If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Thin lens equation and problem solving

Some examples of using the thin lens equation. Created by David SantoPietro.

Want to join the conversation?

  • male robot hal style avatar for user Aaron Dean
    Why does the object distance have to be a positive? Do we not follow the Cartesian sign conventions for thin lenses?
    (26 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user A Person
      I don't actually think he is using Cartesian convention; I think under Cartesian convention, anything to the left of the lens, say, is negative, and anything to the opposite side is positive. So for a convex lens, u (on the left) would be negative and v (to the right) would be positive.
      (19 votes)
  • piceratops tree style avatar for user Bralin Scott
    At how did 1/-8cm minus 1/24cm come out to be 1/6cm?
    (18 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Pratyay Islam
    I think something is wrong..Object distances are always negative aacording to modern cartesian sign system./.!!
    (7 votes)
    Default Khan Academy avatar avatar for user
  • female robot amelia style avatar for user Harshika
    At can't it be explained like.. if the image is real the image distance is positive and if the image is virtual the image distance is negative? :-)
    (7 votes)
    Default Khan Academy avatar avatar for user
  • starky ultimate style avatar for user Anthony Guerra
    At the image was described to be as positive. Using Sal's equation in an earlier video, we have that (do/di) = (ho/hi). In a hypothetical example, let's assume the height of the original image is 8 cm. Using the rest of the values from the video, we get (24/-6) = (8/x). Solving for x yields -2, which means that the image is inverted. Although the magnitude is correct, according to this video, the number should be positive. Why am I getting a negative number using Sal's equation as opposed to the Magnification equation?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user SmithChanteM
    I thought concave focal length is always positive and convex is always negative? that's what my physics book says and what we've been taught...
    (4 votes)
    Default Khan Academy avatar avatar for user
  • aqualine tree style avatar for user dravuja
    i thought the lens formula is ...
    1/v-1/u=1/f

    where u = object distance
    v = image distance
    f = focal length.
    (5 votes)
    Default Khan Academy avatar avatar for user
    • orange juice squid orange style avatar for user santhosh prabahar
      If you did this problem using the equation 1/f=1/v-1/u, you would get the answer as 6 cm.
      According to the same sign convention using which the above mentioned formula was derived, the answer 6 cm means the same as -6 cm when viewed from different sign conventions.
      The sign convention used deriving the above mentioned formula is known as the Cartesian sign convention. According to this, imagine a lens placed such that its optical centre coincides with the origin of the Cartesian plane. In a Cartesian plane rightwards and upwards are considered to be positive directions, whereas, leftwards and downwards are considered to be the negative directions. So, the same thing is applicable to lenses. So its always imagined that light rays are passed through the lens in the rightward direction.
      Hope this helps!
      (1 vote)
  • aqualine ultimate style avatar for user Hussain Mashkouri
    In the video, there was no convex lens example, it just said that the focal length is positive. can someone plz give me an example of it (and the solution), because it is very confusing.
    (2 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Adam Thai
      In this case, the focal length of the convex lens' focal length would be positive, totally opposite with the Cartesian sign convention, If you apply this method then you should be careful not to be confused.
      Both method are correct and the discrepancies are base on where is your point of view, though Cartesian sign convention is being use more widely.
      (1 vote)
  • mr pink red style avatar for user Ansuman Padhi
    Hi, Watch from to , Sal is describing about the object distance but I think that the object distance from every lens would be negative every time, weather it is a concave or convex, Right! Can any one tell me Whether I am Right or wrong.
    (2 votes)
    Default Khan Academy avatar avatar for user
  • primosaur seedling style avatar for user Arul Yalini
    There is another lens equation . U described the old method method , but I need the new method.
    (2 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Robert Feng
      I believe you are talking about the lens-maker's equation, which gives the focal length of a thin lens, given radii of curvature and the refractive index. This video talks about how to find the location of images, so perhaps there is a different video which will talk about the equation you want.
      (1 vote)

