Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why virtual images in telescopes & microscopes?

  1. Sep 28, 2011 #1
    Why in telescopes and microscopes the ocular distance is such that it creates virtual images instead of real images?
  2. jcsd
  3. Sep 28, 2011 #2

    Philip Wood

    User Avatar
    Gold Member

    The rays that come through the eyepiece lens, and appear to be coming from a virtual image are diverging. A normal eye can accommodate (focus) diverging rays, so can 'see' the virtual image, even if it is only a few cm from the eye.

    Each point on a a real image is formed by converging rays. A normal eye intercepting such rays can't accommodate them (you hardly ever get converging rays in the natural world). You can, of course, see a real image on a screen, because light is scattered from the screen, giving rays diverging from each point. But forming and viewing such a real image would have no advantages and plenty of disadvantages.
    Last edited: Sep 28, 2011
  4. Sep 28, 2011 #3
    Ok, thanks, it also would be interesting to know why the eye only just can accommodate diverging rays.
  5. Sep 28, 2011 #4

    Philip Wood

    User Avatar
    Gold Member

    Well, as I said, you hardly ever get converging rays in the natural world, so there is no need for the eye to be able to do this. [In Darwinian terms, no natural selection for the evolution of an eye which can do this.]
  6. Sep 28, 2011 #5

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    The function of the eyepiece is to present an image to your eye that appears to be at infinity. This is so your eye is relaxed, making viewing easier and reducing eye strain. Because the image appears to be behind the eyepiece, the image plane of the eyepiece is on the 'wrong' side of the lens and is thus a virtual image.

    Eyepieces are complex optical devices- very asymmetric and difficult to design. In addition to a virtual image, the exit pupil of the eyepiece is outside of the lens- it is located at the pupil of your eye when viewing.
  7. Nov 29, 2011 #6
    hello forum,
    this leads me to a question:

    if an object is placed exactly on the focal point of a positive lens, will an image form?
    I think so, because the converging lens inside our eyes will be able to focus those parallel rays and form a real image on the retina.

    But some people told me that we would only see a blur instead: the focal point is where a real and virtual image should form at the same time so a blur is what we end up seeing.
    I was told to perform an experiment with a concave mirror: as we move the object towards the mirror the image is real until we get exactly on the focal point where we see a blur.
    If we move the object past the focal point the image become erected and virtual....

  8. Nov 29, 2011 #7
    At its most relaxed the (normal) eye is set to focus on infinity, that is to accept parallel light rays.
    An object placed at the focal point of a converging lens will produce parallel rays and will therefore be easily viewed by the eye.
    This is a magnifying glass in normal use.
    I would agree with Philip woods descriptions..... clear and to the point
  9. Nov 30, 2011 #8
    Thank you technician!

    I am always bugged when the certain notes say that the lateral magnification is infinite, the image location is infinite and no image is formed (undefined case) and blur is seen when the object is on the focal plane....

    But I agree with you and the others: the lens in the eye focuses that bundle of parallel rays and an image is actually seen!

  10. Nov 30, 2011 #9
    The magnification is ANGULAR. The angle subtended by the rays from the image is greater than the angle subtended by the object on its own.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook