*= Doris Fang =*
Diffraction - off the top of my head!
Diffraction occurs because of the wave nature of light. I seem to remember
from school days optics something called the Raleigh Criterion. This states
that light passing through an aperture behaves like a series of point
sources of light across the aperture separated by the wavelength(?) of the
light. Each point source radiates light out in a sphere centered on it.
The effect of this is that when the aperture diameter is close to the
wavelength of the light, or smaller, at the edge of the aperture, light
spreads out away from the original direction of the light. This change in
light direction is diffraction. I presume that imperfections in the lens
glass or its surface behave as apertures thereby causing diffraction. The
changes in light direction will corrupt the photo image. The wavelength of
the orange light from a sodium lamp is 589 nm (0.589 microns).
You can build a model to show how diffraction occurs in a bath of water with
an aperture set to about the same size as the wavelength.
Robin.
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Dr Robin M Allen
Laboratory of Physical Chemistry and Electrochemistry
Helsinki University of Technology
PO Box 6100 tel +358 9 451 2577 (fax)2580
FIN-02015 HUT email rallen@xxxxxxxxx
Finland www http://www.hut.fi/~rallen/
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