Class 12 Physics (India)
- Convex lenses
- Convex lens examples
- Concave lenses
- Paths of light rays through spherical lenses
- Image formation in spherical lenses
- Thin lens intuition
- Thin lens formula
- Solved example on lens formula
- Using magnification formula for lenses
- Using the lens formula
- Convex and concave lenses
- Thin lenses questions
- Virtual Object
- Thin lens formula - virtual object
- Worked example: Image formed by multiple lenses
- Image formed by multiple lenses
- Lens makers formula
- Solved example: Lens makers formula
- Sign convention: radii of curvature (lens maker's formula)
- Power of lens
- Power of lens
- Thin lenses in contact
- Thin lens sign conventions
Convex Lens Examples. Created by Sal Khan.
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- I thought Convex lenses always displayed Virtual images.(7 votes)
- • Convex mirrors always display virtual
• Concave lenses always display virtual.
• Concave mirrors can display real or virtual.
• Convex lenses can display real or virtual.(66 votes)
- Why is it that the ray that goes through the center of the lens is not refracted (~7:26)?(14 votes)
- because it doesn't change direction.... it will however slow down when it enters the medium... and refraction is the change in direction caused by this slowing down, but as the wave is normal to the surface it slows down at the same instant across it's "front" and as such doesn't change direction.(6 votes)
- At7:50, if a ray passes right along the principal axis and through the center of the lens, wouldn't it intersect with the ray that passes through the focus, since the ray that passes through the center of the lens doesn't get refracted?(6 votes)
- I think light along principal axis from the bottom of object is not possible because the bottom is not visible ... so light must be taken from above the principal axis... (according to me) :D
hope this helps(3 votes)
- Do electrons absorb light ? Do they cause refraction?(4 votes)
- Electrons can absorb photons of light but this is not the reason for refraction of light.
Light is an oscillation of electromagnetic field, this interacts with the electric charges of the particles in the material causing an oscillation in the materials electromagnetic field. The two oscillations interfere with each other causing the the combined field oscillation to propagate slower than the speed of light in a vacuum and change direction.
When light exits the refractive material the interference no longer occurs so the light resumes its normal apparent velocity and direction.(6 votes)
- At2:53, How does Sal tell that the image formed is inverted or erect?(3 votes)
- he showed 2 rays coming from the top point of the arrow, when they converge below the principle axis then it is an inverted image and when it it converges above the principle axis(usually in cases of virtual images) then it is an erect image.(4 votes)
- I understand how to draw the ray diagrams and how to solve problems asking whether the image is real or inverted, bigger or smaller, etc. However, my professor likes to ask questions that are very in depth and realistic. (Also, I am genuinely curious). I do not understand what you mean when you say "we will not see any image" or "we will see an inverted/real image, that is bigger" versus "we will see a virtual/upright image that is smaller", etc, in more realistic terms. In real life, like let us say that I am far sighted and I need glasses that allow me to, let's say, read a book. I would need such a convex lens, right. In this scenario, can you explain how/what it means for the image to be at the focal point, or between my center of curvature and focal point? How can I see the image inverted if I am reading a book? That has never happened to me… Nor have I seen things smaller. I am not sure if the question makes sense, but essentially I am asking if you could give a real life scenario of something that is AT the focal point or IN THE MIDDLE of the center of curvature and focal point, etc. Thank you!(3 votes)
- Well, when you are prescribed lenses for glasses, it is such that the rays are slightly bent towards or away from each other, but not that they form a real image or something on your eye.So, when the rays come, since they are not parallel, they do not converge at the focus of your eye's lens.
If you are farsighted, your eye is smaller.Farther objects form images on your retina, but near objects do not as their images form at a point behind your eye.Since you are prescribed convex lens, they converge some of the light from nearby objects(which is why you always wear them while reading,etc.) and this is again converged, but at a closer on another point in your eyes.This point is your retina.So, you can see clearly.
If you are nearsighted, your eye is larger.Closer objects form images on your retina, but farther objects do not as their images form at a point before your retina.Since you are prescribed concave lens, they diverge some of the light from farther objects(which is why you always wear them) and this is again converged, but farther on another point in your eyes.This point is your retina.So, you can see clearly.
Hope you understand and hope this helps!(3 votes)
- how can a lens give a vitual image?.....cannot we put a screen in where the magnified image is formed?...in the last example?(2 votes)
- No. Try it with a magnifying glass. Instead of looking into it with your eye to see the object you are trying to enlarge, put a screen where your eye would be, and look at the screen. Do you see an image?(3 votes)
- In the fourth case why didnt the second light ray get refracted..? at7:25(3 votes)
- if the medium on either side of the lens is changed then what effect would it have on the focal length on the lens ???(3 votes)
- Lens change its focal lenght in the following ways--->
1.If the medium is dense then the focal lenght increases, due to the change in refractive index being lowered
2.If the medium is rare then the lens focal lenght decreases due to the change in refractive index being increased
it will also show its effect on the power of the lens.
power of the lens is given by the formula u/f.
in this formula u is the refractive index of the surrounding medium and f is the focal length of the lens in the surrounding medium.
note that a medium can have a different refractive index for different color of light rays.
