Re: how does light cause interference phenomena?

From: John Kennaugh (JKNG_at_kennaugh2435hex.freeserve.co.uk)
Date: 09/22/04 

Date: Wed, 22 Sep 2004 19:56:04 +0100

In message <df76407e.0409210811.60c550e3@posting.google.com>, Randy Poe
<poespam-trap@yahoo.com> writes
>John Kennaugh <JKNG@kennaugh2435hex.freeserve.co.uk> wrote in message
>news:<h8kcqSFCZsTBFwAV@kennaugh2435hex.freeserve.co.uk>...
>> 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.
>
>You're absolutely correct. A light bulb is not a coherent
>source and it can't be used to generate an interference
>pattern.

I rather assumed it could. My text book describes how Newton's rings can
be produced with white light and how colourful it is.

> Some technique must be used to get an coherent
>source, such as selecting a small portion of the wavefront
>by diffracting through a slit first. Here's an article
>describing the Young experiment from 1805.
>http://en.wikipedia.org/wiki/Double_slit_experiment

If you have a large area source then already you have different path
lengths from the centre and from the edge to any given point. If the
point of the experiment is to produce an exact difference in path
lengths and observe interference you cannot start with multiple path
lengths.

My text book shows a Newton's rings set up without any slit. You seem to
have misunderstood my concern

  Suppose you have a beam of light

  ***A*************************************B************

A and B are arbitrary points marked on the beam. Suppose you split it
into two and delay half of it by an even number of wavelengths.

  ***A*************************************B************
  *****************************************A*************

The question is why should A and B be in phase? If you assume that light
is the equivalent of a regular sine wave, as per a ripple tank there is
no problem but my understanding is that that sort of phase continuity is
only produced by a laser. That most other light sources, even those
which produce monochromatic light such as a sodium lamp produce EM
noise. If that were not the case there would be nothing special about a
laser compared to a sodium lamp. Noise does not produce regular phases
so there would be no reason to assume that any fixed phase relationship
would exist between A and B. My book talks about detecting Newton's
rings at least to the 1000th ring.

>
>I don't know what was known about light as early as 1805
>and why he got the idea to use that first slit, but as you
>point out it is crucial.

I didn't and don't think it is. 

-- 
John Kennaugh
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