Re: 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
Received on Wed Dec 11 20:08:13 2002