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If you have radiation emitting at the focus of a lens, why is it that on the other side of the lens, rays are essentially parallel within a region $\approx \frac{D^2}{\lambda}$ , where D is the diameter of the lens and $\lambda$ is the wavelength.

I have seen this numerous times but never explained.

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Try to derive the paraxial approximation and Fourier propagation from the wave equation this should enable you to see how this approximation works and why. No better way to learn than to do.

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  • $\begingroup$ This is a comment, not an answer. $\endgroup$ – garyp Nov 27 '17 at 16:07
  • $\begingroup$ Yes it is it provides the op with the means to understand what (s)he is requesting to understand. $\endgroup$ – MJC Nov 27 '17 at 16:09

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