The mirror you see in this picture is a concave mirror. An easy way to keep concave and convex mirrors straight in your mind is that a concave mirror caves in on the front side. So we're going to examine in this video how an image of this arrow is formed in this concave mirror. Right now let's remove my image and let's just give a little bit of vocabulary. The line that runs through the center of the mirror is known as the principal axis. When drawing mirror diagrams to try to determine where the image is, you always want to start with the principal axis. Some other elements you want to label on this picture. The focal point, which is where all the rays of light are going to converge. I'll demonstrate that in just a moment. You also want to label the center of curvature. Let's label these two now. In a word problem given on paper for a test kind of thing, you'll typically be given the focal point of a lens. It's a point, I mean I'm sorry, of a mirror. It's a point that occurs somewhere along this principal axis. I'm going to experimentally find it for this mirror so that you can see how it works. To find that, let me use the laser level from our lab the other day and shine a light as close as I can down the center of this principal axis. Let me turn off the overhead light so you can see what's going on. I'm suffering from a little bit of accuracy. Accuracy issues here. What you can see, my incident beam comes in, strikes the mirror. It's coming off at a slight angle. It tells me I might have set this mirror up a little incorrectly when we try nudging. There we go, not too bad. Light comes in on the principal axis, reflects off pretty darn close to it. This mirror is a spherical mirror, meaning that this radius of curvature is constant. You can string a bunch of these mirrors together and they make a perfect circle. As I move my ray of light up and down, parallel to the principal axis, let's observe what happens to the laser beam as it reflects. Moving up slowly. There I'm pretty parallel. So let me move down slowly parallel. Notice that within the tolerance of me being able to hold this darn thing still, this line reflecting off the mirror is pretty much always coming through near this point. Let me come down here. Still suffering from some accuracy issues, guys. Alright, so let me come down on the lower side. So try to remain parallel. There's a point here where the reflected ray crosses over the principal axis that's known as the focal point. Let me label that now. As I said earlier, the focal point will usually be given to you in a word problem. It's the distance from this back edge of the mirror to the focal point. Now center of curvature. There's a mathematical relationship here related to the curvature of this mirror. The focal point is always half the radius of curvature. Let me get something to measure with. And we'll see that the focal point is 3.4 centimeters from the center of curvature. We'll go another 3.4 centimeters. That's the center of curvature there. The focal point we label with just an F, center of curvature, a C. Now let's see about making an image. We're going to worry now about the object and image, the other two abbreviations here on my list. Let me bring back an object to create an image of. I'm going to set this on the principal axis. Yeah. Right now my image is outside of the focal point. It's not between the focal point and the mirror. It's actually beyond the focal point. This means that the image distance is going to appear smaller than actual size. Okay, how do I actually do this? Bring back your laser level or in your case a straight edge. The first line that we want to draw with the straight edge is parallel to the principal axis. And just touching the top edge of the object. Bring back my mirror so I actually get a reflection. Keep in mind you'll be doing this with a ruler, not a laser level. See I pass through essentially the focal point. And I have here this line. This is the reflected image. And we're going to see that we need to sketch in this line. I'll draw both lines, the incident and reflected for our benefit later. This is the incident drawn arrow coming in. This is a reflected ray coming out. Okay, next step. There's a second line you want to draw. You want to pass a straight edge across the top of the object through the focal point onto the mirror. And that's about got it. You want to also draw this line. If you dot's on here, again remember you'll be doing this with a ruler, not an arrow. And then you can actually laser line in mirror. The slabel incident reflected again, incident reflected. The top of the image, remember both of my incident rays passed through that point. Where my two reflected rays cross is going to be an image. And it will be as tall as the distance from the reflected ray back to the principal axis. So let me attempt to draw this sketch. And of course some color would help us out here. This is a reflected image in a concave spherical mirror of a yellow, post-it flag. I'm sorry, that's totally orange. You'll notice a couple of key features. This image is upside down and it's smaller than actual size. Now for you guys, I want you to experiment with the following. Find for me the image of an object that's placed exactly on the focal point of your lens. And find for me the image of an object placed between the focal point and the lens. I'll have answers available for you to check your work.