Question

A zone plate

• A. has only one focus.
• B. can not act as convex lens.
• C. acts simultaneously as a convex lens and concave lens.
• D. has only two foci.

Hint:

The key difference between a lens and a zone plate is how they work: Lens \(\rightarrow\) Refraction, Zone Plate \(\rightarrow\) Diffraction. This fundamental difference leads to the zone plate having multiple foci (both real and virtual), whereas a simple lens has one primary focal point.

Answer 1

The Correct Option is C

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 \( f/3, f/5, f/7, \ldots \). 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).