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Physics33. Geometric OpticsRay Diagrams For Lenses

Hi guys. Happy Monday. How's everybody feeling? Feeling like we have one week left? Not even, like three days left. You guys know your final exams on Thursday, right? Or what we're calling midterm three. Okay, good. We're all in the same place. So, anybody getting anywhere on the chapter 25 homework? Anybody aware that we have chapter 25 homework? Anybody aware that there is homework for the class? What class are we in here? What is this? >> [Inaudible]. >> Yeah. All right, so my first thought was we were going to largely finish discussing chapter 25 today here in the video lecture. But my second thought is there's quite a few people that have labs right now, unable to view the video until later. So I was thinking we just make the homework for chapter 25 do tomorrow? So I'm gonna do that right now, okay? So you got a little bit of a reprieve. And let's make that chapter 25 homework due tomorrow at 11:59, just like normal. And then, why don't we take a look at a couple of the problems? So, let's look at the first misconceptual problem. And see what it's addressing. Okay, so this one says, "If the distance from your eye's lens to the retina is shorter than for a normal eye, You will struggle to see objects that are far away, colorful, nearby, moving fast." Okay, so to answer this question we need t o know a little bit more about the eyeball. And maybe even a little bit more about nearsightedness and farsightedness. So let's -- let's do that. We're going to draw the ideal eyeball. And, remember what the eyeball looks like. You've got a cornea. There's a crystalline lens. On the back is your retina. You want to form images there. The ideal eyeball takes parallel rays. And focuses those parallel rays to a single point on the back of your eyeball. On the retina. Okay? This is what the ideal eyeball does. The relaxed ideal eyeball. So, when you look at a distant star, it looks like a point. The rays coming from that star are parallel. It should all focus down to a single point. But you, of course, rarely have an ideal eyeball. A few of you have ideal eyeballs. But a lot of you are nearsighted. Nearsighted is, of course, called myopic. And myopic means that you're not focusing the light quite at the right spot. So parallel rays coming in, again, will now focus too close. When they go in, they focus there. Not on the retina where your photoreceptors are, okay? Farsighted, which is called hyperopic. Means that parallel rays coming in -- into the eyeball now focus too far behind the eyeball. Okay. This is nearsightedness. This is farsightedness. So, when you think about if you're nearsighted or farsighted, how do you determine that? Well the first thing you do is you take off your glasses. And you hold up your book and you put it really close. If you can read it when you're really close, then you're nearsighted. If you can't read it when it's really close, then you're farsighted. Okay and this is obviously where they get the name. You can see things near or you can see things far. All right, so how do we deal with this in terms of this question right here? It says, "If the distance from your eye's lens to the retina is shorter than for a normal eye, You will struggle to see objects that are (blank)." Here's our normal eye. Let's draw it again, but we're going to make the distance from the lens to the retina shorter. Okay, so now parallel rays that come in are still going to focus to the same spot as this first picture. Except, it's now behind the eyeball. Okay, so this is a shorter eyeball. Does this look like farsightedness or does this look like nearsightedness? It looks like farsightedness, right? It's the same picture that we drew for the farsighted case. So, if you are farsighted, you can see things far but you struggle to see things close.