I do understand the derivation of Newton's rings. But I couldn't realise the meaning to add $\lambda/2$ in the expression :path difference($x$)$=2tn\cos r$ i.e $x=2tn\cos r+\lambda/2$ for obtaining the optical path difference.
Where,n=ref. index
I am pretty sure $\lambda/2$ is taken for the phase change of light by $\pi=180^\circ$ {phase diff$=kx=(2\pi/\lambda)x$}. But, why is $\lambda/2$ directly added on the expression as $x=2tn\cos r$ already considers optical path for the calculation of path difference.
I wish someone could help me to construct the physical picture of the situation. Just I need is why $\lambda/2$ is added in the above equation, I presume I could do rest of the thing myself.
THANK YOU!
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Lamichhane88
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When light reflects from material with refractive index bigger than the medium it is coming from it obtains phase difference $\pi$
Analytical aspect of this phenomena is well explained in first answer to this question. If don't want to go into E&M details just check the picture in third answer to the same question.
stinglikeabeer
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