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Objects in Optics

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  1. Jan 26, 2016 #1
    Guys
    Just wanted to clarify one doubt
    When we consider image formation by objects
    We always assume there's a source of light present ,right?(because all real-point and extended objects can't always be luminous )
    we also know that it's diffused reflection of light or scattering of incident light from a source that allows us to perceive/see objects
    Right??
    Thanks :)
     
  2. jcsd
  3. Jan 26, 2016 #2
    And there's no way virtual objects can ever be luminous as in virtual objects,the light rays "appear"to converge at a particular point
    Or is there??
     
  4. Jan 26, 2016 #3
    Usually, the object is treated as a source of light. You can see an object because light scatters or reflects off the object's surface, so the light is coming from the object's surface.
     
  5. Jan 26, 2016 #4

    sophiecentaur

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    Virtual Images exist and the concept is coommonly used . But I don't know what a 'virtual object' would or could be. Perhaps you are referring to an intermediate virtual image? That's something that multip[e lenses and mirrors can produce.
    An object can either be luminous or illuminated by a light source. An optical system would treat both situations the same way. If an 'object' happens to be a mirror or a lens then the effect on the light falling on it could be to produce specular reflections and that could (unintentionally, perhaps) produce one or more coherent images. Ray tracing software was developed, not many decades ago, for the fraphics industry which will handle specular reflections and the formation of images in shiny objects - quite convincingly sometimes.
     
  6. Jan 26, 2016 #5

    jtbell

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    Are you thinking of something like this?

    lenses.gif

    To me, I1 = O2 is a real image with respect to lens L1, but a virtual object with respect to lens L2.
     
  7. Jan 26, 2016 #6

    sophiecentaur

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    In my experience, an "object" is something you can actually get hold of and light comes from it so intermediate images in an optical system are not objects in my book. I hate to waste too much time in pointlessly classifying things and there are occasions when a projected image on a screen (like in the example of the first post) is, to all intents and purposes, an object but why change the name of an Image, formed by one lens, to an Object when its light passes through another optic in a 'coherent chain' of optics?
     
  8. Jan 26, 2016 #7
    there are recognised text books that refer to the image formed by a lens as being 'virtual objects' for a second lens. There is no doubt that I1 i JTBells diagram is the 'object' for L2 . As far as L2 is concerned the rays'from' this object are diverging as they strike L2.
    Using the sign convention I am familiar with this counts as a 'virtual' entity...Object.
    In optics it would be a strange step to describe an 'object' as something that you can actually get hold of !.
    I1 is optically an 'object' and is treated as such in calculations for L2

    'A collection of points which may be regarded as a source of light rays for a portion of an optical system but which does not actually have this function.'
    a definition of virtual object from an online dictionary
     
    Last edited: Jan 26, 2016
  9. Jan 26, 2016 #8

    sophiecentaur

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    Fair enough. :smile:
     
  10. Jan 27, 2016 #9
    By a virtual object,sophie,I mean an object towards which the light rays "appear" to converge but don't actually do so due to an intervention by an optical element such as a mirror or a lens
     
  11. Jan 27, 2016 #10
    lenses-gif.94846.gif
    That's precisely what i meant :)
    lenses-gif.94846.gif
     
  12. Jan 27, 2016 #11
    That's what i meant
     
  13. Jan 27, 2016 #12
    That's precisely what i meant
     
  14. Jan 27, 2016 #13
    So let me clear up the things finally
    A real object is something from which light rays emanate (in case of an actual source of light) or seem to emanate
    It can be a luminous or non-luminous object right?
     
  15. Jan 27, 2016 #14

    sophiecentaur

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    That's the same definition as a real image but it fits both.
    The problem with your use of the word "objet' instead of 'image' is that there is nowhere that the word 'Image' would be appropriate. A virtual image would be a location where diverging rays would meet, if produced backwards.
     
    Last edited: Jan 27, 2016
  16. Jan 27, 2016 #15
    If the problem is the use of words then it is a problem in semantics.
    Is there any doubt that I1 is an 'image' formed by L1?
    Is there any doubt that the final 'image' I2 is formed by the rays forming I1 passing through L2?
    The rays forming I2 come from the 'object' for L2 yes or no?
    This is not a 'real' object. How better to describe it than the text book term..'a virtual object'.
    There would be no semantic problems if all of these things were just called 'things'
    The thing at O1, the thing at I1, the final thing at I2.
     
  17. Jan 27, 2016 #16
    well said
    But that's how everything is!!:)
     
  18. Jan 27, 2016 #17
    So why do these quesions keep arising,....they were answered when I was a student....10, 20, 30, 40, 50....years ago
    Does Physics Forums really supply an educational resource???? Text books still exist, is our role to challenge the conventional wisdom ofd text books>?>
     
  19. Jan 27, 2016 #18

    sophiecentaur

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    So you seem to be implying that there is only one 'Image' in an optical system and that is the final 'thing' that is seen by an observer. I guess that could be applied consistenty in all systems - a bit like an electronic circuit in which there is a chain of inputs and outputs; it's all inputs until you get to the final output.
     
  20. Jan 28, 2016 #19
    I suppose that some would argue that the final 'thing' seen by the observer is what appears on the retina but then we are a step removed from the system of lenses.
    In the ray diagrams above if no arrows were marked on the rays there would be no difference to the light ray paths and so in some way we must have produced a 'convention' whereby some things are called objects and others images. Presumably this aids description and understanding. Real images and virtual images are well known terms and their formation by converging and diverging rays is understood. Real objects and virtual objects logically follow.
     
  21. Jan 30, 2016 #20
    thankyou sir
     
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