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Reflecting One-way Light Tube

  1. Apr 10, 2007 #1
    Hey everyone, I just got an idea today surfing the web on Wikipedia.

    I noticed that in Berlin (i think there) they have these light tubes that stick out of the ground that take the light from the sun outside and the light goes through the tube and lights up the subway system. I thought it was a genius idea, but I wanted to make one on a smaller scale for an extra credit lab and CAD computer case project.

    To think about throwing the most light, I thought of using mirrors, but it felt wrong. But then, I was watching Law & Order and they were questioning the suspect in an investigation room with one way plexiglass. Then it stuck me, why not create that same effect for the inside of the tube? When I say this, I mean, you look from the outside of the tube and you can see in. And looking from the inside out, it would be a mirror. Have I lost you yet?:grumpy: I looked up the formula on how to do it in my chemistry class, but then I started questioning the whole experiment. Like:

    - Can the light go from the outside of the tube into the inside?
    - Would the light just constantly reflect on the inside or would it escape out?
    - Would the tube glow bright or glow at all?

    So, now I'm here. I asked my bro who is a physics professor in Sacramento, but he takes too long. Can anyone help explain it to me? Thanks.
  2. jcsd
  3. Apr 10, 2007 #2


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    I am very uncertain about what you have in mind. However, under the right conditions even plain window glass acts as a one way mirror. Look through a window from a brightly lit room into darkness, what do you see? Now step into the darkness and look through the same window into the brightly lit room.

    So the direction of visibility for a one way mirror is determined more by the relative lighting levels then specific properties of the mirror (or window as the case may be).
  4. Apr 10, 2007 #3
    I want this to take an outside light and glow. I thought having the reflective part of the one way glass to face toward the inside of the tube. I was wondering if shining a light into the tube would make it glow brighter having that one way glass effect and the light bouncing the mirrors. Gosh!:grumpy: I know what I mean, I'm just having a hard time describing it. I'll see if i can make it in CAD and see if it glows or not.

    Outside looking in: clear side
    Inside looking out: mirror side
  5. Apr 10, 2007 #4


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    I understand what you mean, but I don't think that the additional expense would be justified. The only time that it would really make any difference is at night when your artificial light that you have to produce would 'leak out' through the pipe. You could need lower wattage bulbs if that didn't happen, but you can achieve the same effect by just putting doors over the inside end.
  6. Apr 15, 2007 #5
    had simular idea in 3rd grade

    i was trying to make basically a perpetual energy thing. It was a solar powred light inside of a one way mirror globe. I never made it but i don't think it would hold in all of the light anyway. if you can see in at all then some light must be getting out so its not going to continue circulating. not sure if this is what you were looking for but thought it was kind of simular.
  7. Apr 16, 2007 #6
    So, my brother got back to me and this is what he said (which is basically what you two said):

    "Regarding your physics questions, let me fill you in on how a one-way mirror
    works. The glass in a one-way mirror has a thin coating of a reflective
    substance applied on one side. It is only partially reflective because not all
    of the glass is covered by the coating -- the molecules are spread too thin.
    So, some light passes through and some light gets reflected back. Light
    hitting it from either side has the same amount transmitted and reflected.
    There is no difference between the two sides of the glass.

    When police (or Home Depot) use one-way mirrors, they brightly light the side
    that they want to act like a mirror and they dimly light the other side.
    Imagine that 50% gets reflected and 50% gets transmitted, okay? The 50% of the
    light that is reflected on the bright side of the glass is much brighter than
    the 50% that is transmitted through the glass from the dark room, so it looks
    more like a mirror than a window to a person on the bright side. But, the 50%
    of the bright light that gets transmitted through the glass to the dark room is
    much brighter than the 50% of the light that is reflected back into the dark
    room, so it looks more like a window to a person on the dark side. If you
    switched the lighting around, you could change which side appeared to be a
    mirror or window.

    So, if you shine a light onto a partially mirrored glass tube, some will
    reflect off the surface right away and some will enter the tube. The light
    that makes it into the tube will continue to bounce around in the tube. Each
    time it hits a wall some of it will reflect and some will transmit through the
    glass, and it won't take long before all of the light has left the tube. My
    guess is that it would just end up looking normal -- like a shiny tube. No
    glowing or anything wierd like that."

    My question is answered, but I still wanna experiment. I'll see if I can do it and post some pictures.
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