> On Wed, 24 Oct 2007 21:47:32 -0700, Jerry <Cephalobus_alie
...@comcast.net>
> wrote:
> >On Oct 24, 4:14 pm, HW@....(Dr. Henri Wilson) wrote:
> >> On Wed, 24 Oct 2007 02:13:45 -0700, Jerry <Cephalobus_alie...@comcast.net>
> >> wrote:
> >> >On Oct 24, 3:10 am, George Dishman <geo...@briar.demon.co.uk> wrote:
> >> >> On 23 Oct, 22:32, HW@....(Clueless Henri Wilson) wrote:
> >> >> > Incidentally, this also tends to suggest that the fringe
> >> >> > production in a sagnac interferometer is something to do
> >> >> > with the phase relationship between INCOMING and OUTGOING
> >> >> > rays rather than the rejoining of the two oppositely moving
> >> >> > rays...I know that sounds impossible...but is it?
> >> >> Yes, for two reasons. The simpler is that if
> >> >> you look at the arrangement of the beam
> >> >> splitter, the remaining light goes back to
> >> >> the lamp but the more robust is that there
> >> >> would be a path length difference of nearly
> >> >> a metre (the loop length) between the originated
> >> >> light and that which has bone round the loop.
> >> >> That grossly exceeds the coherence length for
> >> >> a filament source so there is no way to form
> >> >> fringes with a detectable contrast ratio.
> >> >I think that you have gone -way- over Henri's head with
> >> >mention of coherence length.
> >> >To Henri:
> >> >Early experimentalists such as Michelson and Morley, Sagnac
> >> >etc. used monochromatic sources only during the alignment
> >> >stages while setting up their interferometers. Actual
> >> >experimental runs were always performed with white light.
> >> >The reason for this is that white light creates a distinctive
> >> >pattern of a central bright white fringe surrounded by a
> >> >rapidly fading set of colored fringes. The advantage of this
> >> >is that the central fringe of equal path length is always
> >> >readily identifiable, whereas monochromatic light produces
> >> >uniform fringes in which it is virtually impossible to
> >> >determine the central fringe of equal path length.
> >> >The distinctive pattern of fringes formed by white light
> >> >enabled Michelson and Morley, who recorded their observations
> >> >visually, not to "get lost" while figuring out how far their
> >> >fringes were displaced from their fiducial marks. In the
> >> >Michelson and Gale experiment, which was a giant Sagnac
> >> >setup, the central fringe, in the absence of rotation, would
> >> >appear precisely midway between the two images of the slit.
> >> >This enabled them to calibrate their apparatus for zero
> >> >rotational velocity; it was thus not necessary for them to
> >> >halt the rotation of the Earth to get a zero reading, which
> >> >would have been somewhat impractical in the absence of divine
> >> >intervention (Joshua 10:12-15).
> >> >Note that I stated that the pattern of colored fringes
> >> >surrounding the central bright fringe fades rapidly. This is
> >> >because the spacing between the red fringes and the blue
> >> >fringes is different. Within a few fringe widths from the
> >> >central fringe, the colored fringes overlap until the fringe
> >> >pattern is no longer perceptible. Since each fringe represents
> >> >a half wave difference in path length to the two images of the
> >> >source slit, this means that the path lengths must be
> >> >precisely matched, otherwise it would be impossible to see any
> >> >fringes at all.
> >> >This distance to which the path lengths must be matched,
> >> >otherwise fringes are invisible, is known as the "coherence
> >> >length". The coherence length for white light is no more
> >> >than a handful of microns. Your notion that "fringe
> >> >production in a sagnac interferometer is something to do with
> >> >the phase relationship between INCOMING and OUTGOING rays
> >> >rather than the rejoining of the two oppositely moving rays"
> >> >is totally ridiculous to anybody who knows anything at all
> >> >about optics.
> >> The sensible thing to do is use monochromatic light and tilt
> >> the top miror slightly in order to produce an 'optical wedge'
> >> effect. That produces a straight line fringe pattern rather
> >> than circles. Straight lines are easier to count than circles
> >> and in the case of gyros, make the direction of an acceleration
> >> easy to determine.
> >You have COMPLETELY lost the point. Earlier, you made the stupid
> >and asinine speculation that "fringe production in a sagnac
> >interferometer [has] something to do with the phase relationship
> >between INCOMING and OUTGOING rays rather than the rejoining of
> >the two oppositely moving rays."
> >George's point was that since white light Sagnac interferometers
> >are perfectly functional, your speculation is dead in the water.
> You didn't read properly. I SAID IT WAS IMPOSSIBLE.
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