hope that this would help.(1 vote)
- Hyperopia, also known as farsightedness, is a defect of vision caused by an imperfection in the eye, (often when the eyeball is too short or the lens cannot become round enough), causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance.This is my question,Is the "convex lens", can be a treatment in this eye defect called"hyperopia"?(2 votes)
- in hypermetropia, the image is formed at the back side of the retina and to focus it on the retina a convex lens is used(2 votes)
Like we did with parabolic mirrors, what I really want to do in this video is just for put objects at different distances relative to this convex lens and just think about what its image will look like And the whole point of doing that is going through all the different situations but more is getting practice of how to think about it So let's first put an object out here That's more than two focal lengths away from the lens I'll put the object right here We'll do our classic arrow Actually I'll make a point here. When we dealt with parabolic mirrors we talked about the distance of two focal lengths being our center of curvature Over here we're just gonna call it 2 focal length, because it's really not the center of curvature-- or this distance really isn't the radius of curvature of each of these curves. So we're just gonna call it 2 focal lengths So with that said, let's actually try to figure out what the image of this thing would look like as the light from it gets refracted through this lens So like always, it's useful to draw one ray Every point of this object is emitting rays in every direction because it's diffusely reflecting light So we can just pick rays that are convenient So we can go from the tip of the arrow and go parallel to our principal axis just like that I'm not gonna show all the internal refraction within this lens right here But we know that if we enter the lens, parallel to the principal axis when we get refracted, we will go through the focal point on the other side of the lens And on the left side of the lens we'll do another ray that goes through the focal point on the left side and then comes out parallel The incident ray goes through the left focal point and when it gets refracted, it will now be parallel And so the light that came from that point of the object will reconverge right over here So if you did this for every point of this object if you did a point in the middle right over here, it would reconverge right over here If you did this point right over here, it would reconverge right over there So the image of this object is going to look like this So it's going to be a real image. The rays actually converge here. So it's a real inverted image It was pointing up before. Now it's pointing that down And in this situation, it's actually going to be smaller than the original So, real, smaller, inverted image Let's do a couple of other scenarios Let me copy and paste this before starting this video to save on time Let's do a situation now where the object is at the 2 focal distances. I guess we can call that Let's put the object right over here We can do the exact same thing You might want to do it on paper on your own to get some practice doing it We'll do one ray that's parallel When it gets refracted, it will go through the focal point on the other side And then we'll do another ray that goes through the focal point on the left side and then it will become parallel And they can reconverge or they converge right over there So you can see that this is actually a parallelogram This distance right there is going to be the same thing as that distance over here It is actually--I don't want to say symmetry. You can't flip it over But this is a--and this distance right over here is going to be the same thing as this distance over here. I won't go into all the geometry Anyway, this would be an inverted image of exactly the same size at the exact same distance Actually I didn't talk about distance in this one over here Over here, it's a real image; it's smaller and it's also going to be closer in than this one was. It's gonna be closer to lens But here the image is going to be the same size as the original object It's going to be inverted, but it's going to be at the same distance from the lens just on the other side. Once again this is a real image Let's do another one Copy and paste So let's stick something between one focal length and two focal lengths So what's put my object right over there So once again, let's go parallel then we will refract through the focus on the other side And then let's go through the focus on the left side And then we will refract and go parallel So here, we will have--remember, I could use any point. Here I just used the tip because I know the base will converge over there If I took 2 points, it would go converge right back there And then if I did with a point right here and I did that same exercise, it would go right over there And so the whole image would show up over here So here, it's real; it's inverted; and it's larger and it is now further away from the lens than the object on the other side This is really kind of the reverse of the first example The first example, the object was larger and more than two focal lengths away and the image was in this range Now the object is here and now the image's on the other side. So they're kind of-- that's really just the other side of that first examples Let's do a couple more So let's put the object at the focal point and see what would happen And sometimes people memorize this type of things for physics exams You don't need to. All you have to do is remember just think about 2 rays usually from the tip of the arrow that gives you a sense what the image will look like and you do one parallel and one that goes through the focal point Well, with that said we're gonna do something slightly different when something is sitting at the focal point When something is sitting at the focal point one is we can do the parallel It will be refracted through the focal point on the other side And then instead of doing a ray that goes through the focal point on this side because actually you can't. You're sitting on the focal point right there Let's do a ray that does not get refracted. We did a similar thing with the parabolic mirrors What I want do is a ray that'll go right to the center of the lens where it won't get refracted It's just go straight through the center of the lens So what you have here is, both of these rays that were diverging from this point from this tip they don't reconverge anywhere And they don't even look like they're diverging from another point if someone's eyeball is right over here They're just going to see these as 2 parallel rays of light So no image will form. Not a real image or virtual image So we can say no image is going to form Now the last case is, let's put the object less than one focal distance away Let's put the object right over here. And think about what happens Once again, the light ray from this tip where to go parallel. It will be refracted through the focal point on the other side And then let's do another light ray, going in a direction as if it were coming the direction of the focal point on this side So it comes from that direction The light ray would go like that And then it would come out on the other side parallel So clearly, these two light rays are not converging So no real image's going to form But they do look like they're diverging from some point Continue these lines. They both look like they're coming from a point right over there So what's going to happen? If someone's eyes are processing these light rays they're gonna see the tip of this arrow all the way up here They're gonna see the base of this arrow down here And essentially, they're going to see a magnified virtual image of the actual arrow Anyway, hopefully you found that interesting