 Welcome. We're going to talk about ray diagrams for thin lenses. Great thing to practice. Get ready for those exams where you've got to make some ray diagrams for lenses. First type we're going to look at is a, we would often call that like a convex lens or converging is the term that we like to use because that tells us what the light is actually doing. It's going to come together on the opposite side of the lens. Now in the first scenario we're looking at everything where the the object, in this case an arrow, is past f which means it's to the left of the focal point. We only need three rays to ever get one of these images. The first ray always goes parallel. It hits the lens and then it's going to go through the focal point on the other side. That one wasn't so bad to do. The second one is going to go through the focal point first like so and then it's going to go parallel. So it's kind of like opposite of the first one. And then the third one which is almost kind of overkill here but we'll put it in because it's going to come in handy later. It's going to go right through the center of the lens. It's called the optical center and then it's actually not going to do anything. It's not refracted at all, it just keeps moving. The result, we get a real image where the lines meet, where they converge in the other side of the lens. This image is inverted which means it's upside down enlarged because it was a little bigger than the first one if you were to measure the height of it. Past f means it's to the right of f and it's real. Now if you're wondering why is it real, it was because it didn't need to be made out of virtual rays. We're going to look at some virtual rays in a minute and then the real non-real thing will make more sense. Let's look at a situation where if we were to put our object right at f. All right, so same kind of story, parallel through the focal point and now the second one is going to give us some trouble. How are we supposed to go from the top of the object through the focal point? It is not going to ever hit the lens. The third one is even more interesting because if I go right through the optical center you're going to notice that these two rays are parallel, they're never going to meet and so you're getting no image when the object is at the focal point of the converging lens. Now what if you were to put that object right up next to the lens in front of the focal point? Now let's try to make this diagram. Parallel through the focal point. All right, and now the second one it's not really all that helpful because you could actually make it try to go through the focal point and then parallel. I like to skip over that one, save myself some time and I'm just going to draw out the green guy which goes right through the optical center. Look at those two rays. They are never going to meet here in the right hand side of the lens because they're divergent, they're moving apart. So what we're going to do is we're going to take those two rays and we're just going to extend them back along the same angle. This one is not being bent so I'm just going along the same angle here like that and then for the second or the first one that I did I'm going to extend back this refracted ray here. Now the point where those two cross which is somewhere roughly around there is where my image is going to be. Now because this image had to be made out of these extended back virtual rays it's a virtual image. It's erect, it's enlarged, it's behind the object and it's virtual. Now there's only one last scenario we need to look at and that's for your diverging lens. A diverging lens as the name suggests is going to spread the light out. So we're going to kind of stick to the same sort of ideas we used before but with just a little modification. The first line is going to go parallel. It's going to hit the lens but it's not going to go through the focal point because that would be converging. It's going to diverge. It's going to spread out and the way it's going to spread out is it's going to do so in such a way so that if if you were to extend the line as a virtual it would go through the focal point on this side. So I'll show you what I mean. Extend it. I don't know more or less like that. Let's see how close I came. Alright so it's going to look along that same angle. That wasn't bad. Okay like so. So notice this went parallel and then kind of through the focal point diverging extending it back as a virtual ray. I can do my second one here. I'm going to go aiming as if you were to go through the focal point so I'm going to set it up here with a nice solid line like it was going through the focal point on this side. So I want it to do that but it hits the lens and instead goes parallel. It's that parallel or refracted ray which I'm now going to go and extend back as a virtual ray. Like so. You can already see where we're going to get our image and then the last one. The last one is just going to go just like before straight through the optical center and then you could technically also use a virtual ray for this guy to have him come back like that as dotted lines. You can kind of see it's a dotted line one coming right back and there's our image. It is virtual, it's diminished, it's erect and it's in front of the object. That's every scenario you can basically get with a thin lens. For a little more information and some practice questions on thin lenses as well as the math that goes along with this visit my website ldindustries.ca