I Validity of Fresnel Approximation

AI Thread Summary
The discussion centers on the validity of the Fresnel approximation in relation to paraxial-paraboloidal waves, specifically questioning the conditions under which it applies. It notes that the Fresnel number and maximum angle influence the approximation's validity, with a derived condition of a^4 << 4z^3λ being highlighted. The conversation emphasizes the distinction between Fresnel (near field) and Fraunhofer (far field) regions, mentioning the Rayleigh Distance as a boundary that is not precisely defined. Additionally, it clarifies that the near radiation zone should not be confused with the reactive near field, underscoring the importance of terminology in this context. Overall, the discussion seeks to clarify the conditions and implications of using the Fresnel approximation in wave propagation scenarios.
yucheng
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Wikipedia says that Fresnel diffraction is valid if the Fresnel number is approximately 1. What Fresnel number then is the Fresnel approximation for paraxial-paraboloidal waves valid? It's not mentioned...

Oh I just realized that

$$\frac{N_F \theta_m^2}{4} \ll 1$$

So it depends on the maximum angle... Oops

Anyway, we have the validity condition for Fresnel Approximation ##a^4 \ll 4 z^3 \lambda##

So.. for what a is it valid? How small should ##a## be given the relation ##\ll##, that is rather arbitrary?

Suppose ##z = 1 m##, ##\lambda = 633 \\ nm## (Exercise 2.2-1, Fundamentals of Photonics, Saleh & Teich)

Thanks in advance!
 
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My simple understanding as a microwave engineer is that for the case of waves emerging from an aperture, if we are close to the aperture we have Fresnel conditions and if we are far we have Fraunhofer conditions. The two regions are sometimes referred to as the near and far radiation zones. The boundary is ill defined, and occurs at the Rayleigh Distance, which may be defined as (Diameter^2) / 2 lambda. In the Fresnel region we tend to see a parallel beam, but it usually has a waist and can also have hot spots. Maybe it can also have a black dot in the middle. In the Fraunhofer region the beam diverges at an angle defined by the aperture size in wavelengths, and we also see sidelobes, which are mainly defined by the illumination taper of the aperture. The radiation pattern in the Fraunhofer region does not change with distance.
The near radiation zone is not to be confused with the reactive near field, which occurs at fractions of a wavelength from a source, so we need care in using the terminology.
 
I like Fresnel is "intermediate far" and Fraunhofer is "far". But of course the near and far radiation zone is a better description.
 
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