Understanding Optical Path Difference in Optics | OPD Explained

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The discussion centers on understanding the optical path difference (OPD) formula, specifically OPD = n2(AB + BC) - n1(AD). The confusion arises primarily from the path length and the presence of a negative sign in the equation. It is clarified that the line connecting points C and D is perpendicular to the rays, indicating that C and D are on the same wavefront. Therefore, the OPD is determined by calculating the difference in optical path lengths from points A to C and A to D. This explanation helps clarify the underlying principles of OPD in optics.
moatasim23
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SOme one help me with the OPD.
How is here the OPD=n2(AB + BC) -n1(AD)?
I am not confused with the introduction of 'n'..Rather the path length is confusing me..Why is there a negative sign?
 

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They are computing a path difference ...
 
Just to elaborate a little on Simon's response.

Notice that the line connecting C and D is perpendicular to the rays. This indicates the C and D lie on the same wavefront.

Ergo, the OPD is calculated as the difference in optical path lengths from A-C and from A-D.

Claude.
 

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