Journal 1/31/17 Objective Tonight’s Homework In the picture at right you can see a nearly infinite sequence of reflected images in the mirrors. What do you think would happen if we place a bright light in front of the mirror? Objective Tonight’s Homework To learn how to calculate the bending of a light through a lens Copy down and define the terms you’ll need for ray diagrams.

Ray Diagrams: Plane Mirrors At right we see a reflection of a pot in a mirror. We want to examine what’s happening with light here.

Ray Diagrams: Plane Mirrors At right we see a reflection of a pot in a mirror. We want to examine what’s happening with light here. First, we see the light that comes from the sky, bounces off the pot, and into our eyes.

Ray Diagrams: Plane Mirrors At right we see a reflection of a pot in a mirror. We want to examine what’s happening with light here. First, we see the light that comes from the sky, bounces off the pot, and into our eyes. However, we can also have light bounce off the pot toward the mirror, bounce off the mirror, and into our eyes. Since our eyes don’t know that the light bounced, we think we see another pot beyond the mirror. We see a virtual image.

Ray Diagrams: Plane Mirrors That drawing got a bit complex. Physicists use something called a “ray diagram” to model these things more cleanly. Let’s construct one of these things.

Ray Diagrams: Plane Mirrors First, we start with the person actually doing the seeing. This is usually drawn as an eye from the side.

Ray Diagrams: Plane Mirrors First, we start with the person actually doing the seeing. This is usually drawn as an eye from the side. Next, we add what it is we’re looking at. Our object. Traditionally, this is either a candle or an arrow. We’re going to use the candle.

Ray Diagrams: Plane Mirrors First, we start with the person actually doing the seeing. This is usually drawn as an eye from the side. Next, we add what it is we’re looking at. Our object. Traditionally, this is either a candle or an arrow. We’re going to use the candle. Next, we want to add the incoming light rays. We draw this in red. This is where things get tricky. When light hits an object it bounces in all directions. Anyone could see this candle from any angle! (Just draw the red arrow only)

Ray Diagrams: Plane Mirrors We obviously don’t want to draw every light ray as that would fill up our paper. So we only draw the one going from our source to the observer to keep things simple. (Draw the red arrow here)

Ray Diagrams: Plane Mirrors We obviously don’t want to draw every light ray as that would fill up our paper. So we only draw the one going from our source to the observer to keep things simple. (Draw the red arrow here) But things get even more complex than this. If our red arrows were everything, we’d only see the tip of the candle. But we can see the base, too. So we need more light rays hitting the object. (don’t draw)

Ray Diagrams: Plane Mirrors These other light rays also complicate the diagram, so we usually don’t draw them. Just keep in mind that they’re there. This is also why we use a candle for our diagram. We’ll imagine our person is just looking at the point of flame on the top. We also usually don’t draw the top arrow as we always assume light comes in from somewhere.

Ray Diagrams: Plane Mirrors So! Let’s do an example of a person seeing a reflection of an object in a mirror. In our setup we have our observer, our object and our mirror (always color both blue). We know there’s an image behind the mirror somewhere (always in yellow) but we’re not sure where. We’re going to draw a few specific light rays to determine where our image is.

Ray Diagrams: Plane Mirrors Ray 1: Draw a ray from the object to the observer. Let it bounce off the mirror. Remember the 2 angles are equal. Ray 2: Draw another ray straight horizontal to the mirror. This will bounce right back right to the tip of the candle. 2 1

Ray Diagrams: Plane Mirrors Our eyes don’t know that light bounces. Our brains are tricked into thinking that the light has gone straight. Even if that means it must have gone through the mirror to get to our eyes.

Ray Diagrams: Plane Mirrors Now we can draw our image. Why? The person looks in the mirror and sees the candle tip. So the image could be anywhere along this line.

Ray Diagrams: Plane Mirrors But if you were standing where the real candle is, you could see the image anywhere along this line. The only location that works for both is where they intersect. Ray diagram complete!

Tonight’s Homework Ray diagrams and optics have a ton of vocab terms you guys will need to know. Your homework for tonight is to look in your book (starting at page 368) and define the following terms: Object Principal Axis Image Focal Point Virtual image Real Image Lens Magnification Mirror Convex Concave You have the rest of class to finish your lab write up and work on your homework.

Exit Question Look at the diagram. Which letter can the observer NOT see? a) A b) B c) C d) D e) All of them are visible to the observer f) None of them are visible to the observer A B C D