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B Will the person see the image?

  1. Jun 15, 2017 #1
    I have a very basic doubt in optics . The picture below is used only to make question clearer .

    ?temp_hash=066d4e69d5c59c793f398fc35a655663.png

    Suppose ocular lens was not present and position of the eye remains same , then will the person see the real image formed by the objective lens ?

    Thanks
     

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  2. jcsd
  3. Jun 15, 2017 #2

    BvU

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    Yes. It may be too close to his/her eyes to see it sharp, but: yes.
    You can experiment with one simple lens (*).

    Not clear to me why you consider this a doubt in optics, though.

    (*) with an old (photographic film) SLR camera it's insightful: open the camera; you know where the real image is (in the plane where the film should be) and you can examine it with and without an ocular lens.
     
  4. Jun 15, 2017 #3
    OK .

    Please answer one more , then may be I can put forward my confusion .

    ?temp_hash=a5173cccb62af1894c6adadd1c63a463.jpg

    Suppose the person was to the left of the biconvex lens , then will he be able to see the virtual image formed by biconvex lens ??
     

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  5. Jun 15, 2017 #4

    BvU

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    No. The dashed lines are just a continuation of the solid (and real) rays to the right of the lens. The dashed lines represent virtual rays.
     
  6. Jun 15, 2017 #5
    Sorry . I know this is very basic stuff . But I am not getting how does a person see the image of an object ??

    Could you please explain why the person sees the real image in first condition but not the virtual image in the second condition .
     
  7. Jun 15, 2017 #6

    BvU

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    To the left of the ocular in the first condition, the image formed by the objective lens is real, i.e. actually present. If you place a screen there, the image is formed on the screen.

    To the left of the biconvex lens in the second situation, the virtual image is not really there: a screen would not have an image formed on it.

    In the same way as there is nothing behind a mirror: we use dashed lines to find the position of the virtual image.
     
  8. Jun 15, 2017 #7
    I understand all what you have said .

    My confusion is regarding how a person sees the image of an object .

    Human eye can see both real and virtual images .Right ?

    Then why can't the person see the virtual image in second case ?

    Is it because the refracted rays directly enter the eyes in first case but do not directly enter in the second case ??
     
    Last edited: Jun 15, 2017
  9. Jun 15, 2017 #8

    jtbell

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    The location of an image as seen with our eyes, depends on the directions of the light rays as they enter the eyes.

    If you place your eyes to the right of the lens, and look towards the left, the directions of the rays that enter the eyes are such that they extrapolate back to the image location.

    If you place your eyes immediately to the left of the lens, and look towards the left, the directions of the rays that enter the eyes are such that they extrapolate back to the object location.
     
  10. Jun 15, 2017 #9

    BvU

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    Yes. They can't see virtual rays (the dashed lines) because they are not there. A virtual image can be seen if the virtual rays become real rays before they reach the eye. Virtual rays are just auxiliary lines we draw to find their intersection points: the points where they seem to come from, but never passed.
     
  11. Jun 15, 2017 #10
    Ok .

    In the second picture , what happens if we place the eyes to the left of object and look towards left ?? Why don't the eyes see the image ?
     
  12. Jun 15, 2017 #11

    jtbell

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    In that case, do any light rays at all from the object enter the eye, whether directly from the object, or after refraction through the lens?
     
  13. Jun 15, 2017 #12
    No .

    But if it looks toward right , the light from the object enters the eye . So it sees the object .

    Don't the refracted rays ,when extended backwards , enter the eyes ??
     
  14. Jun 15, 2017 #13

    sophiecentaur

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    Those (dotted) rays do not exist. They are only construction lines to show the virtual position from where the rays appear to originate. You appreciate that there no light coming from the lens from right to left??
     
  15. Jun 15, 2017 #14
    Ok.

    So , the eye sees the image , whether real or virtual , when actual refracted light rays enter the eyes ??
     
  16. Jun 15, 2017 #15

    sophiecentaur

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    Of course. The retina needs Energy to make it work!
     
  17. Jun 15, 2017 #16

    jtbell

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    Correct.
     
  18. Jun 15, 2017 #17
    Virtual images are virtual, they do not exist in real life, these are only illusions. And I am not kidding here :).
    When you put a lens in front of the object and the lens creates virtual image, this lens creates the illusion that an object is in different place and/or in different size than it is in reality, but this illusion works when you only look through the lens. In example, when you look at something under the water you know that this object is not in exact spot as you see it, but its position is shifted due to air/water refraction. Therefore you see a virtual image of the real object.

    Now return to your first picture of microscope. I think you were confused here. If you remove the ocular from system (last lens) you will not able to see the object, even if the image is real. Because in this case the first lens (object lens) creates the illusion that your object is to the right of the lens and too close to the eye. We call the image real, because you can put sensor (paper, CCD matrix, etc) in the real image place and record that image, you cannot do that with virtual images.
     
  19. Jun 15, 2017 #18
    Ok .

    Back to the first case in the OP i.e oculur lens not present .

    Eye sees the real image formed by the objective lens because the refracted rays from the objective meets/converges at the point where real image is formed and thereafter the same rays diverges and enter the eyes .

    This is similar to as if a real object is present at the point where the real image is formed , and , real light rays emanate from the object and enters the eyes . This is how eye sees the real image of the object .

    Makes sense ?
     
  20. Jun 15, 2017 #19
    Sort of yes. Only remember that real (and virtual) images do not produce "new rays". You can see only rays that originated in object.
     
  21. Jun 15, 2017 #20
    In that case what I wrote in the previous post is correct .

    Is this correct ??
     
  22. Jun 15, 2017 #21
    Yes. That's correct.
     
  23. Jun 15, 2017 #22

    sophiecentaur

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    Sometimes it is difficult to make yourself see the real inverted image that's formed by a single lens. It wobbles from side to side in an unfamiliar way as you move your head. We did not evolve to make sense of such images so our brains struggle a bit.
     
  24. Jun 15, 2017 #23

    Drakkith

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    Yes. I can look down into the eyepiece barrel of my telescope without an eyepiece in place and see an image of the scene in front of my telescope. The view may not look very good compared to the view with the eyepiece in place, but I can still see something.

    An interesting exercise may be to take a simple magnifier (magnifying glass), mount it a set distance from an object such that the distance is larger than the focal length of the lens, and then move your eye closer to or further away from the magnifier and seeing how the view changes. This setup will ensure the image's magnification and distance from the lens remains constant so you can focus on understanding what happens when viewing a real image.

    You can then move the magnifier closer to the object such that the object is now inside the focal point of the lens. Now the viewed image is virtual.
     
  25. Jun 15, 2017 #24
    I think this is where the confusion began. All the time I was trying to emanate new light rays from the images . It worked in the second pic where real image was formed . But failed in the first picture where virtual image was formed .

    But as you say , if and when only light rays originated from objects are considered , then virtual image in the second picture cannot be seen .

    @sophiecentaur ,@jtbell ,@Drakkith , do you all agree with Domullus's post#19 ??

    I am asking this , because therein lies the root source of my confusion .
     
  26. Jun 15, 2017 #25

    Drakkith

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    That's right. All rays originate at the object. A real image is formed where the rays that originate from a particular point on the image end up intersecting. Diverging rays do not intersect, but it is still very useful to trace imaginary lines back to where they appear to intersect. A virtual image is formed there.
     
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