On Sep 11, 2009, at 1:29 PM, Harry Veeder wrote:
Thanks for this detailed reply. Attached is a screen capture from the video. The red dashes indicate the location of the laser. Also attached is your diagram with a few additions. (Hope both images get through vortex)Positions B and C on the screen capture correspond to the beam splitter'sstages of displacements B and C on your diagram.
We can assume so here.
A on the diagram is the stage when the difference in path lengths is momentarily zero. My question is this: Should the direction of motion of the pattern change whenever the beam splitter passes through stage A or will it only change when it passes through stages B and C?
The beam splitter reaches maximum sag twice in each rotation, when the device and laser is on about a 45 degree angle, and the beam splitter is parallel to the horizon plane. We can thus assume now the beam splitter is at maximum sag at locations B and C in the photo below, however it is important to note that the value of delta D is still a *continuous function* of *all positions* of sagging motion of the beam splitter. At maximum sag, the motion of the interference lines is stopped, and then reverses. The beam splitter then begins its motion over to the other maximum sag position, which is 180 degrees away. Note that the interference pattern speed of change has only to do with the rate of *change* in the difference between the two paths, and thus of delta D. For half the rotation cycle we can see that delta D, and thus the difference, is moving in one direction, the other half delta D moves the other direction. The interference line motion matches the *change* in delta D, which is the displacement of the beam splitter. For half the rotation cycle we can see that delta D, and thus the difference, is moving in one direction, the other half it moves the other direction. This matches what is on the video.
The direction of motion of the interference pattern only stops and changes direction at the maximum sag positions, because these are the only points at which delta D changes its direction. When the beam splitter is at point A, its normal rest position, and vertical with respect to the horizon, the speed of the interference pattern motion should be at a maximum, regardless of which half cycle it is on.
Best regards, Horace Heffner http://www.mtaonline.net/~hheffner/
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