# Plane wave conditions

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Which conditions have to be fulfilled in order to approximate a light beam by a plane
wave (i.e. $\phi(x)\approx \phi(0)e^{ikx}$)?

I am looking for both mathematical and experimental conditions. At the moment, the only thing I can think of is to consider that the source is at a very distant point and the use of collimators.

Closed as per community consensus as the post is high-school/UG level.
recategorized May 27, 2015

Is this question maybe more appropriate for physics stackexchange?

I agree with @JoshBurby.

Voting to close as high-school/UG level. PO is for graduate-level+ questions.

A spherical wave has an exact expression (with $1/r$ factor). Within certain precision $\varepsilon$ the denominator can be considered constant. Thus, the plane wave approximation has the corresponding limits in the volume.