Okay, I am not a physicist, but let's see if we're talking about the
same thing. Could it be that the author of this Wikipedia entry is
referring to Fraunhofer diffraction rather than Fresnel diffraction?
See
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/fraunhofcon.html
and
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/fraungeo.html
for a description of the differences between Fraunhofer and Fresnel
diffraction.
The simple high-school physics experiment that utilizes a shallow
wave pool (and a device that generates a plane wave) and an
adjustable slit to reproduce this phenomenon would seem to me to be
an example of Fraunhofer diffraction, as described and illustrated on
the website (http://www.phy.hk/wiki/englishhtm/Diffraction.htm)--or
am I wrong in this assumption?
Diana
On Jan 25, 2007, at 6:06 AM, Marc SCHILTZ wrote:
Diana Tomchick wrote:
The most conceptually simple example of diffraction is single-slit
diffraction in which the slit is narrow, that is, significantly
smaller than a wavelength of the wave.
What ? Diffraction when the slit is much smaller than the
wavelength ???
After the wave passes through the slit a pattern of semicircular
ripples is formed, as if there were a simple wave source at the
position of theslit. This semicircular wave is a diffraction
pattern."
Most physicist would not agree that this should be called a
diffraction pattern. This "pattern of semicircular ripples" is
simply an instantaneous image of a spherical (in 3D) or circular
(in 2D) wave that emanates from the aperture. But if the intensity
of such a wave is recorded as a function of scattering angle, a
constant value is obtained, whereas diffraction is generally
associated with the presence of so-called diffraction fringes (Born
& Wolf, section 8.1), i.e. a variation of the intensity as a
function of scattering angle. This can only occur if the wavelength
is of the same order or shorter than the size of the aperture.
Wikipedia is great, but use it with care...
--
Marc SCHILTZ http://lcr.epfl.ch
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Diana R. Tomchick
Associate Professor
University of Texas Southwestern Medical Center
Department of Biochemistry
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