How can we visualise a half period zone inside a zone plate?

[Vipanj]

how can we consider/ visualise a half period zone inside blackened portion of +ve zone plate ?

 

[Ibix]

It's just a ring of black paint, basically. What more is there to know?

 

[Vipanj]

By definition (n)th half period zone is the annular area between (n-1)th and (n)th circle in a plane wavefront. Then how can a black region in zone plate can have a half period zone ?

 

[Ibix]

I'm not sure I understand what you are trying to say.

The zone edges lie at ##r_n=\sqrt{n\lambda f+n^2\lambda^2/4}##. The ##n##th zone is the region between ##r_{n-1}## and ##r_n##. So to make a zone plate you paint the region between ##r_0## and ##r_1## black, leave the region between ##r_1## and ##r_2## transparent, paint the region between ##r_2## and ##r_3## black, etc.