Rami Arieli: "The Laser Adventure" Chapter 10 Section 2, page 19
Physics behind Fresnel Zone Plate
Figure 10.6 show how a Fresnel zone plate serves as a hologram.
The focal point of Fresnel zone plate is at a distance of "f" from the first zone, "f+l/2" from the second zone, "f+l" from the third zone, "f+3l/2" from the forth zone, etc.
 

Figure 10.6a: Physics of focusing Light with Fresnel Zone Plate.

If all zones were transparent, the beams from adjacent rings would interact destructively at the focal point, since there is l/2 difference between them.

Since every other ring is blocked, only path difference of l is created, and only constructive interference between all the beams from the transparent rings is achieved.


Figure 10.6b: Focusing Light with Fresnel Zone Plate.

The result is a point of light at the focal point.