Re: Diffraction Angle form a Moving Grating.

From: Androcles (dummy_at_dummy.net)
Date: 11/01/04 

Date: Mon, 01 Nov 2004 10:24:35 GMT

"sal" <pragmatist@nospam.org> wrote in message
news:pan.2004.11.01.03.39.09.722424@nospam.org...
: On Sun, 31 Oct 2004 20:33:00 +0000, John Kennaugh wrote:
:
: > sal writes
: >>>
: >>>>Stars are definitely not.
: >>>
: >>> *- Sparked a thought. Correct me if I am wrong but a diffraction
: >>> grating works on the principle of constructive interference does
it
: >>> not? In an earlier thread I asked about interference and had
some
: >>> useful answers which I followed up. Basically light from a
thermionic
: >>> source is produced by what appears to be a random process but
turns out
: >>> to be a quasi random process. If it was truly random then light
would
: >>> not be coherent and would not produce interference OTOH it is
not
: >>> really accurate to think of light as waves as the coherence
length is
: >>> only about 2cm (typically).
: >>>
: >>> Essentially light consists of short bursts of coherent light and
if you
: >>> built a MM interferometer with one arm a couple of inches longer
than
: >>> the other you wouldn't get interference fringes because you
would be
: >>> combining two unrelated bursts. That is referred to a temporal
: >>> coherence. There is also spatial coherence. In the double slit
: >>> experiment the double slit is illuminated by a single slit and
the
: >>> reason for that is to select a small region of the sources
output
: >>> because different parts of the source will be giving out their
own
: >>> bursts independent and not coherent with other regions of the
source.
: >>>
: >>> Now if you have a large source and you place yourself a long
distance
: >>> away from it so that it looks like a point of light then that
light
: >>> should consist of loads of different spatial groups so if they
do
: >>> diffract when put through a diffraction grating there is a
question to
: >>> be answered as to why?
: >>
: >>Yes, absolutely, it diffracts. That's how stellar spectroscopes
work.
: >>
: >>That's what limits the resolving power of telescopes, even on
stars that
: >>are so far away they look like "point sources". Blow 'em up
enough and
: >>you get diffraction rings all around the central maximum.
: >>
: >>
: >>> Question - if you put star light through a diffraction grating
do you
: >>> get a spectrum ? If so any suggestions why?
: >>
: >>Because ONE photon diffracts. ONE photon interferes with itself.
: >
: > If individual photons diffract then why has a light source got a
coherence
: > length?
: >
: >
: >>This experiment was carried out a long time ago:
: >>
: >>Set up a two-slit interference experiment, using a candle, a long
box,
: >>and some film. Some details remain to be filled in, but you get
the
: >>idea.
: >>
: >>Do the experiment once. You get an interference pattern.
: >>
: >>Now, put a piece of smoked glass between the candle and the slits,
to
: >>reduce the intensity. Increase the exposure length to compensate,
and do
: >>it again.
: >>
: >>You still get an interference pattern.
: >>
: >>Add more density to the smoked glass, and repeat, and keep
darkening the
: >>glass until you are certain (statistically) that no more than ONE
photon
: >>will ever hit the film at a time.
: >>
: >>PREDICTION: As the light is made dimmer and dimmer, the
interference
: >>pattern will gradually fuzz out, until at the dimmest intensities
there
: >>will just be a bright spot in the middle of the film and no
interference
: >>pattern at all.
: >>
: >>ACTUAL OBSERVATION: The interference pattern is _unaffected_ by
the
: >>intensity. In the final run, IIRC, they had to expose the film
for
: >>around a week to get an image (I guess they changed the candle a
few
: >>times -- not sure) but no difference in the pattern was observed.
: >>
: >>This came as a great surprise, or so I have been told. I don't
know in
: >>what year it was done.
: >>
: >>(I was told about this in school but I have never looked it up to
check
: >>the details.)
: >
: > I do hope the experiment has been repeated with something a little
more
: > sophisticated than smoked glass.
:
: Me too! :-)
:
: > Scott Murray suggested that this experiment is flawed. He
suggested that
: > photons are not produced purely randomly but that a large number
of
: > atoms which have enough thermal energy to release a photon may
: > accumulate before one random photon is produced and that one
photon
: > triggers a numerically massive burst from the other 'pregnant'
atoms.
: > This seems to tie in with idea of coherence length and that light
is
: > generated in bursts. If this is the case it totally cocks up the
: > statistics as in:
: >
: >>you are certain (statistically) that no more than ONE photon will
ever
: >>hit the film at a time.
: >
: > It seems to me that with modern technology a fascinating area of
study
: > is opened up trying to answer the question 'what is a photon' and
how do
: > they produce wavelike properties. Physicists seem more interested
in
: > calculating what would happen if you fell thorough the centre of a
: > rotating black hole.
: >
: > Then again if you look at your own statement:
: >>you are certain (statistically) that no more than ONE photon will
ever
: >>hit the film at a time.
: >
: > If that is true surely it means that a dark fringe is where no
photons
: > arrive. Classically destructive interference involves two things
anti
: > phase arriving and cancelling each other. What you describe seems
more
: > like certain angles are 'permitted angles' and some are not.
:
: Yes, exactly. I'm hardly an expert in this field, to put it mildly,
but
: to me the effect appears identical to the diffraction of particles,
such
: as electrons. The probability of finding the particle someplace
behaves
: as a wave, and that probability wave diffracts and/or interferes.
The
: particle, on the other hand, either is or isn't at any particular
spot.
:
:
: > A purely random thought. Have you studied a tuning fork? What you
have
: > is a seemingly solid lump of metal with two prongs sticking out of
it.
: > Superficially it looks as if there is no reason why you couldn't
have
: > just one prong vibrating, yet if you stop one the other stops.
:
: Not exactly. If you stop one prong _dead_ -- you freeze it,
suddenly, so
: it can't move, by, say, grabbing it with a rigid clamp attached to a
very
: heavy table -- the other prong will keep right on vibrating. But
that's
: very hard to do. Normally you stop it with something, like a hand,
that
: has a certain amount of "give" to it. The object which is "stopping"
it
: actually acts as a damper, rather than a rigid clamp. The "stopped"
prong
: then actually goes on vibrating, a little, but the energy of the
: vibrations is sucked out rapidly by the object which is damping it.
The
: energy from the "free" prong leaks into the "stopped" prong, and is
also
: absorbed by the damper.
:
:
: > A photon must interact with the slot it goes through, otherwise it
would
: > go straight through can it be that a photon sets up a sympathetic
field
: > vibration in the other slot. Then again what is the effect of
varying
: > the width and spacing of the slots. I have seen comments such as
'slots
: > of x width spaced about y give best results'. That is fine if all
you
: > are trying to do is demonstrate a phenomena but surely someone has
: > investigated further.
:
: You'd better believe it. This has been a bread and butter issue for
: astronomers for many decades, and a great deal is known about
exactly
: what diffraction patterns result from various apertures.
:
:
: > I suppose that it is the edges of the slot which have the affect.
:
: That's certainly not how it's _modeled_. The edges of the slot have
: nothing to do with it -- the wave function actually interferes with
itself.

