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Spoon Optics-Virtual Inverted image with a concave mirror

  1. Feb 14, 2010 #1
    When you look at a spoon, like at a mirror, the image of yourself appears to be on the other side of the mirror/spoon, where light does not go! So the image must be virtual!

    So tell me, since a concave mirror must produce a real inverted image when you are far from the focal point http://en.wikipedia.org/wiki/Real_image how does a spoon produce a virtual inverted image?
    OR is it that it only appears to be virtual and is really real? Which is problematic since you definitely see yourself on the other side of the spoon, just like with a plane mirror.

    p.s. conceptual answers are preferred. I have been able to find values with the thin lens/mirror equation which work (f=-3, s=-4, s'=-12) but cannot ray trace anything like it or comprehend it. Criticisms on my f,s, and s' numbers are welcome too (and yes i know a negative s is weird but still, it works).

    Thanks to anyone who dares answer!
     
  2. jcsd
  3. Feb 14, 2010 #2

    Doc Al

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    Why do you think the image appears to be on the other side of the spoon? The surface is curved, so how can you tell?

    I'm looking into a spoon right now. Looks like the usual inverted real image to me, on this side of the spoon (in the bowl, not behind it).

    Try pointing a pencil at the spoon and look at its image.

    For a ray diagram, look here: http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/mirray.html#c3"
     
    Last edited by a moderator: Apr 24, 2017
  4. Feb 14, 2010 #3
    I am at a college campus so I haven't used metal spoons for months.
    I just got ahold of one. It is amazing. You're right, the image is real and not behind the spoon. The pencil tip helped too.

    Thank you Doc Al!
     
  5. May 27, 2012 #4
    I will say, that image is actually formed on this side of the spoon -- means image is real, it is only we see on the background of spoon.
    This can be confirmed by moving our eyes slightly, and the position of image changes drastically on tha background of spoon, proving that image is actually closer to eyes than spoon.
    In case of virtual image formed by convex side of spoon, if we move our eyes little, the image position doesn't change to that extent prooving that image is farther from eyes compared to spoon.
     
  6. Dec 3, 2013 #5
    I also have the same question and i still didn't understand the reason of a virtual inverted image.
    Since we see the image in the spoon so it will be forming behind the mirror and the image will be virtual.
    So why can't we say that the image is virtual?
     
  7. Dec 3, 2013 #6

    Doc Al

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    The image is in front of the spoon, not behind it (as a virtual image would be).
     
  8. Dec 3, 2013 #7

    sophiecentaur

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    You can get both a real and a virtual image with a concave reflector (same as with a convex lens). 'Shaving mirrors' are concave and produce a virtual (erect) magnified image (behind the wall, of course), when you are fairly close to them. If you move your head away, the image will suddenly 'burst' and then become a real, inverted image. This effect is much easier to see when the radius is large then in a spoon with a radius of only a few cm.
     
  9. Dec 4, 2013 #8
    If the image is in front of the spoon (or real) , then it should only appear on a screen in front of the spoon.But ,to our eyes, the light rays appear to come from behind the spoon.So, how can the image be real?
    (I more thing sir. I have posted on my blog "the dispersion of sunlight by our eyelashes" , but i don't know the reason of it.Can you please help me in that?)
     
  10. Dec 4, 2013 #9

    sophiecentaur

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    You may be confused because the spoon has such a small radius of curvature. The {inverted) image you see is real and in front of the reflecting surface. You can verify this by holding a pencil point just in front of the spoon and, getting the right position, you can use the no-parallax method to put it where it moves with your head (image) as you move from side to side.
    The parallax method is described (though not very well) in this link.

    If you can find a concave reflector with a less curved surface, you will find it a lot easier to see what's going on.
    Taks some time to reflect on the problem, :biggrin:
     
  11. Dec 4, 2013 #10

    Doc Al

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    The light is reflected off the spoon and is headed for your eyes. So you don't need a screen to see it.

    If you look closely, the image appears to be in front of the spoon, just within the bowl. (We are talking about the image formed at the concave side of the spoon.)

    I recommend that you get a shaving mirror, as suggested by sophiecentaur, and play around with it. The larger radius of curvature allows you to move in and out of the focal length.
     
  12. Dec 4, 2013 #11

    sophiecentaur

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    Yes. There are several optical instruments which produce a real image that's not on a screen, although there is often an eyepiece to take that real image and put it, as a virtual image, somewhere more easy for the eye to focus on.
     
  13. Dec 4, 2013 #12

    Doc Al

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    Here's an example of a real image that's viewable without a screen (or an eyepiece), the classic "mirage" toy:

    https://www.youtube.com/watch?v=hBzjKHEJCaY
     
  14. Jan 5, 2014 #13
    The piggy illusion illustrated in the video above makes the piggy appear at a distance away from the surface of the concave mirror and not on it. The point raised in the beginning is why does appear that we see an image on the spoon or a concave surface.

    Apart from a real image, a virtual image is also formed with a concave surface. Think of the concave mirror as made up of a large number of small pieces of plain mirrors, each such plain mirror will form a virtual image of its own, however since all the reflected rays converge , a real image is formed as well.
     
  15. Jan 5, 2014 #14

    sophiecentaur

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    Not at the same time though. Real or virtual will depend upon were the object is placed. This is basic book work stuff and there is hardly any question about it.
     
  16. Jan 5, 2014 #15

    Doc Al

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    You might want to reread the first few posts again. The image in question is a real image that appears in front of the spoon--within the bowl, but in front of the surface of the spoon, not behind it.

    The piggy illusion was just given as an example of a real image.
     
  17. Dec 20, 2014 #16
    But why it is said ,to view a real image a screen is needed ,when we can see real image with our eyes in this spoon case in air?I thought that we can only see virtual images directly by our eyes.
     
  18. Dec 20, 2014 #17
    This means that if we directly look in projector that forms real image we can watch movie?
    Also it would be great help if one can answer how

    Our retina can form real image of both virtual and real objects.
     
  19. Dec 20, 2014 #18

    sophiecentaur

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    That is not "said", by people who 'know' the subject. What is said is that you can project a real image on a screen. Inside many optical devices you will find real images with no screen. At the other end, you may or may not find a projected real image to look at.
    This is yet another example where the way things are classified is regarded as more important than understanding of the Physics.
     
  20. Dec 20, 2014 #19
    Thanks sophiecentaur for clearing my doubt.This real images sometimes confused me.I would be pleased if you can solve my other doubt that is above your reply.
     
  21. Dec 20, 2014 #20

    sophiecentaur

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    If you stand in the right place, in front of a projector you can, in fact, see a real image. It is hard to identify what you are seeing but if you focus the projector to as short a distance as possible and stand well beyond the focal plane, you can actually see parts of the film slide. But it is not easy or pleasant to watch as the image moves about and looks distorted. Why are you trying to find exceptions to a 'rule' that you have found? Just consider that the rule might be an oversimplification of real life. (Like so many other things that teachers have to tell you in School.)
    Look on this link and you will see the ray diagrams for various lens arrangements. The bottom three diagrams show what I mean. Note: you need to have your eye in the right place for the light in the narrow cone to enter it, if you look from the side the light will just go straight past you.
     
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