Re: [Phys-l] A demo for biologists?

[ludwik kowalski <kowalskil@mail.montclair.edu>]

On May 16, 2009, at 1:23 PM, Brian Whatcott wrote:

> ludwik kowalski wrote:
> > I was not thinking about "interference fringes;" I expect the
> > glass area covered by a thin foil to be either much darker or much
> > brighter that the area that is not covered. The contrast will depend
> > on the film thickness. That would be an excellent demo for students.
> >
> > - - - - - - - - - - - - - -
> > Ludwik Kowalski
>
> Perhaps you were overlooking that the fringe effect is caused by  
> cyclic
> darkening of the field by phase differences in path length - the  
> sort of
> effect that could be increased if part of the field is phase changed  
> in
> one leg with a plastic film?

Dear Brian,

Yes, indeed; I was assuming that the beam consists of parallel rays.  
Cyclic darkening should be expected in a diverging or converging beam,  
even in a glass plate without a film. Will this prevent one from  
converting an invisible phase image into a visible amplitude image?  
This remains to be seen. A glass plate covered by a this film might  
not be the best possible approach. Suppose a glass plate is replaced  
by a thin film stretched over a hole, in a rigid frame. Suppose one  
half of the hole is also covered with the second thin film. In this  
case cyclic darkening will probably be absent. Do you agree?

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Ludwik Kowalski, a retired physics teacher and an amateur journalist.  
Updated links  to publications and reviews are at:

http://csam.montclair.edu/~kowalski/cf/   http://csam.montclair.edu/~kowalski/my_opeds.html 
   http://csam.montclair.edu/~kowalski/revcom.html

Also an ESSAY ON ECONOMICS at: http://csam.montclair.edu/~kowalski/economy/essay9.html