Question

What is the shape of the air film formed in the Newton's ring experiment?

• A. Plane
• B. Cylindrical
• C. Spherical
• D. Wedge-shaped

Hint:

Remember: Variable thickness air film = Wedge-shaped (Newton's rings).

Answer 1

The Correct Option is D

Concept: Newton's rings are formed due to interference of light waves reflected from the top and bottom surfaces of a thin air film formed between a plano-convex lens and a glass plate.
Step 1: Formation of air film.
When a plano-convex lens is placed on a flat glass plate, a thin layer of air is trapped between them.
Step 2: Nature of the air film.
The thickness of this air film gradually increases from the point of contact outward.
Step 3: Shape identification.
Since the thickness varies continuously in one direction, the air film behaves like a wedge-shaped film.
Step 4: Evaluating the options.

• Plane $\rightarrow$ Uniform thickness (incorrect)
• Cylindrical $\rightarrow$ Not applicable (incorrect)
• Spherical $\rightarrow$ Lens is spherical, but film is not (incorrect)
• Wedge-shaped $\rightarrow$ Thickness varies gradually (correct)

Step 5: Additional insight.
The varying thickness of the air film causes constructive and destructive interference, producing concentric circular fringes known as Newton's rings.
Step 6: Conclusion.
Thus, the air film formed in Newton's rings experiment is wedge-shaped.