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Pinhole Train

  1. Sep 3, 2009 #1
    Hi This is regarding the pinhole camera concept regarding apparent angular size.

    If we take the simultaneity train and assume the lightning strikes are behind translucent ends of the observers car and that the observer has two pinhole cameras facing forward and back: In the view of the track observer the forward camera has moved toward the front of the car during the lights transit, and therefore the end appears bigger , subtends a larger angle.
    Looking into the camera it is seen that as the light moves from the pinhole to the back, the film plane has moved forward, thus producing a smaller image. equivalent to moving a darkroom projector closer to the film plane.
    Of course the exact opposite effect takes place with regard to the rear.
    So in both directions this is logically consistent wrt the angular size.

    But what about luminosity?? Photons per unit area?
    The forward camera already has a larger area and less falloff that is intering the camera. Then the effect of the forward motion would concentrate that even more , just like moving a projector closer to the film plane.
    And of course the effect would be exactly opposite toward the rear , where the motion away from the pinhole would tend to diffuse the image even more.
    Yet I have to assume that this wound not actually take place but cannot figure out a mechanism that would prevent it from occuring . Any ideas???? Something obvious I am missing equivalent to doppler self cancellation??.
  2. jcsd
  3. Sep 3, 2009 #2


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    Keep in mind that if a flash produces an expanding sphere of light which is uniform in all directions in one frame (same number of photons per unit area on all parts of the sphere), then in other frames although the distribution will still be a sphere expanding at c, it will no longer be uniform in those frames (if you're talking about the frame of an observer moving to the right in the frame where the distribution is uniform, then in his frame the light should be more concentrated on the left side of the sphere than the right side). So, although I haven't really thought about the details of the pinhole setup, it might well be that both frames could agree the luminosity is greater for the light coming through one pinhole than the other, though the reasons would probably be different in each frame.
  4. Sep 4, 2009 #3
    I had never thought of the light sphere in that regard,,it is a good tip . I will give it some thought thanks
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