33. Geometric Optics

Ray Diagrams For Lenses

Physics33. Geometric OpticsRay Diagrams For Lenses

It says, "While you are photographing a dog, it begins to move away. What must you do to keep it in focus?" Okay so let's go back to the camera. The cameras just like an eyeball, right? It's the same idea so let's draw a camera. Here's the lens of the camera. Here is the film plane or the CCD plane. Okay, this is where your retina would be in your eyeball. And it says we got a dog that we are photographing and it's in focus. But then it's going to start to move away. All right so we draw a dog. Here's our dog. Happy dog. Okay, and we're taking a picture of this dog and he's in focus. All right, so if I think about the rays coming off the dog when that dog is close, how do I figure out where those rays are going to end up? Well let's draw some focal points. Let's say there is a focal point here. And a focal point there. And now this is just like our good old object. So we're gonna erase the dog and just draw an arrow like we usually have. Okay, and now let's see if we can figure out where it's all in focus. Parallel rays go through the focus. Rays through the focus go parallel. Everything is in focus right here, at the film plane. Good. But now this thing is going to move further away. Okay, and if it moves further away, we're gonna have to do something to keep it in focus. So let's draw a picture again. It looks like that. But now parallel rays go through the focus. Rays through the focus go parallel. And look what happens. The image should be in focus right there. And yet our film plane is fixed. Okay, we can't move where the film is, but we can move where the lens is. So if I want to move that image to the film plane, which way should I move this lens? Should I move it closer to the film or further away from the film? I have to move it closer to the film. If I move the lens to the right, the image is going to move to the right. Everything is going to move to the right. So I want to move the lens closer. Okay, and let's see if that is the right answer. It says, move the lens closer to the sensor, or the film sensor's CCD array if it's like your digital SLR. And it does say that that is the right answer. Okay, so when things get further away, you want to move the lens closer to the film. And, in fact, when they get out at infinity, the focal point should be at the film plane. Okay.