Size of image in a mirror problem

  1. 1. When we talk about flat mirrors, it says that the size of image appears in the mirror is the same as of the object. Is the long rectangular mirror at home that is hang on the bedroom door to see how you look like when you dress up a flat mirror? If so, then when we approach the mirror, our image is bigger but we probably can not see our whole body; when we step away from the mirror, we look smaller and can see our whole body. So does the size of the image change? Then does it sort of relate to concave mirror which is used for a magnifies image. I am confused with these concepts.
    2. Can real image and virtual image both be seen in the mirror by our sight?
    thanks for help.
     
  2. jcsd
  3. andrevdh

    andrevdh 1,786
    Homework Helper

    When you look at a real object how will its size be influenced by your distance from it? The same situation happens with a mirror. It just seems that there is someone (you) behind the mirror (allthough we all know that there is'nt). This is what we call a virtual image. It can be seen but it is not really there. A real image can actualy be projected (on a screen). The light comes to an actual focus. Yes, curved mirrors can focus light and form real, and virtual, images of objects placed in front of them. When you look in a flat mirror and wave to yourself with your right hand your image waves back at you with its left hand though.
     
  4. When we look at a big mirror at home, Is that flat mirror? But the image has different sizes depends on our standing distance from it. What is an image point?
     
  5. Doc Al

    Staff: Mentor

    If you stand 3ft from a flat mirror, your image will be 3ft on the other side of the mirror, thus 6ft away. Your image is exactly the same size as you, but it appears smaller since it's 6ft away. If your twin brother stood 6ft away, he'd look the same size as that mirror image.

    And, as you know, you can change how big the image appears by standing closer or further from the mirror.
     
  6. So you talk about the size that the image appears in a flat mirror depends on the distance where we stand. But according to its definition, its size shouldn't change. If so, then is it like a concave mirror which is used for a magnifies image, it can produce big image or small image; its image changes. I think I have misunderstanding of some concepts. Hope you can correct me.
     
  7. Doc Al

    Staff: Mentor

    The actual size of your image in a plane mirror does not depend on where you stand. It just appears smaller if you stand farther away. (If your twin brother is far away from you he appears small, but he's still the same size; that's the same idea.)
     
  8. So the concept of concave mirror however is that th actual size of the image appears bigger or smaller?
     
  9. Doc Al

    Staff: Mentor

    It's confusing when you use the terms "actual size" and "appears" in the same sentence! Better to say: In a curved mirror the image can be larger or smaller than the object, depending upon curvature and object distance. (In a plane mirror, the image is always the same size as the object, regardless of object distance.)
     
  10. Ok, I get it. So it's like in a flat mirror, image has the same size as the object, but it appears bigger or smaller depending on the distance. However, image in concave mirror is bigger or smaller than the actual size (not 'appear') BTW, do you know what is an image point?
     
    Last edited: Aug 7, 2007
  11. andrevdh

    andrevdh 1,786
    Homework Helper

    When the size of the image is mentioned in textbooks it refers to the actual size of the image not how it appears to one, which changes depending from which point the image is viewed as you rightly mentions. What this means for a virtual image is rather difficult to understand, but one can construct a scaled ray diagram and actually determine the real size of the image. Generally (but not always) an optical component (lens/mirror) will form a smaller image of an object when the object is further away from it.

    As far as your question about the image point goes give us a sentence in which it is mentioned.

    It is most likely a point in space where the image is formed (real or virtual). Just like one finds the statement "... the focal point ..." in textbooks.
     
    Last edited: Aug 8, 2007
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