Ray Diagrams For Lenses - Video Tutorials & Practice Problems
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Ray Diagrams for Converging Lenses
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Video transcript
Hey guys. Now what we're gonna talk about are ray diagrams for converging lenses. OK. We talked about ray diagrams for converging mirrors. But a mirror's job is to reflect light and produce an image in front of it. A lens job is to transmit light and produce an image behind it. So we're gonna see how that works now conceptually with ray diagrams. All right, let's get to it when light strikes the surface of a mirror it reflects, right. This is already something we talked about a bunch. But when light strikes the surface of a lens, it transmits lenses are gonna be made out of transparent material that allows the passage of light through it. The transmitted light undergoes refraction just like the reflected light off of a mirror obeys the law of reflection. OK. Converging lenses right? As the name implies are lenses that allow the convergence of light when you have initially collated light like you do here, when it passes through the lens, those light rays all bend towards the central axis and therefore they converge on a point. This is a point of convergence, the point on the opposite side of the lens where the light converges is known as what guys we know it as the focus same as we had for mirrors. The thing about lenses though is that in order to draw ray diagrams properly, we have to represent focuses on both sides of the lens. OK. So whatever this focal length is F, we're going to have a second focus that same distance F on the front side of the lens as well. It's just a tool that we need to use in order to draw ray diagrams properly. OK. The most common type of converging lens and the one shown in the figure above is called a biconvex lens, right? It's biconvex because both sides are convex surfaces. OK? And it looks convex either way you look at it, you could rotate this mirror, uh sorry this lens and it's gonna look convex no matter how you look at it. OK. Just like with mirrors, we can draw ray diagrams to find information, qualitative information about the images formed by lenses. But we need an associated set of rules for lenses just like we had a set of rules for mirrors. OK. So those rules are gonna be presented here to draw ray diagrams. For converging lenses, you need to draw two of the following lines just like the same thing for mirrors. OK? A line parallel to the central axis then through the lens towards the far focus by that, I mean the focus on the other side of the lens. Second aligned through the near focus, the focus on the side of the lens of the object then through the lens parallel to central axis. OK. And lastly aligned to the very center of the lens that passes through, un deflected, that line will not get refracted. It's gonna pass through at the exact same angle. OK. And let's do an example to illustrate this process. Draw the image location for the fallen converging lens is the image upright or inverted. And in order to draw ray diagrams, you need some sort of ruler or some sort of straight edged object. What I have is my trusty protractor because that's what I'm using instead of a ruler. Now we're gonna draw two of the lines and find where they intersect. We could draw the third line and it would intersect where the other two do as well. All we need to know where the image is located is to find a point where two lines intersect. So the first line I'm gonna draw is parallel to the central axis. And for ray diagrams, you always draw them to the center of the lens. These types of lenses that we're gonna be dealing with are called thin lenses, which means that compared to the radius of curvature of the lens, they are very, very thin. OK. So they're essentially occupying a central line. OK. So you're always going to that center line that I have indicated. Then from the center line through the focus. OK. The next line is going to be from the object through the focus to the center line of the lens and then parallel to the central axis of our limbs and look you there, I just barely caught it. So here's a point of convergence because this Blu ray, uh Blu ray is just gonna continue. And right there is clearly the point of convergence. Now is this image upright or inverted? We're gonna use the same convention that we used for mirrors. If the conversion of light is below the h the central axis, the horizontal axis, then it's inverted. If it's above the central axis, then it's upright. This is clearly below the central axis. So this image is inverted, all right. And that wraps up our talk on ray diagrams for converging lenses. Thanks for watching guys.
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Problem
Problem
If an object is placed within the focus of a converging lens (it's at a distance of less than the focal length), will a real image form? If so, does it form at a distance less than or greater than the focal length?
A
The image is virtual. Distance is greater than f.
B
The image is real. Distance is greater than f.
C
The image is virtual. Distance is less than f.
D
The image is real. Distance is less than f.
E
No image is formed
3
concept
Ray Diagrams for Diverging Lenses
Video duration:
6m
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Video transcript
Hey guys in this video, we're gonna talk about ray diagrams for diverging lenses. We just took a look at ray diagrams for converging lenses. So we know that these should be similar. But since the light diverges, we know ahead of time, some things about the images that are gonna be formed. All right, let's get to it. A diverging le lens will never focus light ever because when light rays pass through it, they spread further apart, they don't come closer together. So they will only produce virtual images. I have a picture here of initially collated light passing through a diverging lens. And what it turns out to happen is that if you were to look at the diverging light after it had passed through the lens, it all appears to have come from a point. So we have an apparent convergence. This is almost identical to convex mirrors except mirrors, reflect light and lenses transmit light. Remember just like with those convex mirrors, the light appears to focus on a point which we call the apparent focus. For good reason. Though oftentimes, we'll just refer to it as a focus because physicists tend to be lazy now, because light can pass through either side, we need to have a focus that exists on either side of the lens just like we did for converging lenses. OK. This is also important in how we're gonna draw our ray diagrams. All right, the most common type of diverging lens is the one shown above which is called a biconcave lens. It's biconcave because it's a concave surface. Either way you look at it. If you were to flip this lens, it would still look concave. OK. Just like with mirrors, we can draw ray diagrams for these lenses to find out information about the images. We did it for converging lenses. Now we want to do it for diverging lenses and the rules are gonna be very similar with slight differences to draw red diagrams. For diverging lenses, you need to draw two of the following lines, a line parallel to the central axis then through the lens away from the near focus. OK. Second a line towards the far focus, the focus on the other side of the lens, then through the lens and parallel to the central access. All right. And lastly just like for converging lenses, a line through the center of the lens that passes through un deflected. Those are gonna be our three rays that we're gonna draw. We only need to draw two of them to find an intersection of light. But those are gonna be the three possible rays we can draw for ray diagrams of diverging lenses. OK. Let's do an example, draw the image location for this. This should say diverging lens is the image upright or inverted. OK. So before we even begin, is the image gonna be upright or inverted? What do you guys think this is a diverging lens? So the only images it can produce are virtual images and virtual images are always upright. So before we do a single thing, we know just through rationalization in our physics knowledge that this image is gonna have to be upright. I don't have to draw a single line. If the question was is the image upright, you'd be done. But where is the image located for that? We do need to draw a ray diagram. So the first ray is going to be from the object parallel to the central axis right to the center of the lens. That's where we always draw and then away from the near lens. OK. So I'm drawing it on a line parallel to the near lens but away from it. And then I'm gonna trace the line back to the origin, the apparent origin of that line because that's where your brain is gonna see that line coming from. Next. What I need to do is draw a line towards the far focus. OK. But once it hits the center of the lens, then it becomes parallel to the central axis. OK. And where does this line appear to come from, I need to trace it backwards. It appears to come from this direction. So you can see right there, there is an apparent convergence that is our virtual image. And because it's above the horizontal axis, because it's above the central axis, we know that it's upright exactly as we had predicted. All right guys, that wraps up our discussion on ray diagrams for diverging lenses. Thanks for watching.
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Problem
Problem
If an object is placed within the focus of a diverging lens (it's at a distance of less than the focal length), where will the image form? If so, does it form at a distance less than or greater than the focal length?
A
A real image is formed at a distance larger than f
B
A real image is formed at a distance less than f
C
A virtual image is formed at a distance larger than f
D
A virtual image is formed at a distance less than f
E
No image is formed
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