Understanding Optical Path Difference in Optics | OPD Explained

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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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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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