At 7:03 AM -0800 1/31/00, *- DORIS FANG -* <sfsttj@xxxxxxxxxxxxxxxxxxx> wrote:
So, what exactly causes diffraction ? I'm not sure I understand
the reason.
Diffraction is a basic result of the wave nature of light. For the most
part, you can think of light as particles (which it also is) but whenever
you get to scales near the wavelength of light (about 1/2000 of a millimeter
for green light in the middle-short of the spectrum) the wave stuff comes
into play.
In particular, when light travels through an aperture it tends to spread
out because the edges of the waves (where it has been cut off by the edges
of whatever aperture it just went through) act as little point sources
radiating light in all directions. (So do the waves in the middle of the
beam, but thanks to interference effects all of their sidewise propagation
cancels out) The smaller the aperture, the greater the contribution of the
edges and hence the more diffraction. (This is the version minus the integrals
and stuff.) The standard formula for a diffraction spot is roughly 2L/A where
L is the wavelength of the light and A is the aperture (and the spot size is
as a fraction of the distance from aperture to wherever you're measuring,
usually the focal plane.)
So if you had a 5mm aperture (f10 for a 50mm lens) you would have a diffraction
spot of roughly 1/10,000 x f.l. or in this case about 1/100 mm.
paul
Paul Wallich pw@xxxxxxxxx
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