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Anderson Video - Law of Reflection

Professor Anderson
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 which is the following. Let's say we have a mirror. Okay, we know that a mirror is made up of shiny metal. And let's say that we have a ray coming in at some particular angle. And when we draw these angles, we always draw it relative to the surface normal. That incident angle, we call it theta I. Now, going out the other side, we of course have a reflection. And if this is a nice flat metal surface then that reflection is at theta R and the law of reflection says the following. Theta I is equal to theta R. Incident equals reflected. Remember, they're always relative to that surface normal. Okay, so in the case of the mirror that we just talked about we said, "where is the image?" Using this law now, we have to think about how do you form an image in a mirror? So let's draw the following picture. We've got a metal mirror that we're going to make vertical. Let's put in what we call the optic axis. And let's put an object right here. What do the rays from that object do? Well first of all how are those rays generated at all? Light illuminates the object just like light from these lamps is illuminating me. Every single point of me is reflecting rays in all directions. So if this is an object, if this is an arrowhead object light that is hitting the tip will go off in all directions. Light that is hitting the base will go off in all directions. Just like when you look at this image of me light from my cheek is different than light from my nose or my hair and yet those rays are going off in all directions. You intercept a few of them with your eyeball. So let's think about where the rays will be going from here. Okay, light comes in, bounces off. We don't have to worry about the light coming in. What we have to worry about is the light bouncing off. So one ray will do this. It will bounce off of the mirror and come back. And if I continue with a dashed line that is where that ray appears to be coming from. Now, if I think about an observer Here's my observer. Then there will be a ray from the tip of this object that comes up and bounces off and goes into the eyeball. And now if I take a dashed line and continue the point where those two dashed lines intersect each other is where the image is located. So that object has an image that is right here. Okay, and so everybody got it right. The image is equidistant from the mirror on the other side. And you know this already, right, when you stand in front of a mirror you see an image of yourself on the other side of the mirror the exact same distance away. If you're standing two feet away from the mirror it looks like you have an image that's two feet on the other side of the mirror. If you back up further your image gets further and further away. Okay, as you get closer it comes closer in. So everybody got that one right, their intuition was correct that it was indeed answer B on that clicker question. And the way you see that is this idea of the law of reflection. If this ray comes in at this angle it's going to go off at the exact same angle relative to the normal and now you just have to follow those lines back. It's kind of weird but when you, when you see something you see what you think is where those rays are coming from. Right? If the mirror wasn't there, you wouldn't see an image over there. Putting the mirror there means that it looks like those rays are coming from there.