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Observing perpendicular light

  1. Jan 12, 2009 #1
    Ok, this is a question I've been pondering lately.. I'm sorry if it's in the wrong forum, I just don't know where I should put it! So... Without further ado...

    Imagine you are looking at yourself in a mirror. There are particles of light that are moving from your body to the mirror, and back again to your eye. Those particles are moving through space on a straight path to and from the mirror. Right?

    Ok.. Now here's the question.. If I am at a 90-degree angle to the person looking at himself in the mirror, why can't I see the person's image as it travels to the mirror and back again?

    So, if you imagine that it looks something like this:


    X ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~O
    --------------+  ~  +-----------------------
                  |  ~  |
    If X is the person looking at the mirror (O), and Y is the person perpendicular to the path of the light. The ~ is the light, and - and + are walls. So, Y can NOT see the person or the mirror visually.. But, shouldn't Y be able to see the light particles containing the image of the person? Now, obviously, I can NOT see the light, but... Why not?
  2. jcsd
  3. Jan 12, 2009 #2


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    Because that light never hits your eye?
  4. Jan 12, 2009 #3
    But even if it doesn't hit my eye, shouldn't I see the effects of particles crossing my field of vision? Why is it completely translucent--shouldn't it be opaque? So, if I couldn't see the detail, shouldn't I at least see some interference (at least)?
  5. Jan 12, 2009 #4


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    Your field of vision isn't like a net that gets cast out and pulls in everything in front of it. It only sees light that hits it.

    A similar question would be: Why doesn't the mirror reflect the light from person Y?
  6. Jan 12, 2009 #5
    So, if I understand this correctly, since light only moves in a straight line, light moving in any direction other than directly towards my eye, should be invisible and completely translucent.. Ok, but, why doesn't the invisible light particles, at the very least, disrupt my view of the wall directly in front of person Y? It is intersecting the path between X and the mirror. Shouldn't that cause a disruption? Light has some "property" to it.. Cameras collect photons on film to create an image, solar cells collect light photons. So, there has to be something to COLLECT (which means it's something physical). So, why doesn't it disrupt what it passes through?

    As to why the mirror doesn't reflect light from Y.. It's because light doesn't bend. But, in my scenario, I'm not suggesting the light bends or not; just that I should be able to observe the light as it passes between X and the mirror.

    I know from Quantum Physics that we can observe particles going through a slit. The observations are performed perpendicular to the stream. Why can they see the stream if I should ONLY be able to see things that are moving directly towards my eye/my stream detector?
  7. Jan 12, 2009 #6


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    I come from a computer graphics background, not a physics one, so here is my limited understanding:

    Imagine you are a deaf and blind man in a room full of ping-pong balls bouncing around. Let's say that one of the ping-pong balls is headed directly towards you and is deflected by another while traveling towards you. You would never know it existed. The only ones you know about are the ones that hit you.

    In reality it's a lot more complex than that, but it seems to solve the conceptual problem you have. (I think).

    I think in real physics:
    1) The probability of two photons hitting each other is close to zero
    2) When they do, they cancel each other out or something

    correct me if i'm wrong.
    Last edited: Jan 12, 2009
  8. Jan 12, 2009 #7
    The simple answer is that photons don't interact with each other. They can, however, interact with electrons, such as the ones in photosensitive molecules in camera film or your eye.
  9. Mar 1, 2009 #8
    Uhh... I'm pretty sure photons do interact with each other.

    That's what the EPR paradox, quantum entanglement, and the double-slit experiment are all about.
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