also called a positive lens. All right? So, biconvex, converging, positive, those are all essentially synonymous. So, let's draw the focal point f, for this lens. And now, let's do the following: let's take our object and let's put it inside the focal length. So, we brought the object all the way through the focus and now it's sitting right there. Okay? This is not hard to do. Right? If you take an object and hold it right up next to your eyeball that's essentially the same idea. How is this going to form an image, and where is that image? Let's follow our rules. It says that rays that are parallel are gonna go through the focus. That's number one. It says that rays through the focus are gonna go parallel. Well, that one's gonna be a little hard to draw because if I come from this focus, it's gonna miss the lens entirely. Okay? That would be our ray number two, but we're not going to worry about that. Remember, there's only two of the three rules that you need. The last rule, I'm gonna erase that one just for clarity. The last rule was rays through the center. Do not bend. All right. We can draw that: one rays through the center do not bend. That's number three. So, where's the image? Well, it's where those two rays meet. That looks like a problem, right? It looks like they are separating. They're not getting closer together. But, that means somewhere, back over here, it looks like they were coming from the same point. And so, all we have to do is extend this dashed line back, take this one and extend this dash line back, and they meet right there. This is the object. This is the image. Remember, dash lines mean virtual rays. It's not real light over here. It's just where you perceive it coming from. So if you're sitting over here looking at this, you see these rays 1 & 3 coming from this point over here. So, is this a real object, a real image, or a virtual image? It's obviously virtual. In other words, I can't take my piece of film or a piece of paper and form an image on it there. That doesn't make any sense. It's upright. Okay, not inverted because it's also pointing up, above the optic axis. And, it is magnified. So, this is a way to see objects very magnified, is to put the object very close to the lens. And, this is what you do with a magnifying glass. Ultimately, we see it works with a microscope. Okay? That's what the image looks like in this case.