🚀 go-pugleaf

WHAT FOLLOWS IS MY REPLY TO A MESSAGE ON ANOTHER SERVER (PHYS-L). THE QUOTE
CONSISTS OF THE FIRST TWO PARAGRAPHS FROM THE ORIGINAL MESSAGE. WHAT GEORGE
DID WAS TO ASK STUDENTS TO MEASURE THE WIDTH OF A HAIR WHEN LAMBDA IS GIVEN
AND WHEN DISTANCES BETWEEN THE MINIMA ARE MEASURED. IT IS A GREAT ACTIVITY.

> The last year I had  my students determine the width of a hair by
> diffraction.  According to Babinet's Principle (a very nice discussion of
> this is given in Ohanian's Physics, p 892-4) a 'plug' will produce a
> diffraction pattern identical to a that of the complementary 'perforation';
> that is, a hair of width w produces the same diffraction pattern as a single
> slit of width w.
>
> It is quite easy to mount the hair or hairs on a 3 x 5 card and shine a laser
> on it and the diffraction pattern produced is quite sharp - one can see out
> to n = 8 or so in a moderately darkened room.  From the distances between the
> central maximum and the nth maximum and from the hair to the screen you
> calculate the angle, and from this and the wavelength of the light, the width
> of the hair.
>                 George Goth  Skyline College    Goth@AOL.COM

I want to thank George Goth for the description of "diffraction patterns"
activity. It came at right time; I was looking for a demo on diffraction.
I used it in the class today, as a demo, not lab. Works as described; a
thin wire was used instead of a hair. The only added twist was a "predict
what will happen" question which helped to review the wave superposition
principle. I simply asked how would another laser beam, passing through
the wire, affect the pattern created by the first beam. Nothing profound;
just the way of reviewing the essence of the law of superposition. The
two laser beams were at about 30 degrees with each other and patterns
created on the white wall (about 7 meters from the wire) were really
independent. Twelve minima were at once seen on each side of the central
maximum when the room was darkened. Distances between these minima could
be measured quite accurately with a ruler.

                  Ludwik kowalski    kowalski@apollo.montclair.edu