On page 462 in the chapter summary (aka «glossary») for chapter 17 the entry for planar wavefront asserts that All wavefronts are approximately planer when the distance to the source is large relative to the wavelength.
I very much doubt it. Here's a counterexample:
Source = sunlight ... condenser ... pinhole ... lens ... image of pinhole
I claim the wavefronts near the image point are not even remotely planar, even though that point is 0.3 million million million λ from the source and one million λ from the nearest tangible component.
I don't have any constructive suggestions. I have no idea what point this sentence was trying to make. I suppose you could say that the wavefront can be treated as approximately planar if the region of interest is small compared to the radius of curvature of the wavefront ... but that sounds tautological and uninformative ... and does not even depend on the fact that it's a wave.
Also I'm pretty sure that the wavelength is not one of the relevant variables (except maybe in peculiar extreme cases). I reckon the size of the region of interest is relevant (almost always).
On page 462 in the chapter summary (aka «glossary») for chapter 17 the entry for
planar wavefrontasserts thatAll wavefronts are approximately planer when the distance to the source is large relative to the wavelength.I very much doubt it. Here's a counterexample:
Source = sunlight ... condenser ... pinhole ... lens ... image of pinhole
I claim the wavefronts near the image point are not even remotely planar, even though that point is 0.3 million million million λ from the source and one million λ from the nearest tangible component.
I don't have any constructive suggestions. I have no idea what point this sentence was trying to make. I suppose you could say that the wavefront can be treated as approximately planar if the region of interest is small compared to the radius of curvature of the wavefront ... but that sounds tautological and uninformative ... and does not even depend on the fact that it's a wave.
Also I'm pretty sure that the wavelength is not one of the relevant variables (except maybe in peculiar extreme cases). I reckon the size of the region of interest is relevant (almost always).