Video transcript

- [Voiceover] When you're dealing with these thin lenses, you're going to have to use this formula right here, one over f equals one over d-o plus one over d-i. Not too bad, except when are these positive or negative? Let's find out. F is the focal length. The focal length, when you've got a thin lens, there's a focal point on each side of the lens. The focal length is the distance from the center of the lens to one of these focal points. Which one, it's doesn't actually matter, because if you want to know whether the focal length is positive or negative, all you have to look at is what type of lens you have. In this case, we've got a convex lens, also known as a converging lens. It turns out, for these types of lenses, the focal length is always, always, going to be positive. If this focal length right here was, say, eight centimeters, we would plug in positive eight centimeters. It doesn't matter, we could have measured on this side. This side will be eight centimeters. We still plug in positive eight centimeters into this focal length if it is a converging, or a convex, lens. If you had the other type of lens ... Here's the other kind. This one is either diverging or it's going to be concave. If you have a concave or diverging lens, it also will have two focal points typically drawn on either side. These will be a certain distance along that principal axis to the center of the lens. If you measured this, by definition for a concave or a diverging lens, the focal length is always going to be a negative focal length. So, if this distance here was eight centimeters, you'd have to plug in negative eight centimeters up here into the focal length. All you need to look at is what type of lens you have. D-o, d-i, doesn't matter. D-o and d-i could be big, small, positive, negative. You could have a real image, a virtual image. It doesn't matter. All you have to look at is what type of lens you have. That will tell you whether you should plug in a positive focal length, or a negative focal length. All right, so focal length isn't too bad. How about d-o? D-o represents the object distance. If I had an object over here, and we always draw objects as arrows. That lets us know whether they're right-side up or upside down. Here's my object. The object distance refers to the distance from, always measured from the center of the lens to where the thing is, and in this case the thing is the object, so here's my d-o. This object distance ... this one's even easier ... object distance, just always positive. So my object distance, I'm just always going to make that positive. If this is 30 centimeters, I'm plugging in positive 30 centimeters over there. If it's 40 centimeters, positive 40 centimeters. Always going to be positive unless ... there is one exception. If you had multiple lenses it's possible you might have to deal with a negative object distance, but, if you're dealing with a single lens, whether it's concave or convex, I don't care what kind of lens it is, if it's a single lens, your object distance is going to be a positive distance if you only have one lens. Okay, so object distance is even easier. Always positive, no matter what the case is, if you have a single lens. How about image distance? Image distance is the tricky one. This refers to the distance from the lens to where the image is, but your image can be on one side or the other. Let's see here, let's say for this case over here I ended up with an image upside down over here, something like this. Say this is my image that was formed by this object in this converging, convex, lens. Image distance is defined to be from the center of the lens to where my image is, always measured parallel to this principal axis. Sometimes people get confused. They think, well, am I supposed to measure from the center here on this diagonal line? No, you never do that! You always go from the center, parallel to the principal axis, to where the image is. This is defined to be the image distance. When will this be positive and negative? Here's the tricky one, so be careful. Image distance will be positive if the image distance is on this other side of the lens than the object. One way to remember it is image distance will be positive if it's on the opposite side of the lens as the object, or, the way I like to remember it, if you're using this lens right, you should be looking, your eye should be looking through the lens at the object. Putting your eye over here does no good at all. Really, your lens is kind of pointless now. If my eye's over here, I'm looking at my object, and I'm just holding a lens in front of it. This is really doing no good. So I don't want my eye over there. If I'm using this lens right, my eye would be over on this side, and I'd be looking at this object, I'd be looking through. I'm not shooting light rays out of my eyes, but I'm looking in this direction through the lens at my object. I wouldn't see the object. What I would actually see is an image of the object, I'd see this image right here, but still, I'm trying to look through the lens. A way to remember if the image distance is positive, if this image distance has been brought closer to your eye than the object was, if it's on the side of this lens that your eye is on, that will be a positive image distance. So if it's on this, in this case, the right side, but what's important is it's on the opposite side of the object, and the same side as your eye, that's when image distance will be positive. That'll be true regardless, whether you've got a concave, convex, converging, diverging. If the image is on the same side as your eye over here, then it should be a positive image distance. Now, for this diverging case, maybe the image ended up over here somewhere. I'm going to draw an image over here. Again, image distance from the lens, center of the lens, to where your image is, so I'm going to draw