Re: how does light cause interference phenomena?
From: EL (hemetis_at_hotmail.com)
Date: 09/23/04
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Date: 23 Sep 2004 02:51:00 -0700
[EL]
Greetings John, please realize that I am in agreement with you
generally, and that that was further confirmed by reading your reply.
See below for some comments.
John Kennaugh <JKNG@kennaugh2435hex.freeserve.co.uk> wrote in message news:<tav$TAIzBZUBFwrv@kennaugh2435hex.freeserve.co.uk>...
> EL writes
> >[EL]
> >Although your post seems very logical, intuitive and rather obvious in
> >conclusion, I have never read such analysis any where in one and the
> >same page to focus on this issue.
> >
> >Opticians would use different words such as "monochromatic" to
> >describe single-frequency-waves that may interfere to give finite
> >interference patterns.
>
> I think perhaps we should define a little closer here what we mean. In
> electronics noise is a useful signal for measurement purposes. 'White
> noise', if analysed in the frequency domain, contains all frequencies at
> equal power levels. If you filter it using a narrow band filter then you
> will produce a narrow band of frequencies. If it is in the audio band it
> will sound like a single frequency. If the bandwidth is carefully chosen
> it will sound very like a violin playing that note. Why a violin and not
> a flute? A flute produces a near perfect sine wave. Essentially when a
> violin is bowed the string tries to resonate at a single frequency but
> the bow keeps re-stimulating it so that it is like a sine wave
> constantly being restarted at different phases which gives its
> characteristic sound. One would still describe a violin as playing a
> particular note, a particular frequency.
>
> The term 'monochromatic' simply means single colour this may or may not
> be 'phase coherent' in the sense a sine wave is. If you pass white light
> from a filament lamp (or the sun) through a red filter it reduces the
> bandwidth of the light it does not make it phase coherent. Even if you
> use a monochromatic light source such as a sodium lamp# the process for
> producing the light is still essentially random but because it involves
> a particular transition of energy states produces single energy photons
> = single frequency it is not the same as phase coherent light from a
> laser. To produce interference lines you surely need phase regularity.
>
> # I know sodium has two lines close together not one but I can't think
> of a source which is truly 'single line'.
>
> >An oscilloscope is a machine founded on the "time" domain, tracing
> >amplitude variations over a steady time scale we call the time-base.
>
> Correct and if you trigger a scope on a sinewave you get a stable
> picture. If you put the same signal into the other scope input and
> select 'add' it will add the two together and produce a sine wave twice
> the amplitude. If you delay one input relative to the other the
> amplitude will reduce until one is delayed by half a period compared to
> the other when the amplitude becomes zero. A useful way of demonstrating
> interference.
>
> If you try it with a noise source then there is no stable picture just a
> blur. No matter what the delay you put in the average amplitude is the
> same (actually = sqr(2) x what it was with one signal).
>
> >An interferometer would be founded on the "space" domain from that
> >perspective, such that what is being displayed is the instant
> >"projected-steady-space-scale"; tracing amplitude variations too, but
> >in this case it is the spatial distribution rather than the temporal
> >distribution that matters.
>
> Delaying one by a specific distance is related to delaying a specific
> time by c.
[EL]
Yes, but the difference between the two cases is not in the fact that
the wave-phase-spatial-location is being shifted when time is shifted
but that the wave-phase-shift (when comparing the two interacting
waves) is rather in a plane perpendicular to the direction of the wave
in the case of "interference patterns" rather than being in the same
plane of the wave direction-line in the case of
electronic-amplitude-modulation.
That is why phase-coherence of monochromatic waves is not a sufficient
prerequisite for the production of interference patterns but the
spatial polarity of the phase-shift too is essential, which may be
observed as a polarization of the pattern. Of course non-polarized
waves will also show a polarity in the interference pattern
corresponding to the direction of the phase shift.
