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What physics causes this mirror reflection?

  1. Mar 2, 2012 #1
    I just bought an iPad mirror screen protector and I'm really puzzled about how it works.
    You can see a video of the protector here
    The mirror quality is very good, of the order of 95% reflectivity, and it's not a metalized film as transmission is ~95%
    You can see in the video that there is a diffraction type of coloring at grazing angles.
    If I rotate it in front of an LCD screen there is a weak polarization effect.
    The manufacturer claims it comprises three layers but doesn't explain the physics.
    What is going on?
  2. jcsd
  3. Mar 3, 2012 #2
    if a material is made up of layers of different refractive index it will selectively transmit or reflect a range of wavelengths - lookup dichroic effect in a search engine
  4. Mar 3, 2012 #3


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    Welcome to Physics Forums cahillj :smile:
    I'm not sure (yet) what is going on, but these are contradictory statements. Reflectivity and transmission cannot add up to more than 100%. Can you clarify how you got these values? Was it just an "eyeball estimate"?

    From the video, it looks to me like reflectivity is a lot less than 95%. I could believe 10%, but that is just an eyeball estimate on my part, and from cheap video footage at that.
    Last edited: Mar 3, 2012
  5. Mar 3, 2012 #4


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    Anything like this must have equal transmissions and reflections in either direction. The thing is useless if it reduces the screen brightness by more than a few % and there is an 'assumed' benefit, I guess in having a mirror finish in that you can eliminate annoying individual external light sources by tilting the screen. It could be very good in some circs but in very bright light, you can't afford to chuck away the brightness from the display.
    The screen in the movie is not highly reflective - it's just very well polished.

    It could be argued that the screen should be lenticular for personal viewing as it channels all the light towards the viewer. It's not for famliy watching.
  6. Mar 8, 2012 #5
    Thanks to all for your comments
    Redbelly98 - a good comment about the sum of intensities.
    I crudely measured reflectivity at less than 50% with transmission around 80%, much closer to "sum not greater than 100%"
    Crudely = DSLR+digitally generated image of bands of grey which I also printed+mirror+iPad part covered by mirror +different ASA+ImageJ
    Any better suggestions to make measurements at home?
    Sophiecentaur - as a screen protector this stuff is terrible! I wouldn't recommend it BUT it is an interesting material. Next time you are on Amazon buy a cheap sample <$5
    Sambristol - I think you got the closest, I'm thinking it is made up of materials of many layers of different RI and I've just found this paper, which I intend to read through.
    Giant Birefringent Optics in Multilayer Polymer Mirrors
    Science 31 March 2000:
    Vol. 287 no. 5462 pp. 2451-2456
    this link might work here
    fun, fun, fun!
  7. Mar 8, 2012 #6


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    Using a camera is a good idea. But I would look at the actual pixel brightness values in the images -- try to take a rough average over some area of the image. Camera settings (aperture, exposure time) should be such that the reference image (looking directly at the light source) has pixel brightness less than the full 255. If the brightness is 255, you are saturating the sensor and you'll need to reduce the aperture or exposure time until it is less than 255.

    EDIT added: make sure your reflection and transmission measurements are both as close to normal incidence as possible. Tricky to do for reflection, since at normal incidence the camera is blocking the light source, but try to get as close to normal incidence as you can.
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