that line. This would be my image distance. In this case, my eye still should be on this side. My eye's on this side because I should be looking through my lens at my object. I'm looking through the lens at the object. I'd see this image because this image is on the opposite side of the lens as my eye, or, another way to think about it, it's on the same side of the object. This would be a negative image distance. I'd have to plug in a negative number, or if I got a negative number out of this formula for d-i, I would know that that image is formed on the opposite side of the lens as my eye. Those are the sign conventions for using this thin lens formula. But notice something. This formula's only giving you these horizontal distances. It tells you nothing about how tall the image should be, or how tall the object is. It only tells you these horizontal distances. To know about the height, you'd have to use a different formula. That other formula was this magnification formula. It said the magnification, M, equals negative the image distance. If you took the image distance and then divided by the object distance you'd get the magnification. So we notice something. We notice something important here. If the image distance comes out negative, we'd have magnification as negative of another negative number, object distance always positive, so we'd have a negative of a negative, that would give us a positive. If our image distance comes out negative like it did down here, then we'd get a positive magnification and positive magnification means you've got a right-side up image, if it's positive. If our image distance came out to be positive, like on this side, if we had a positive image distance, we'd have a negative of a positive number, that would give us a negative magnification. That means it's upside down. So it's important to note if our image distance comes out negative, negative image distance means not inverted, and positive image distance means that it is inverted from whatever it was originally. Let's look at a few examples. Say you got this example. It said find the image distance, and it just gave you this diagram. We're going to have to use this thin lens formula. We'll have to figure out what f is, f, the focal length. We've got these two focal lengths, here, eight centimeters on both sides. Should I make it a positive eight centimeters or a positive eight centimeters? Remember, the rule is that you just look at what type of lens you have. In this case, I have a concave lens, or another way of saying that is a diverging lens. Because I have that type of lens it doesn't matter. I don't have to look at anything else. I automatically know my focal length is going to be one over negative eight centimeters. One over negative eight centimeters equals one over the object distance, here we go, object over here, 24 centimeters away. Should I make it positive or negative? I've only got one lens here. That means object distance is always going to be positive. So that's one over positive 24 centimeters. Now we can solve for our image distance. One over d-i. If I use algebra to solve here I'll have one over negative eight centimeters minus one over 24 centimeters, and note, I can put this all in terms of centimeters, I can put it all in terms of meters. It doesn't matter what units I use here. Those are the units I'll get out. I just have to make sure I'm consistent. So if I solve this on the left-hand side, turns out you'll get negative one over six centimeters equals, well, that's not what d-i equals. That's what one over d-i equals, so don't forget at the very end you have to take one over both sides. If you take one over both sides, my d-i turns out to be negative six centimeters. What does that mean? D-i of negative six centimeters. That means my image is going to be six centimeters away from the lens, and the negative means it's going to be on the opposite side as my eye or the same side as my object. My eye's going to be over here. If I'm using this lens right, I've got my eye right here looking for the image. The negative image distance means it's going to be over on the left-hand side, where? Six means six centimeters and away from what? Everything's measured from the center of the lens, and so from here to there would be six centimeters. This tells me on my principal axis, my image is going to be right around here, six centimeters away from the lens, but it doesn't tell me, note, this does not tell me how high the image is going to be, how tall, whether it's right-side up ... Actually, hold on. It does tell us whether it's right-side up. This came out to be negative. Remember our rule? Negative image distances means it's got to be right-side up. I'm going to have a right-side up image, but I don't know how tall yet. I'm going to have to use the magnification equation to figure that out. I'll come over here. Magnification is negative d-i over d-o. What was my d-i? Negative of d-i was negative six, so I'm going to plug in negative six centimeters. On the bottom, I'm going to plug in, let's see, it was 24 centimeters was my object distance. What does that give me? Negative cancels the negative, I get positive, and I get positive one-fourth. Positive one-fourth. Remember, here, positive magnification means right-side up. One-fourth means that my image is going to be a fourth the size of my object. If my object were, say, eight centimeters tall, my image would only be two centimeters tall. I'm going to draw an image here that's right-side up, right-side up because I got a positive, and it's got to be a fourth as big as my object, so let's see, one-fourth might be around here, so it's got to be right-side up and about a fourth as big. I'd get a really little image. It'd be right around there. That's what I would see when I looked through this lens. That's an example of using the thin lens equation and the magnification equation.