Re: how does light cause interference phenomena?
From: John Kennaugh (JKNG_at_kennaugh2435hex.freeserve.co.uk)
Date: 09/29/04
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Date: Wed, 29 Sep 2004 20:04:21 +0100
Tom Roberts writes
>John Kennaugh wrote:
>> If you design an oscillator and you want to produce as pure a
>>sinewave as possible you make the overall gain = 1.
>
>An "oscillator" with gain=1 won't reliably oscillate at all.
If it is EXACTLY 1 and it is oscillating then the O/P will remain
constant. The problem is making anything with an exact amount of gain.
>
>
>> Any more gain and the amplitude will continue to increase until the
>>waveform distorts.
>
>You need to control the amplitude, not the gain.
No there are two ways.
1/ You can start with a gain higher than 1 (say 2) and allow it to
'distort'. Gain is (change of O/P)/(change of I/P). When the waveform
starts to clip you are getting less O/P change than your gain implies so
the effective gain is reduced. If the tuning element is high Q e.g. a
crystal, this will not show up too much as the distortion will be
largely filtered out and you will still get a reasonable sinewave.
2/ You can detect the amplitude (rectify it) and used a control loop
(negative feedback) to control the gain. It starts off with gain >1 the
amplitude increases. When the amplitude reaches the required level the
loop reduces the gain to 1 and the amplitude doesn't get any bigger.
Increase in amplitude reduces gain, decreased amplitude increases gain.
When stable gain = 1. - normal servo loop type action.
>
>
>> If less than 1 it won't oscillate at all.
>
>Yes. Which is why a circuit designed for gain=1 won't work reliably,
>because components have tolerances....
Explained in detail above. BTW I make my living as an electronics
consultant. I normally charge for this sort of advice.
>>> Yes, a filament produces light via a random process: thermionic
>>>emission. But all sources of any type of signal are "band limited
>>>noise" -- the differences are in the bandwidth, not the principle.
>> The output of a crystal (or any) oscillator is not a random process
>
>Sure it is.
Absolutely not.
>All electronic oscillators amplify noise in a feedback loop with high
>gain
Gain = 1
>and limited bandwidth (determined by the frequency-determining
>element). Just think about how such an oscillator starts up and you'll
>realize that startup is always from random noise.
That is no different to saying a watch won't start unless you give it a
little shake. Once started a balance wheel isn't amplifying vibration.
IF you could make a noiseless amplifier then you would have to inject
some sort of signal to kick start it. Once started it does not rely on
noise any more than you have to keep shaking a watch to keep it ticking.
As I explained in the lump you snipped you can never get a true sine
wave by filtering noise because in the limit you get a perfect sinewave
of zero amplitude.
> Once started, a well-designed oscillator will have the desired signal
>dominate the circuit's operation so that out-of-frequency noise has no
>chance.
>
>BTW a laser starts up with random noise, too. But only noise within the
>bandwidth (and direction) gets amplified.
>
>
>>>> 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.
>>>
>>> That is indeed the case. But individual "bursts" are basically a
>>>single photon.
>> I don't accept that because you can get at least 1000 rings in a
>>Newton's ring experiment. Can a single photon extend its influence
>>over 1000 wavelengths?
>
>See the discussion on coherence length. Don't attempt to discuss "an
>individual photon", discuss coherence length.
You were the one who brought in a single photon.
>>> The coherence length of a common He-Ne laser is usually a few
>>>centimeters, which means you can construct a hologram over a volume
>>>of about that size. A singlemode laser can have a coherence length
>>>of many meters. IIRC a lightbulb has a typical coherence length of a
>>>few microns -- long enough to generate Newton's rings in soap
>>>bubbles and oil films, but not any larger-scale interference phenomena.
>> You seem rather confused. Newton's rings have nothing to do with
>>soap bubbles or oil films.
>
>Sure they do. On a sunny day one can see splotches of color in an oil
>slick or a soap bubble.
Interference yes, Newton's rings No. - hint - they don't form 'rings'.
[...]
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