Are you sure about that? Seems to me there would be a few fuzzy
electrons lurking around at the edge of anything.

:
: If you have a "slice" of a plane wave, with nothing on either side,
it
: will spread. It won't spread evenly, however; it'll actually show a
: spread pattern with maxima and nodes in it. That's the diffraction
: pattern. In the wave model, you just treat the entire oncoming wave
front
: as an infinite set of infinitesimal sources, each radiating in all
: directions, and integrate the contributions from all sources at
every
: point of the target, with appropriate phase shifts, to find the
pattern.
: The result shows good agreement with experiment. (No, I don't know
"how
: good". I think it's perfect, within error bars, in all experiments
which
: have been conducted, but I don't _know_ that. Any reasonably well
: informed astronomer should know, however.)
:
: Normally, you take the integral over the slit width, because you
know the
: form of the wave there. There's nothing magic about the slit,
however.
: You just need to find a cross section of the wave where you know, a
: priori, what the wave must look like, and integrate over that wave
: "surface".
:
:
: > If you have the slot too wide some light will blatt straight
through the
: > middle unaffected and spoil the contrast. If you have the slots
too
: > narrow then what? Do you reach a point where a photon is affected
by
: > both edges at the same time. How wide is a photon?
:
: It appears that a photon is infinitesimal (or anyway, very small)
when it
: actually hits something. One electron is kicked loose from one
silver atom
: by one photon when it hits the film. Neighboring atoms are
unaffected.

Huh? In agreement with experience over-exposed film has fuzzy edges
as photons "bleed" into neighbouring grains.

: But going through a slit -- who knows? As far as I can tell, the
: probability wave for a photon is as wide as whatever aperture it's
passing
: through, which is enormous compared to the size of the "particle".
:
: And what about radio wave photons? Dunno -- I don't think they can
be
: detected individually: too little energy.

Long baseline interferometry was alive and well the last I heard.

:
: And now I have ventured so far beyond what I actually know about
this
: subject that I should just stop talking.
:
:
: --
: I can be contacted through http://www.physicsinsights.org
:

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