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's stages of displacements B and C on your diagram. 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? Harry ----- Original Message ----- From: Horace Heffner <hheff...@mtaonline.net> Date: Friday, September 11, 2009 12:38 pm Subject: Re: [Vo]:Michelson-Morley Interferometer experiment finally done correctly? > One more typo correction and some changes in wording for clarity. > > > I don't know why you refer to the direction of the beam splitter > w.r.t the laser. Direction is not what changes significantly. The > > beam splitter splits the beam 50/50, always pretty much in the same > > directions with respect to the base and laser. It is the change in > > path length that affects the interference pattern. The laser path > lengths are probably affected by motions of both the beam splitter > and the mirrors, but my main assumption was that it was the beam > splitter that was contributing the most. Considering the beam > splitter only, it 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. At maximum sag, the > motion > of the interference lines is stopped, and then reverses. The mirror > > then begins its motion over to the other maximum sag position, > which > is 180 degrees away. When the beam splitter sags by a few > wavelengths the light *direction* is affected almost not at all. > However, the path length to the mirror in line with the laser is > significantly affected. > > The laser hits the splitter at a small spot roughly in its middle. > Call the distance from the laser to the splitter spot D. Call the > distance from the Splitter spot to the mirror in line with the > laser > D1. Call the distance from the splitter spot to the mirror to one > side of the main beam D2. The path to final exit of the beam > slitter > toward the lense is D + 2 D1 for the straight path, and D + 2 D2 > for > the side path. See attached drawing. > > When the beam splitter sags towards the laser it moves the spot on > the mirror slightly to one side on the mirror, because the mirror > sags in a direction 45 degrees to the laser beam. This leaves the > path length D2 to the mirror to the side of the beam unchanged, > even > though the spot on that mirror is laterally displaced, toward the > laser. This lateral displacement is not detectable because that > displacement is only a small number of wavelengths of light, a few > microns since the wavelength is 532 nm. However, the movement of > the > mirror toward the laser reduces the path length D from the laser to > > the beam splitter by delta D, while increasing the path length from > > the beam splitter to the in-line mirror, D1, by delta D. Since the > > light travels distance D1 twice, the total path to the in-line > mirror > and back to the splitter is changed from D + 2 D1 to D - (delta D) > + > 2 D1 + 2 (delta D) = D + 2 D1 + delta D. The path to the side > mirror > and back to the splitter is changed from D + 2 D2 to D + 2 D2 - > delta > D. The total difference in path lengths is 2 (delta D). When the > mirror swings the opposite direction, the path length difference > changes to -2 (delta D). Note that the interference pattern motion > > has only to do with the change in the difference between the two > paths. For half the rotation cycle 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. > > Horace Heffner > http://www.mtaonline.net/~hheffner/ > >
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