I 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.