Re: What is Diffraction?

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?
----- Original Message -----
From: "Guillermo" <penate@rogers.com>
To: <pinhole-discussion@pinhole.com>
Sent: Wednesday, December 11, 2002 7:06 PM
Subject: Re: [pinhole-discussion] What is Diffraction?

>
> ----- Original Message -----
> From: "Lisa Reddig" <lisa@julianrichards.com>
> >
> > I've been hunting on the web for a good description of diffraction, but
it
> > all talks about physics and x-rays. Could some one give a real simple,
> > basic description of diffraction and how it shows itself in pinholes? I
> > would appreciate no equations if possible. An example with a picture
> would
> > be cool.
>
> You already got answers to your question, so I'll try to be as terse as
> possible:
>
> Diffraction is the bending light suffers when it "skims" an opaque object.
> This java applet shows you exactly that:
> http://micro.magnet.fsu.edu/primer/java/diffraction/index.html
> By selecting (with the slider) different colors you can see how they are
> diffracted. Red diffracts more than violet. Green/yellow in the middle
of
> the slider (550nm) is what represents the average of all colors, that's
why
> people using formulas to find the optimum pinhole size use that color
> wavelength in their formulas.
> By moving the size of the aperture you can see that the smaller the
aperture
> the more diffraction it causes.
>
> In a perfect world, the image a pinhole would produce of a point source of
> light would be a perfect dot the size of the pinhole, but because
> diffraction, the image of that point source of light is a dot of light
> surrounded by alternate dark and light concentric rings, very much like
what
> you see here:
> http://www.microscopy.fsu.edu/primer/java/microscopy/airydiscs2/
> BTW, the white rings represent the dark rings.
>
> So we where expecting a dot of light and we have a "bulls eye" like dot
> instead, as you can imaging this causes the image to be unsharp.
>
> As for showing you a sample, it'd be very difficult with a pinhole image.
> Fortunately, light doesn't care if you are using a pinhole or an expensive
> lens, diffraction is always there. So I'd suggest you do the comparison
> yourself, take your SLR, an take an image using the widest aperture your
> lens have, then place a pinhole in front of your glass lens and take an
> image, compare the images, any difference you see will be due to the
> diffraction smaller aperture is causing.
>
> Guillermo
>
>
>
>
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Received on Wed Dec 11 21:30:12 2002