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Understanding Light in a Sheet of Glass

  1. Jul 7, 2010 #1
    I need help to understand what is happening when I project a image in a sheet of glass.
    Imagine this picture, a projector as the light source projecting a image in a sheet of glass:
    1: I have high transmission. (the light that pass away the sheet of glass)
    2: Low absortion
    3: A so so reflection (part of the light is reflected at my wall)
    4: Low Scattering
    5: Refraction ?

    The light that pass the sheet of glass make a image on the other side of the projector(transmission), but the image appears too in the sheet of glass. Is the image that appear in the sheet of glass a absortion or a refraction?

    Attached Files:

  2. jcsd
  3. Jul 8, 2010 #2
    Neither. It's a reflection, about 4% from each side.
  4. Jul 8, 2010 #3
    Reflection won't cause an image to form at the reflecting surface. What you see is impurities in the glass, dust on the surface etc. acting as a rather nebulous projector screen. A bit like a layer of very thin smoke.
  5. Jul 8, 2010 #4
    I agree with you in part. I think it isnt only impurities, probably its a propertie of the arrange of the electrons of the glass. If its only impurities or electrons arrangement, the image is form from the fluorescent absorption, right?
  6. Jul 8, 2010 #5


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    Doubtful - I would guess it's all dust on the glass or surface imperfections resulting in scattering.

    Fluorescence would not show the same color as the image being projected - it would absorb the energy and emit it at some other wavelength.

    Also, understand that this sheet of glass (which I assume is very close to the projector) is intercepting the light when it is much, much stronger.

    For example, our movie projector projects an image which appears very, very bright when spread out to shine on a screen that is, say, somewhere in the neighborhood of 30 feet long by 15 feet tall, but the glass that separates the projector from the theater intercepts the light right next to the projector, when the image is only 4 inches across. So you're looking at light that is ~8000 times brighter where the glass is than when it hits the screen (which is really, really bright, which is why any diffuse scattering from surface imperfections and dust is so bright, and why you may see light scattering after one or more internal reflections inside the glass).
  7. Jul 8, 2010 #6
    Partial reflection is a QM effect that varies with the ratio of the glass thickness to the wavelength of the light. The reflective variance will actually oscillate between minimums and maximums as the thickness of the glass is continually increased.
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