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How are virtual images formed?

  1. May 15, 2014 #1
    I understand that virtual images are formed when two rays APPEAR to meet and do not actually meet (lenses and mirrors) and that is why we extend the reflected (or refracted) rays behind. But that is as far as the drawing goes. What is actually happening to the rays to form a virtual image? (They cannot be extend behind in reality, right?)
     
  2. jcsd
  3. May 15, 2014 #2
    After passing the lens of the eye they form a real image at the retina.
     
  4. May 15, 2014 #3
    Thank you, DrStupid...do you mean to say that the only time the rays are intersecting (in case of a virtual image) is when they reach the retina? I thought the retina was forming an image of the image formed by the mirror.
     
  5. May 15, 2014 #4
    Yes, unless you are shortsighted.
     
  6. May 15, 2014 #5

    sophiecentaur

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    Your eye is not stupid but it can only work on the information it gets. (:wink:) If the light coming into the eye can be formed into an image on the retina than you 'see' something that looks the same as an actual object in some position. The only time you can 'see' anything is when rays focus on the retina and form an image. That image is not to be confused with the image formed by an optical device and is in a different place from some originating object.
    In the simple case of a plane mirror, (you must have seen the 'two ray' diagrams) the rays spread out from the original object, hit the mirror and keep spreading out until they enter your eye. Your eye can only conclude that there is something 'behind the wall' ( we did not evolve in a world of mirrors, for a start, so that's all we can ever know - unless we are used to using mirrors, in which case we learn about how to find our face to shave and put makeup on). That image is a virtual image because the rays only appear to diverge from a position. The same happens when you are close up to a concave mirror; you see a virtual image. If you use a concave mirror and hold your head far enough away from it, the rays will come to a point between you and the mirror. The image (a real one this time) will actually appear to be in front of the mirror - you can put your finger right next to it. You could put a screen there and an image would be formed at that point. You could never do that with a plane or convex mirror; those images are always virtual.

    A concave lens (on its own) will never form a real image but a convex lens can form either real or virtual images, depending where you place an object. Spectacles never form a real image but (concave or convex) they take an object and produce a virtual (displaced) image at a distance that your dodgy eyes can focus on it.
     
  7. May 16, 2014 #6
    thank you...:)
     
  8. May 19, 2014 #7

    sophiecentaur

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    That Wiki definition is 'legally' correct but I found it hard to see what it was getting at, at first. It's a good 'confirmatory' definition, once you have actually seen how to do the ray tracing.
     
  9. May 19, 2014 #8
    To see anything sticking the mirror, you feel harder. But to see anything in the mirror, you feel as easy as seeing it directly. In reality, the image stays as behind as indicated by your ray diagram.......

    I have read this in one of the web page. But I don't remember it now.
     
    Last edited: May 19, 2014
  10. May 19, 2014 #9

    sophiecentaur

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    I sympathise with your problem with English (I would have great problems in posting in any other language but English aamof). There are a few words here that, I imagine, are straight translations with a dictionary and the meaning is very hard to understand. Could you try to rephrase the pieces in blue, please.
     
  11. May 19, 2014 #10

    sophiecentaur

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    You really shouldn't worry too much about what to call the images as the 'definition / classification' only really applies in very simple cases. In most complicated optics, each lens element will be doing its own thing and some of the intermediate images could be treated as virtual and some as real. It is not particularly relevant - as long as you do the sums right. :smile:

    People can waste a lot of time on what to call things when it's the understanding that really counts.
     
  12. May 19, 2014 #11
    See Mister Mystere's answer in this link: http://physics.stackexchange.com/questions/101627/does-a-mirror-help-a-near-sighted-persion-see-at-a-distance-clearer/101656#101656
     
  13. May 20, 2014 #12

    sophiecentaur

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    I'm afraid that link doesn't tell me what your post means, actually.
    The link is far too long winded, imo, and, when I read things like
    ,
    I have to doubt its usefulness.
    Changing the brightness of an image (if your pupil has not changed in diameter) cannot change the resolution of your eye lens.

    Which part of that whole post says what you want to say?
     
  14. May 20, 2014 #13
    See Mister Mystere's answer's example portion: http://physics.stackexchange.com/questions/101627/does-a-mirror-help-a-near-sighted-persion-see-at-a-distance-clearer/101631#101631

     
    Last edited: May 20, 2014
  15. May 20, 2014 #14

    sophiecentaur

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    Mister Mystere seems to be using a lot of words to replace a very simple equation. The basics of how an image forms in a plane mirror are in all basic books and websites. (Hyperphysisc says all you need to know - get to love the site) He is just chewing the fat and adding very little to the argument. I get very tired of 'instance-based' Science because it tends to follow a divergent path instead of getting nearer and nearer to understanding the problem.
     
  16. Feb 13, 2017 #15
    a virtual image is an image formed when the outgoing rays from a point on an object always diverge. The image appears to be located at the point of apparent divergence. Because the rays never really converge, a virtual image cannot be projected onto a screen. In diagrams of optical systems, virtual rays are conventionally represented by dotted lines. Virtual images are located by tracing the real rays that emerge from an optical device (lens, mirror, or some combination) backward to a perceived point of origin.
     
  17. Feb 13, 2017 #16

    sophiecentaur

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    This thread is v. old!
    It's such an old chestnut and it's a topic that only worries people who's Science revolves around definitions and a concern about answering simple exam questions 'correctly'. If anyone really wants to get this in their head, all they have to do is to look at a few examples (reputable sources, old course) and to spot the pattern that's always there for real and virtual images. For a start, it becomes clear that a 'screen' does not have to be there for a real image to be formed. For some reason, that seems to bother people.
    Being able to name the images 'correctly' is of no consequence at all. Once you start doing the sums, the result will be the result and will tell you where to put your lenses and mirrors in an optical system. What more would you want?
     
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