>
> >Waves from a hot filament definitely interfere all the time, but what
> >is it that we should expect from countless patterns overlaid randomly,
> >both in time and space!
> >If we use a painting roller (with a pattern engraved on its surface)
> >in all directions on a wall and use it repeatedly, we shall end up
> >painting the wall completely.
> ## See comment below.
>
> >The hot filament certainly emits an undetermined number of "mixed"
> >monochromatic rays.
>
> Again from electronics there is a difference between a comb spectrum and
> a noise spectrum. On a spectrum analyser the first will appear as a
> series of peaks close together each being an individual frequency the
> second has no definite peaks just one broad peak corresponding to the
> noise bandwidth.
>
> >We can verify that by using "monochromatic-color-filters" and
> >polarization filters.
> >
> >In other words, we can "extract" pure-frequency-rays from that
> >"noise".
>
> I have explained above that narrowing the frequency range does produce
> pure frequency in the sense of regular phase. If you do a Newton's ring
> experiment with white light you still get rings but the rings are
> coloured. ## I am not sure in what you write above whether you are
> implying that you don't. If you view such coloured rings with a
> monochromatic filter I believe you will see the same result as you would
> had you used monochromatic light i.e. put the same filter in front of
> the light source.
>
> >As for destructive-constructive interferences, they do not occur from
> >rays emitted from a single source hot filament because of the nature
> >of the spatial propagation over time.
> >Multiple sources and/ or bounced rays are the ones that interfere and
> >superimpose when they "collide" eventually.
> >
> >A single photon may not interfere with itself due to the unique
> >spatial-temporal characteristics of each and every photon.
> >Photons of the same ray too are in different loci at an instance of
> >time or at the same locus but at different time-interval-slots.
> >
> >All manipulations begin with beams (bundles of rays); we may split
> >them, change their path, delay them, polarize them and make them react
> >at a predetermined point in space-time.
> >
>
> >In LASER devices, it is not true that a single photon could
> >"stimulate" a complete beam.
>
> I never said it did.
[EL]
Yes, I apologize if I sounded like I implied that you did.
There are two major sequences of events in the production of LASER
beams.
The first sequence involves the source high-energy-photons bouncing
randomly in planes inclined on the direction of LASER beam
target-direction. This sequence of events includes the atomic energy
level excitation of valence-electrons preparing them for action
(warming-up), where the bouncing is mainly off the cylindrical
half-mirrored surface of the LASER-tube.
The second sequence of events involves the full-mirrored back and the
half-mirrored front of the tube on which (emphasizing)
resultant-emission-photons bounce to-and-fro until phase coherence
randomly happens along the lattice, at which point a single photon
literally "collects" all the energy contained in its path (from all
the pre-excited-electrons) before exiting the LASER tube.
I only cared to clear a common confusion concerning the difference
between a wrong "stimulated-excitation" and a correct Stimulated-
Emission.
The total energy of the beam is governed by the efficiency of the
energy conversion applied to the excitation of the core by the source
photons.
Stimulated emission precisely means that the returning of excited
electrons to their ground states is stimulated by the event of the
being amplified photon entering the spatial domain of the excited
electron such that the being emitted photon is coherently in-phase
with stimulating photon hence "adding-up" amplitudes constructively.
This sequence of events where an amplified-stimulating-photon could
pick-up more energy along its path (which is parallel to the axis of
the LASER tube) may happen many times before the ray exits the lattice
and onto the absorbing target.
>
> >There is always an excitation source of high energy photons that
> >determines the energy of the beam. It is the resonant emission that
> >"magnifies" the constructive interference, producing an extremely high
> >amplitude waves-beam.
> >
> >The law of energy conservation demands that we understand that there
> >can be no successive stimulation in a chain reaction scenario.
>
> It is surprising that we still rely on cloud chambers to view the
> progress of particles and never assume that the particle looses energy
> when it initiates the formation of a bubble.
