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

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

On May 17, 2009, at 7:34 PM, Bernard Cleyet wrote:

> Please Brian!
>
> The fact I replied several times should belie that.
>
> OTO thank you** for expanding my telegraph.
>
> ** Note, I didn't write Die Panzer.
>
> bc was planing to expand before reading B.W. had.
>
>
> p.s. They must be process clean ( fresh from the box).
>
>
> On 2009, May 17, , at 09:24, Brian Whatcott wrote:
>
> > or perhaps he felt you were unworthy of
> > his time and effort:

Dear all,

1) Today I followed Brian's advice and tested my hypothesis with the  
Michelson-Morley interferometer at school. Unfortunately, this old  
instrument is in very poor shape. I had to move the platform by hand,  
instead of using the micrometer. Semi-transparent mirrors have  
scratches and are covered with spots. This created all sort of masking  
effects. But the effect I expected was clearly recognizable. I hope  
someone will confirm my observations. Here are some details:

a) He-Ne  0.95 mW laser from Spectra Physics, Model 155 SL (1987)
b) Distance from the interferometer 13 meters (input beam diameter 1.5  
cm
c) A standard microscopic slide was partially covered with the clear  
polyethylene "Cling Wrap." The mean thickness (of a 300 cm^2 sheet)   
was 1.36 mg/cm^2. This translates into 13.6 microns, if the density is  
1.0 g/cm^2 (or 6.8 microns if the density is 2 g/cm^3).
d) The part of glass covered with polyethylene was not distinguishable  
by visual examination under day light, except the border line.
e) A sheet of  white paper (screen) where the beam spot was observed  
was located about 50 cm from the interferometer.

2) I wish I could say that the area of glass not covered with the  
plastic film was red while the covered area was black (or vice versa).  
But that is not what I saw. The covered areas (single layer and double  
layers of polyethylene) were only slightly darker than the uncovered  
area. Another teacher, who had no reason for being biased, confirmed  
this fact. I believe that more convincing results (higher contrasts)  
would be obtained with a  well-performing interferometer. A pinhole,  
suggested by Bernard, would probably help to eliminate some masking  
effects.

3) The explanation of the expected effect (turning the invisible phase  
contrast into visible amplitude contrast) is simple. A beam spot seen  
on the screen was the superposition of two beams, one passing through  
the glass slide (leg 1 of interferometer) and another not passing  
through the glass slide (leg 2 of interferometer). The phase shift  
between these two parts of the beam (resulting from the initial laser  
beam) depends on the glass thickness. The glass covered with a plastic  
film acts as a slightly thicker glass plate. Constructive  
interference, as always, happens when the phase shift is n*2*Pi, where  
n is an integer. Destructive interference happens when the phase shift  
is Pi, 3*Pi, 5*Pi, etc. In general glass areas of different thickness  
would produce beam spots of different brightness.  Is there anything  
wrong with this reasoning? Yes, my English is horrible; I am tired.

- - - - - - - - - - - - - -
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