On Oct 26, 6:57 am, Jerry wrote:
On Oct 26, 6:34 am, HW@....(Dr. Henri Wilson) wrote:

On Wed, 24 Oct 2007 02:13:45 -0700, Jerry
wrote:
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.

I know. I once made a michelson interferometer. I was quite
easy to adjust.

Using a monochromatic light source, yes. A white light
Michelson interferometer is rather finicky because of the
short coherence 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.

The deliberate tilting of the top mirror to create an optical
wedge was a later innovation that produced almost straight line
fringes.

Nope. See my next comments.

It is obviously easier to measure the sideways
displacement of a line than to estimate the shade of
fairly uniform image.

Tilting doesn't work with a white light interferometer.
The interference pattern doesn't extend far enough out to get
"straight" fringes, and the fringes would be colored. The early
experimentalists used SLIT sources of light.

Obviously you are accustomed to monochromatic light and lasers.

Maybe circles are still preferred in metrology.

get it yet?

Sure. But YOU sure haven't.

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.

It's all irelevant anyway since light moves at c wrt its source
and everything at rest wrt the source.

Anything you don't understand is "irrelevant"?

Jerry
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Jerry