What Does Plane Wave of Infinite Extent Mean in Optics?

[manimaran1605]

From Optics by Hecht
He says "only a plane wave of infinite extent will propagate as a plane wave"
What does it mean by " plane of infinite extent" in this context?

 

[Drakkith]

I believe he means that a perfect plane wave would need to extend to infinity in all directions perpendicular to the direction of propagation. If not, the ends of the wavefront would diffract and the wave would no longer be a perfect plane wave.

 

[sophiecentaur]

From Optics by Hecht
He says "only a plane wave of infinite extent will propagate as a plane wave"
What does it mean by " plane of infinite extent" in this context?

It may be easier to think if it as the limit of a spherical wave as the radius increases without limit. Nothing has 'interfered with' the edges of the wave.