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Mirror and white surface

  1. Jun 20, 2005 #1
    What is the difference between a mirror and a white surface?What I know is:
    1. A mirror reflects light such that an image is formed. The angles of reflection and incidence are equal. It reflects all wavelengths of light.
    2. A white surface reflects all visible wavelengths of light. It causes the light to be diffused.

    This list, I think, is right, though not complete.
    What I'm wondering about is: Why can't one see his image in a polished white surface. We do see an image, but it does not have the same colours as the object.
  2. jcsd
  3. Jun 20, 2005 #2


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    A mirror reflects light at "equal angles": that is, a beam that comes in at 30 degrees to the mirror will be reflected at 30 degrees. A white surface will reflect most of the incoming light, equally at all frequencies, but at many different angles.
  4. Jun 21, 2005 #3
    I think I've already mentioned that.
    Have you seen an acrylic whiteboard. Its white, polished and one can see one's image in it, but it doesn't reflect colours properly. White is supposed to reflect all visible wavelengths of light.
  5. Jun 23, 2005 #4
    this is a guess but anyway...white surfaces absorb most of the light incident on it but reflect equally well for all frequencies(I think the term involved here is the reflection coefficient. The reflection coefficient of a white wall is the same for all visible frequencies). For example if I keep a flashlight with the light having a yellow colour next to the wall I would see the yellow colour reflected from the wall, since the intensity of light from the flashlight is high, so the intensity of reflected light of the flashlight is still higher than the intensity of the 'usual' white light reflected. But if I tried to see the image of, say, my red shirt, I couldn't because the light incident from my shirt is simply of too low intensity. If the surface is highly polished, you may be able to distinguish images in the surface, but the light reflected showing the red shirt would probably have only a slightly higher intensity than the intensity of the reflected white light, so the image is not in its 'proper' colour. However mirrors hardly absorb any light. So the images are seen in their 'proper' colours.
    At any rate I don't think it is technically right to say that white surfaces are perfect reflectors. They reflect most, but not all light.
  6. Jun 24, 2005 #5
    Each part of a mirror reflects light in one direction.
    Each part of a white surface scatters light in all directions.

    The frequencies don't enter into this aspect of it.
  7. Jun 24, 2005 #6


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    I'd think that the reason you can vaguely see your reflection in a whiteboard is because, as you say, it's got a shiny smooth surface. The fact that it's white doesn't come into play, (you can't see your face in a sheet of A4), - because it's polished it will generally reflect light at equal angles (as explained by HallsofIvy).
  8. Jun 24, 2005 #7


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    Do you see your image reflected in it, or do you see merely a silouette of yourself? A silouette is simply the grossest, most highly contrasty elements reflected (such you unlit face blocking a bright-walled room.)
  9. Jun 25, 2005 #8
    Not into this aspect of it. Definitely a polished white surface will give a sharp image. But if a surface is white does not mean that it will neccessarily scatter light in all directions.

    But when you say 'white' you mean that it is reflecting all colours of light towards you. So why isn't your image as clear as in a mirror? You only faintly see the colours, the contrast depending on the intensity of reflected light. I think that's what ssj5harsh's question was.
  10. Jun 25, 2005 #9


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    What is common to all materials that make good mirrors ? Do any of these materials ever look white ? Can you pick out a possible correlation there ?
  11. Jun 26, 2005 #10
    Yes, this was what I intended to ask.

    Well, one correlation is that it is not simply lustre, but the greyish shiny colour that makes metals good reflectors. I, for one, haven't seen a white mirror and I doubt one can be made. As for the correlation, I see none(except for the colour). I would rather like to ask why such a correlation exists.

    For that matter, I should also mention that my experience indicates that all surfaces which are highly polished show images, irrespective of their colour. But even a black surface(which supposedly absorbs all frequencies), shows more colours than a white surface in the image.
    Further, why don't frequencies enter into this aspect of it.
  12. Jun 26, 2005 #11


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    You have the keyword there - "metal". Essentially, it appears that no matter what the polish, you can make a good mirror only out of a metal. Do you concur ?
  13. Jun 26, 2005 #12

    Could I not also make a good mirror entirely out of glass? For example, a right angle prism or similar shape which has the effect of internally reflecting the light.
    But maybe that's off the topic.
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