how does this aproximation reduse the integral
Where are lambda and k_z in the integral?
The approximation simply assumes the integrand to be constant, evaluates it at the midpoint (z'=0) and multiplies it with the range z' takes.
so its because r'=0 ??
then why we need
The exponential function should not change its argument too much, so kzl/r should be small compared to 1.
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