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

Concave lenses

Concave Lenses. Created by Sal Khan.

Want to join the conversation?

  • spunky sam blue style avatar for user Pavitra  Shetty
    what is the difference between reflecton and refraction
    (15 votes)
    Default Khan Academy avatar avatar for user
  • aqualine sapling style avatar for user Si
    what if the lens is not symmetrical? is it even possible to have a non symmetrical lens? if it was possible, what would happen?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user Tom Bradley
    What is the difference between refraction and diffraction? is there is a difference in angle of light change or velocity? Thanks.
    (4 votes)
    Default Khan Academy avatar avatar for user
  • marcimus pink style avatar for user Jaibun
    as the refractive index of concave mirror is higher than that of air, why do the rays not converge after being refracted? why r the diverged although the lens is a denser medium?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • male robot donald style avatar for user Sourish Ray
      Very very good question dude.
      When rays of light enter the lens , due to change in absolute refractive index , they get converged. But as they exit the lens , once again they enter a rarer medium and thus diverge . Now the question arises that why don't the refracting forces equalize each other? This is because in a plano-concave lens , the angle between the refracting surfaces is maximum (at the edges) and hence the diverging power is maximized .
      (5 votes)
  • blobby green style avatar for user igbaram
    At , why does sal use a ray going through the middle that doesn't get refracted instead of one going through the focal point
    (5 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user jimstanley49
      He could have drawn a line toward the right focus. This would have exited the lens parallel to the principal axis. If you follow that path back through the lens like he did with the one that shoots up to the right, you would find it intersects the same point. My teacher pointed out that drawing a third ray is a good way to be sure you haven't messed up somewhere, and through the center is pretty hard to mess up.
      The thing to remember here is that the foci of a concave lens work opposite of a convex lens. Parallel rays entering the lens exit directly away from the near focus, and rays that move toward the far focus exit parallel.
      (4 votes)
  • blobby green style avatar for user Sara S.
    how do we know if an image is virtual or not?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Satwik Pasani
      Virtual images do not have any real place in the physical world where the light rays converge/meet. So when you analyse a situation (say by a ray diagram), if you find that the rays meet at a point, that would be a real image. If they do not actually meet, but only seem to diverge from a point, it is a virtual image.
      (6 votes)
  • primosaur ultimate style avatar for user Jerry
    What type of lens is used in contact lenses (e.g. hard contacts, soft contacts, etc.)?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Anna Sharma
      If you have myopia or nearsightedness, you would use diverging lenses(concave) to shift the focus of your eye lens backwards so that it can focus on the retina. Just the opposite goes for hypermetropia or farsightedness, in which you would use converging(convex) lens to bring the focus closer. In astigmatism, you would use cylindrical lenses. There are various other types of eye problems that require other shapes of lenses.
      (6 votes)
  • piceratops tree style avatar for user Sam Zachary
    So basically, the properties of the image formed using a concave mirror is identical to the properties of an image formed by a convex lens, and vice-versa, right?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Tom King
    Why doesn't the beam going through the center get refracted as it leaves the glass?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • blobby blue style avatar for user Isabella Mathews
      Cool question.
      If you really zoom in, you can find that at the center of the lens, the two edges are parallel and that kind of acts like a parallel-sided glass slab. Well, you know, when light emerges out of a glass slab after refraction, the emergent ray is parallel to the incident ray, but the ray would be displaced laterally.

      Since, we're assuming that the lens is very thin, this lateral displacement is very, very, very small and hence, we feel like it doesn't get refracted. (Actually, it does.)

      But for simplicity's (and our) sake, the refraction is not shown. Or else it would've been way, way more complicated.

      Hope this helps. :)
      (4 votes)
  • blobby green style avatar for user Alex M
    So, why would a ray going through the left focal point not go through the concave lens parallel as it did in convex lenses? I drew it and it messed up the whole drawing.
    (2 votes)
    Default Khan Academy avatar avatar for user

Video transcript

We've talked a lot about convex lenses, so I thought I would do a quick video on concave lenses, although there aren't as many combinations of what a concave lens can do. So just remember, concave has the word 'cave' in it so I always imagine that it's kind of caving inwards caving inwards It's a little bit of an exaggerated drawing but you get the general idea this is a concave lens right over here and let me draw its principal axis so this is its principal axis shouldn't curve at the beginning just redraw it, so that's its principal axis and let me draw the two focal points let me draw one over here on the left side and then an equal distance focal distance on the right side and I'm assuming that the two surfaces are both concave and they are symmetric over here Just an assumption I'm making Now lets think about what's gonna happen if I put an object some place on the left side of this concave lens So I can put it, let me just stick it anywhere So let me just stick it right over there So, if I do and like always, we will do our two rays but with a concave lens, I'm only gonna do two rays one that is parallel to the principal axis and then I'm gonna do one that does not get diffracted so it goes right through the center of the lens and let's think about what happens. So, if we go parallel, to the principal axis, this will get diffracted away, since it's coming in parallel and it will get diffracted outwards so that it looks like it's coming from this focus. So it will get diffracted in a way so it looks like its coming from that focus So its getting diverged outwards Let me draw another point or another ray, I should say I will do that in yellow and this will just gonna straight through the centre of the lens and not get diffracted So its just gonna go straight through the lens and not get diffracted So what will the image be here? Well clearly, these two rays don't converge so we won't have a real image but they do both look like they are diverging from someplace and they look like they are diverging from right over there so look to an observer out here if their eyeball is right over there I could draw their nose, just make it clear where the eye is If they look at from this side, they are going to see a virtual image of the image right over here Just like that So you're actually gonna see is a virtual image of the object that is not inverted and it's gonna be smaller its gonna look like it's closer to the lens than it really is Anyway, that's all it really is to know, obviously there's a lot you can know about anything. but that's the general gist about the concave lenses