Convergence of light rays and Optical Path Length (OPL)

AI Thread Summary
Rays from a distant source do not necessarily travel the same Optical Path Length (OPL) when converging at a point, as the OPL depends on the intended use of the rays. For planar sources, the maximum difference in OPL can be estimated based on the source's dimensions and the distance to the convergence point. This difference, while small relative to the distance, can be significant compared to the wavelength of light. The discussion emphasizes that rays originating from the same point source may still exhibit variations in OPL. Understanding these principles is crucial for applications in optics and imaging.
pardesi
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Why do all rays coming from far off distance from a source(may be planar)... converging at a point need to travel the same Optical Path Length(OPL)
 
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They don't have to - it depends on what you want to do with them at the point.
 
pardesi said:
Why do all rays coming from far off distance from a source(may be planar)... converging at a point need to travel the same Optical Path Length(OPL)
Usually they don't travel the same OPL.
 
pardesi said:
Why do all rays coming from far off distance from a source(may be planar)... converging at a point need to travel the same Optical Path Length(OPL)
For a truly planar object with its plane is normal to the direction of the rays, then you can estimate the maximum difference in the OPL from its largest dimension (say 2d) and the distance to the point of convergence (call this D):

\delta L= D[(1+(d/D)^2)^{1/2}-1] \approx d^2/2D

This number is tiny compared to D, but it's much harder to make it small compared to the wavelength of the light.
 
but is it true if they start from the same point source
 
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