33. Geometric Optics

Ray Diagrams For Mirrors

# Ray Diagrams for Plane Mirrors

Patrick Ford

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Hey, guys. So far we've done convex mirrors and we've done concave mirrors. But now the final type of mirror plane mirrors, not plane, is in simple but P L. A. And E plane is in flat. Okay, these are the types of mirrors that you would hang on your wall. These are your bathroom mirrors, etcetera. So, by far the most popular kind of mirror. All right, let's get to it. Column mated. Light coming off of a plane mirror doesn't converge or diverge, right? The law of Reflection says that if you're hitting a flat surface perpendicular to that surface, you bounce off at the same angle. Right? So all the blue raise those initially column mated. Rays of light all bounce off colonnaded. Okay, 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 will cover in the future just to make those equations work, the focal length of a plane mirror is said to be infinity. It said that infinitely far away hypothetically, those lines could converge okay. It's just a mathematical tool to make equations that we'll see in a little bit work better. Okay. To draw raise diagrams for plain mirrors, we need to draw two of the following lines. Okay, there are two types of line, but the second line is actually a new infinite number. Now get to that in a second number one, a line parallel to the central axis, then reflected off of the mirror parallel to the central axis. Right. 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 that's reflected at the same incidents angle okay for convex and concave mirrors. The second point was on Lee, true for lines that went to the apex, but for plain 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 m tall person stands 0.7 m 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 really 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 gonna return parallel. So this one is a rounded 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. Right now, what I'm gonna do next is I'm going to draw a line from the head toe halfway down the body, because then it's gonna reflect at a 45 degree angle and reach the feet. Okay, so from the head, I could choose any point on the mirror, and it will reflect at the same angle that it hits. But I'm strategically choosing toe. Have it reflect at a point halfway down the person's body. Okay, okay. 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, right? What type of image is it? A real image Or is it a virtual image? This is absolutely a virtual image. Okay, this is not really because light is not actually converging on that point. It only appears to converge on that point. Furthermore, the Onley types of mirrors that can produce riel images are mirrors that can actually converge light, which air con cave mirrors. Convex mirrors diverge light so they can never form a real image. And plain 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? Well, look at this particular green line. It's at the same height as the person. So the height of the image is just 16 m. Now, the question is, how far away is this? Well, this angle is actually going to be the same as this angle. Okay, These two triangles are identical triangles. That means that this distance has to be the same. Okay, so you're gonna find that whenever a knob jek tis in front of the plane mirror, that mirror produces a virtual image off the same height as the object upright and the same distance behind the mirror that the object exists in front of the mirror. Okay, This wraps up our discussion on Ray diagrams for playing mirrors. Thanks for watching guys.

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