How Does a Plane Electromagnetic Wave Exist in Three Dimensions?

[j-lee00]

A simple question

How can one talk of a Plane Electromagnetic wave, when it moves within 3 dimensions, i.e. E and B?

 

[Andy Resnick]

A "plane wave" is taken to mean a propagating electromagnetic wave that has flat planes as surfaces of constant phase. This implies that the direction of propagating is in a single direction (normal to the planes). As such, a plane wave is a nonphysical idealization of real electromagnetic waves which diffract.