[Phys-l] A demo for biologists?

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

What follows was posted last night. But I did not receive it. I am  
reposting because others might be in the same situation.

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 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 resulting 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 and double  
layers of polyethylene) were only slightly darker than the uncovered  
area (and only slightly different from each other). 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. The  
interferometer splits the original laser beam into two beams (leg 1  
and leg 2) which recombine (interference) on the screen. Leg 1  
(without a slide) is a reference beam. Leg 2 (containing the slide) is  
usually out of phase with the reference beam. 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. Glass plates of different thickness (different  
phase shifts) would produce beam spots of different brightness. The  
part of the slide covered with polyethylene is slightly thicker than  
the area that is not covered. That is why a contrast between two areas  
(covered by polyethylene and not covered) is expected, except when, by  
coincidence, the shift is n*2*Pi. Is there anything wrong with this  
reasoning?

To avoid a coincidence, two areas on my slide were covered; one with a  
single layer and another with a double layer. Only small differences  
of contrasts were observed. A better experiment is was performing,  
unless something is wrong with my reasoning. Do you agree?

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