Step 1: Understanding the Concept:
A zone plate is a diffractive optical element that focuses light. Unlike a refractive lens which uses refraction, a zone plate uses diffraction and interference from a series of concentric opaque and transparent rings. Its properties are different from a conventional lens.
Step 2: Detailed Explanation:
Let's analyze the properties of a zone plate:
Multiple Foci: A zone plate does not have a single focus. It produces a series of focal points along its axis. The principal focal length is the most intense, but other, weaker foci exist at . Therefore, statement (A) "has only one focus" and statement (D) "has only two foci" are incorrect.
Focusing Action: The zone plate creates these focal points by constructive interference of diffracted light. This focusing of light is analogous to the action of a convex (converging) lens. Therefore, statement (B) "can not act as convex lens" is incorrect.
Converging and Diverging Action: In addition to the real (converging) foci, a zone plate also produces a series of virtual (diverging) foci on the opposite side of the plate. This diverging action is analogous to a concave lens. Because it produces both real and virtual foci, it can be said to act simultaneously as a convex lens and a concave lens. Therefore, statement (C) is correct.
Step 3: Final Answer:
The most accurate description among the choices is that a zone plate acts simultaneously as a convex lens (producing real foci) and a concave lens (producing virtual foci).