Hey guys. So far we've discussed convex mirrors and concave mirrors, but now the final type of mirror, plane mirrors. Not plain as in simple, but plane as in flat. These are the types of mirrors that you would hang on your wall. These are your bathroom mirrors, etc. So by far the most popular kind of mirror. Let's get to it. Collimated light coming off of a plane mirror doesn't converge or diverge. The law of reflection states that if you're hitting a flat surface perpendicular to that surface, you bounce off at the same angle. So all the blue rays, those initially collimated rays of light, all bounce off collimated. The light doesn't converge or diverge. That means that there's no focus for a plane mirror, not on the front side of it and not an apparent focus on the back side of it. Sometimes just for equations which we'll cover in the future, just to make those equations work, the focal length of a plane mirror is said to be infinity. It's said that infinitely far away, hypothetically, those lines could converge. It's just a mathematical tool to make equations that we'll see in a little bit work better.

To draw ray diagrams for plane mirrors, we need to draw 2 of the following lines. There are 2 types of axis, then reflected off of the mirror parallel to the axis then reflected off of the mirror parallel to the central axis. And that's exactly what I showed in the image above. If you come in parallel to the central axis, you leave parallel to the central axis. And then any line from anywhere to any points on the mirror is reflected at the same incident angle. For convex and concave mirrors, the second point was only true for lines that went to the apex. But for plane mirrors, because they're flat everywhere, not just at the apex, this applies to any line drawn at any point on the surface.

Let's do an example. A 1.6 meter tall person stands 0.7 meters away from a plane mirror. How tall does the person appear in the mirror? How far from the mirror does the image appear? Is this image real or virtual? So, just a whole bunch of information about the image that they want to know. So let's draw our lines: first, parallel. And this is going to return parallel. So this one is a round trip. It goes both ways. What this also means though is when it's coming back to you, it appears as if it came off the other side of the mirror parallel. Now what I'm going to do next is draw a line from the head to halfway down the body because then it's going to reflect at a 45-degree angle and reach the feet. From the head, I can choose any point on the mirror and it will reflect at the same angle that it hits, but I'm strategically choosing to have it reflect at a point halfway down the person’s body. And so I need to draw where this line appears to come from. And you can see right away there is an apparent convergence of light. So, there is an image here. What type of image? Is it a real image or is it a virtual image? This is absolutely a virtual image. This is not real because the light is not actually converging on that point. It only appears to converge on that point. Furthermore, the only types of mirrors that can produce real images are mirrors that can actually converge light which are concave mirrors. Convex mirrors diverge light so they can never form a real image and plane mirrors don't converge or diverge but since they don't converge, they cannot form a real image either. What's the height of this image? Look at this particular green line. It's at the same height as the person. So the height of the image is just 1.6 meters. Now the question is, how far away is this? Well, this angle is actually going to be the same as this angle. These two triangles are identical triangles. That means that this distance has to be the same. So you're going to find that whenever an object is in front of a plane mirror, that mirror produces a virtual image of the same height as the object, upright, and the same distance behind the mirror that the object exists in front of the mirror. This wraps up our discussion on ray diagrams for plane mirrors. Thanks for watching guys.