Re: What is Diffraction?

erickson@hickorytech.net writes:

> Here's a question that has troubled me for a long time, since we've
gained
> the attention of the physicists among us. If the optimal image occurs
when
> all light waves are 'in phase", which the Young article says occurs at
the
> junction between the nearfield and farfield diffraction patterns
(whatever
> that is), is there another point further on where the various waves
again
> come into simultaneous phase sync, and thus optimal sharpness?
Theoretically
> there should be, but how far?

I don't think so. Once you get into the far-field, the image of a point
is the
classical Airy disk and rings. They just keep getting bigger as you get

farther from the pinhole.

I also don't think there is any point where all the waves are 'in
phase'.
You can compute (and I have) what the image of a point looks like by
dividing up the pinhole into an array of small sections. Then for any
place
on an image plane, you sum up the contributions of all the sections of
the
pinhole, taking into account the phase of the contributions. The result
is
the intensity of the light at that place in the image. When you are far
away
from the pinhole, you get the classical shape of the diffraction disk
and rings.
This is also why it isn't a good idea to talk about bending of light
next to the
edge of the pinhole, because the diffraction disk and rings are the sum
of
the contributions from all parts of the pinhole, edge and middle.

If you want to photograph the diffraction disk, mount a very small
pinhole
much farther than the optimal distance and shine a laser pointer at it.
I took
a 50 micron pinhole, put it 100mm away from the film and got a big 3mm
diffraction disk with a red laser pointer, which is what the formulas
tell
you is the size of the disk for that pinhole, distance and wavelength.
Received on Wed Dec 11 21:51:50 2002