1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why is virtual image not seen when a defective eye is corrected?

  1. Jan 22, 2013 #1
    Hello Everyone!

    I have a question related to the ACTUAL APPEARANCE of the virtual image/object formed when a person wears a lens.
    When a lens (spectacle) is used for correction of vision to make the rays meet at retina, my book says a virtual image seems to form before the lens. This takes place in both myopia(concave) and hypermetropia(convex). But, we don't see that happen in real life?

    Consider the following ray diagram in myopes where concave lens is used for correction:
    http://dl.dropbox.com/u/27434897/light2.jpg [Broken]

    Here, in the third case, the image of object at infinity should seem to be formed at F(far point). But, we don't see that happen in real life?

    That said, for a person wearing spectacles - the location of image of object should be different from the location of actual object! WEIRD! Because, we don't see that happen in real life?

    If this is true, the sun(which is at infinity) would appear at far point(which is usually 1m) to a myopic person. Now, that's weird! Because then the sun would appear at 1m to a myopic person! But, we don't see that happen in real life? THAT'S A HUGE DIFFERENCE BETWEEN THE TWO LOCATIONS! ONE IS MILLIONS OF MILES AWAY AND THE OTHER IS JUST A METRE IN FRONT.

    So, the actual position of the object should be significantly different from the position of the image seen by a spectacle-wearing person.
    I don't understand how? It would make life difficult for spectacle-people! :)

    NOTE: Though I have used myopia as an example, the difference in locations of image and actual object still would persist in case of hypermetropia(convex lens).
    I believe there's something wrong in here. The theory says something, but practically things are different.
    Thank you all in advance.
    #HappyPhysics! :)
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jan 22, 2013 #2


    User Avatar
    2017 Award

    Staff: Mentor

    Usually, you estimate the distance of objects via their different position for your two eyes - this difference is present independent of the correction.
    Glasses are correcting eyes which are "wrong" - so afterwards, you see what you would see with ideal eyes. In the third example, the eyes would see an object 1m away as if it would have an infinite distance.
  4. Jan 22, 2013 #3
    Your brain uses several different cues to judge distance.

    You are thinking of the fact that your eye has to change shape to focus on objects at different distances, but that is only one cue.

    Another one already mentioned is the difference between the image your left eye sees and the image your right eye sees.

    Another cue is the apparent size of an object compared with the size we expect it to be from experience.

    Another is that your brain notices when 1 object partially obscures another. You are seldom aware that your brain notices this, but it does.

    Yet another is parallax. If you are moving, objects which are closer change their position in your field of view more quickly then objects which are further away. This even works if you are walking directly toward both objects because your head bobs up and down as you walk, creating a vertical movement that your brain uses for parallax comparison.

    I'm pretty sure there were 1 or 2 more, but I can't remember them. The bottom line is that your brain uses a wide variety of cues to judge distance. When one cue is out of agreement with the others your brain quickly learns and makes adjustments to keep your perception consistent with reality.
  5. Jan 25, 2013 #4
    Thank you for your replies mfb and mrspeedybob!

    So , are you agreeing with my argument that the location of image is significantly different from the actual location of object itself?
    And that it is some secondary factors like parallax, experience, etc. which leads us to perceive that the sun is not 1 metre infront but far away even though the image seems to be formed at far point?

    Please tell that is my argument in "image location" right? (even though according to you the human perception is different).
  6. Jan 25, 2013 #5


    User Avatar
    2017 Award

    Staff: Mentor

    The corrected image is at the correct distance.
    The uncorrected image is wrong.
  7. Feb 9, 2013 #6
    The corrected virtual image is NOT at correct distance.
    The corrected real image is being formed on the retina but the eye should perceive that the object is just a metre in front(because of the virtual image at far point), even though the object actually is at infinity.
    That's the point for my whole argument!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook