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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