theMaréchal plates of the actual wave, they must be bright. Basic geometry and the formula for a
circle reveal
∆z
f
D
f≅
=
()
22
2
2
λλ/#
that is, the depth of focus is also set by the f/# and wavelength of the beam, this time with a square law
dependence on f/#.
Once again, the physical limit of resolution is about one wavelength. is combined with the dirac-
tion limit sets the resolution limit in all three spatial dimensions to about a wavelength.
1.5.6 Rayleigh Range
In applications where beams must travel signicant distances, it is important to understand when
simple ray tracing will no longer predict where t
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