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About convex lens

  1. Feb 23, 2016 #1
    For an object at focal length in front of a convex lens, the image formed will be at infinity and the refracted rays are parallel (refer to the figure). Since our eye lens is convex in nature, why don't it converge these parallel rays so that an image can be formed?
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  3. Feb 23, 2016 #2


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    That is exactly what happens.
  4. Feb 23, 2016 #3
    If it's the case, what does 'an image is not formed' in the figure mean?
    I've searched some Physics books, some say that the image is in infinity and others say no image is formed.
    So, I really feel confused as there seems a contradiction.
    If our eye can see an image, how will this image look like? Blurry or clear? Magnified?...
  5. Feb 23, 2016 #4


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    The lens forms an image "at infinity" ONLY if the object is at the focal point of the lens. If this lens IS your eye, then you'll just see a blurry mess. If this lens is NOT your eye, then you can place your eye in the path of the parallel rays and your eye will focus them onto your retina and an image will be formed.

    An "image at infinity" and "no image is formed" mean the exact same thing in this context. They both mean that the rays neither converge nor diverge.
  6. Feb 23, 2016 #5
    What about if I put an object at the focal length at one end of the lens, and take an photo at the other end of the lens using focus at infinity, will the image become blurred? I'm not a professional photographer, what I think is that our eye works like an camera, both with a convex lens...
    To me, if the rays get parallel by the convex lens, then as it is photographed, these light rays will be converged by the lens of the camera and get focused, and a clear image is supposed to form. But some pictures used to illustrating the concept of object putting at the focal length of convex lens show that blurred (instead of a clear) image is formed. Still, there's another contradiction....
  7. Feb 24, 2016 #6


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    Just speaking of focal lengths and such, no, the image shouldn't be blurred.

    If the focus of the camera is set correctly the image should be sharp.
  8. Feb 24, 2016 #7
    I am also thinking in that way but it seems that I can't get a whole picture...
    The following question is obtained from a secondary school physics textbook.
    A boy holds a magnifying glass at arm's length. He looks at a poster through the glass and sees a magnified erect image. What happens to the image if he moves the lens closer to his eyes?
    A. It gets larger till it gets totally blurred at some distance.
    B. It gets larger, keeping erect all the way.
    C. It gets smaller and becomes totally blurred at some distance.
    D. It gets smaller, keeping erect all the way.

    Which is correct?
  9. Feb 24, 2016 #8


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    It looks like you are describing the situation where a 'magnifying glass' (convex lens) is brought up to an object at the right distance for a virtual image to be formed infinitely far behind it. You will see that image because your eye lens can bring parallel rays to focus on your retina. However, if you move just a bit closer to the object then the rays will converge too much for the eye to focus them on the retina. The image will form in front of the retina and look big and blurry, I think. Try it.
  10. Feb 24, 2016 #9
    I'm sorry that I can't get it comprehensively.
    You've mentioned the virtual image to be formed infinitely far behind a convex lens. I'm not sure whether you're referring to an object at exactly the focal length or object lying within the focal length. For the former, the image should at infinity, while for the latter, it should be a magnified image. Am I correct?
    Also, you've mentioned moving just a bit closer to the object. In this case, does the distance between the object & the lens remain unchanged (i.e. simply the observer moves closer to the lens) or the lens put closer to the closer without any change in observer's position?
    I know it may be too long and complicated for you to read, but I'm really very confused and everything's a mess!!!
    For a ray to converge too much, maybe the lens is too thick. But doesn't moving closer implies the eye accommodates for a thinner lens to let the rays focus on the retina? If it's possible, can you show me a picture?
  11. Feb 24, 2016 #10


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    Lens creates parallel rays:


    Eye lens focuses the parallel rays on the retina to create a sharp image.

  12. Feb 24, 2016 #11
    ah, I've met this picture before;
    I want a picture that shows:
    when an object is put at focal length of a convex lens, it gets refracted and becomes parallel. When an observer sees, these parallel rays will be converged by the eye lens onto the retina so that an image is formed.

    Correct me if I'm wrong.
  13. Feb 24, 2016 #12


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    In A.T.'s second diagram, "tilt" the incoming parallel rays so they are at an angle to the horizontal axis (but still parallel to each other). They now meet on the retina at a point below its center.

    Now combine the two diagrams together, with the parallel rays emerging from the lens in the first diagram becoming the incoming parallel rays in the second diagram.
  14. Feb 24, 2016 #13


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  15. Feb 24, 2016 #14


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    It means that with just the lens that is shown in the figure -- and no additional lenses present -- then no image is formed. Of course, adding another lens -- such as a person's eye -- would alter the direction of the rays so they are no longer parallel, and thus an image would be formed.
    I understand the confusion. As Drakkith said, those two statements ultimately mean the same thing.
    If an eye is able to make the rays come to a focus on the retina, the image is clear. (See A.T.'s figures)
    If an eye cannot do this, then the image is blurry.
    Different people's eyes will have different results.
    That depends on how the eye's focal length compares to F for the lens in your original figure.
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