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How does a mirror work

  1. Oct 27, 2009 #1

    Fuz

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    I've been pondering this for a while. 2 questions. The first question is how does a mirror work. Think about it, why and how does it reflect a near perfect image? My second question is why don't other things reflect light like mirrors do. More specifically, why doesn't a white piece of paper reflect an image like a mirror if white is supposed to reflect light?


    Thanks in advance peeps,

    Fuz
     
  2. jcsd
  3. Oct 27, 2009 #2
    Re: Mirrors

    Good question regarding mirrors vs white paper.

    True, a perfect white piece of paper will reflect 100% of the light that hits it, so why can't you see yourself in a sheet of white paper?

    Because white paper diffuses the light.
    That means that light is scattered off in all directions, unlike a mirror that reflects light always at the angle at which it hits.
     
  4. Oct 27, 2009 #3

    DavidSnider

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    Re: Mirrors

    Why does a piece of paper emit a solid color rather than the random bits of all the stuff that hits it though?
     
  5. Oct 27, 2009 #4

    ideasrule

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    Re: Mirrors

    A piece of paper is much less smooth than a mirror, and its tiny bumps and valleys reflect light in all directions. This is diffuse reflection (whereas mirror-like reflection is specular).
     
  6. Oct 27, 2009 #5

    Fuz

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    Re: Mirrors

    Hmmm... So if the paper was substantially smoother, would it become a mirror?
     
  7. Oct 27, 2009 #6

    russ_watters

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    Re: Mirrors

    The solid color reflected by a white piece of paper is a conglomeration of the colors of all the light that hits it!
     
  8. Oct 27, 2009 #7
    Re: Mirrors

    The image in the mirror seems near perfect. But in your image, you are probably left handed, not right handed. It you look at the image of palm of a right hand, the thumb is on the left side. If you look at the mirror image of a right-hand thread, it is left handed. This so-called parity transform is common in various physics fields. Protons have + parity; anti-protons have - parity.
    Bob S
     
  9. Oct 28, 2009 #8

    A.T.

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    Re: Mirrors

    Have you never seen reflections on glossy paper?
     
  10. Oct 28, 2009 #9
    Re: Mirrors

    It will be red in a red room! Although like all things it will also emit radiation like a black body according to it's temperature.
     
  11. Oct 28, 2009 #10

    Fuz

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    Re: Mirrors

    Paper was just an example. How about a perfectly white, impossibly smooth wall. If you stand in front of the wall, would you see a reflection of yourself, or a white wall. Please explain.
     
  12. Oct 28, 2009 #11
    Re: Mirrors

    Speaking about mirror, I have this question: metal surface also reflect lights well, but only when it is smooth, especially after polishing. So what is the reason for a polished surface to shine compared to a rough one?
     
  13. Oct 28, 2009 #12
    Re: Mirrors

    this topic has got me thinking today, and as a result I believe the answer to your question is "yes".
    I associate shinyness with metal, and that’s because its shiny, right? Yeah, but why is it shiny I ask myself… well, to create specular, not diffuse relflections, its all down to smoothness (achieved by polishing the surface or making it perfect ot begin with), the limits of which, for any particular material, is all down to density, ultimately. My thoughts only, I am looking forward to somebody answering that question with confidence.
     
  14. Oct 28, 2009 #13

    sophiecentaur

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    Re: Mirrors

    To get a good specular reflection, the structure (small scale) must be much finer than the wavelength of the radiation. The surface electrons on a polished metal plate behave more or less as a continuum. It is these electrons which are moved by the alternating fields of the incident radiation. Because of their motion, they will radiate the energy and the geometry is such that the 'ray optics' works. I appreciate that this may be an over simplification of what actually goes on but it can certainly be regarded this way when dealing with radio wave being reflected by metal objects.

    With a polished non-metal or metal compound surface - like a paint pigment or paper, there may also be re radiation of the incident waves but the structure is still granular and has a structure which is larger compared with the wavelength and so you are effectively getting reflections from multiple, relatively large, objects. This will diffuse the light, although there may be a strong 'peak' in the specular direction, making the white surface appear a combination of shiny and matte.
     
  15. Oct 28, 2009 #14
    Re: Mirrors

    Good mirror-like specular reflection is also possible of of non-metallic surfaces like glass or surfaces with even higher index of refraction. One distinct difference of reflection off of non-conducting surfaces is that the reflection can be partially (or completely?) polarized at some angles.
    Bob S
     
  16. Oct 28, 2009 #15

    sophiecentaur

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    Re: Mirrors

    Oh yes - I had forgot the reflection at dielectric interfaces but to be specular, you still need an effectively fine structure (and a very flat surface).
     
  17. Oct 28, 2009 #16
    Re: Mirrors

    "Float" glass plate is cast on a surface of molten tin.
    "To make plate glass, liquid glass is spread out on a metal table while it is still extremely hot, and then rollers are passed over the table to smooth and polish the glass as it cools, creating the desired uniform sheet of glass. Depending on the precise details of the manufacturing method, the glass may need to be polished again once it has cooled. Plate glass is also produced through a floating process, using a bath of molten tin to float liquid glass as it cools and sets."

    So, how shiney and flat is molten tin? See

    http://www.wisegeek.com/what-is-plate-glass.htm

    Bob S
     
  18. Oct 28, 2009 #17
    Re: Mirrors

    Does it mean that the porosity or bumpiness of a surface gives it its color?
    What about transparent materials. They soak up Light, let it go through. This is neither scattering nor diffusion. Wait, Black is yet another quality. It also soaks up Light, but does nothing to it less catch it.
     
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