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