>
> The suggestion was that a number of atoms have reached an energy state
> where an electron could drop from one energy level to a lower energy
> level and produce a photon, all energy involved coming from the atom
> itself. That the first photon to do so spontaneously, may trigger others
> which are 'ready' in an 'excited state' - whatever the terminology might
> be - none of the photons so produced taking any energy from the
> initiating photon. That a burst of photons so produced ends up a burst
> of phase regular 'wave' - what you call 'pure-frequency-rays'.
>
> >The
> >mirrored surface and the partially mirrored surfaces are responsible
> >for the accumulation of energy at first, until a steady state is
> >reached, where the incoming number of photons (from source) equals the
> >outgoing number of photons. The lasing material is responsible for the
> >monochromaticity and polarization of the resulting beam. The frequency
> >of the waves in a LASER beam is the "lower-energy-bound" corresponding
> >frequency.
>
> If you say so!
[EL]
Assuming that I know what I am talking about.
<smile>
EL
>
> >
> >John Kennaugh
> >wrote
>
> >> How does light cause interference phenomena? Silly question. Light is a
> >> wave when you add waves together they interfere. Ripple tanks and all
> >> that. I don't think it is as simple as that. As an electronics engineer
> >> I am familiar with different sorts of signal and the sinusoidal wave
> >> shape, similar to what we see in a wave tank plays an important part.
> >>
> >> However there is another signal familiar to electronics engineers and
> >> that is a noise signal. Pure noise has a random structure. One can still
> >> talk about intensity (power level) but this is a mean level, the
> >> amplitude varies randomly and the interval between zero crossing points
> >> is also random so concepts such as phase do not have meaning. It does
> >> not have a frequency but it may have a bandwidth. If you put it through
> >> a band pass filter then when looked at in the frequency domain the power
> >> is evenly distributed across the band. If you look in the time domain
> >> the upper frequency cut off will limit the maximum rate of change in
> >> amplitude.
> >>
> >> I am under the impression that very early radio transmitters, which used
> >> sparks were in effect EM noise generators. It was not until the
> >> thermionic valve oscillator was invented that sinusoidal EM radio waves
> >> could be generated. My understanding is that something like a filament
> >> light bulb generates light 'noise' and that it was not until the
> >> invention of the laser that light joined radio in producing a pure
> >> waveform.
> >>
> >> If I have that right, and it seems logical that a filament should
> >> produce light via a random process then it is the em equivalent of band
> >> limited noise. What we see is the upper frequency limited by the
> >> temperature of the filament and the lower frequency by the limitation of
> >> our eyes in the infra red but no regular 'waves' in the wave tank sense.
> >> The problem is that if you take a noise signal. Split it into two. Delay
> >> one path and mix with the undelayed path then there seems no reason why
> >> 'a bit of it delayed' should have any fixed relationship with the 'bit
> >> which is not delayed' - essential to give a static interference pattern.
> >>
> >> Scott Murray suggested (from theory underlying the laser) that when a
> >> photon is randomly emitted it acts like a catalyst and stimulates the
> >> emission of a mass of other photons. This would suggest that light from
> >> a filament is not in fact random but in short bursts of coherent light
> >> with no fixed phase relationship between one bust and the next but a
> >> fixed relationship within the burst.
> >>
> >> If one takes the sinusoidal wave tank model then one expects to get
> >> constructive interference if the path difference is n wavelengths (where
> >> n is an integer) and destructive interference if the path difference is
> >> (n+0.5)wavelengths. If my suggestion is correct there would be a maximum
> >> value for n beyond which this would not be true. i.e. when n is
> >> sufficiently large that you start mixing different uncoherent bursts
> >> together. This in turn would give a rough idea of how long a burst
> >> lasts.
> >>
> >> I may of course in my ignorance be describing well know phenomena.
> >> Comments please.
- Next message: Eugene Stefanovich: "Re: Download a new book on quantum mechanics and relativity."
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