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: Concept Introduction:
A zone plate, a diffractive optical element, focuses light through diffraction and interference from alternating opaque and transparent concentric rings, distinguishing it from refractive lenses.
Step 2: Detailed Analysis:
Zone plate properties are examined:

Multiple Focal Points: Zone plates possess numerous focal points along their axis, with the primary focus being the most intense. Additional weaker foci exist at \( f/3, f/5, f/7, \ldots \). Consequently, claims of a single focus (A) or only two foci (D) are erroneous.

Focusing Mechanism: Constructive interference of diffracted light generates these focal points, mirroring the focusing action of a convex lens. Thus, the assertion that it cannot act as a convex lens (B) is incorrect.

Convergent and Divergent Behavior: Beyond real (converging) foci, zone plates also produce virtual (diverging) foci on the opposite side, analogous to a concave lens. This dual capability allows it to function simultaneously as both a convex and a concave lens, validating statement (C).
Step 3: Conclusion:
The most accurate description is that a zone plate concurrently acts as a convex lens (real foci) and a concave lens (virtual foci).