Correct Answer: C
Explanation:
The rings are formed by having one ray undergo refection at the bottom surface of the convex lens while a refracted component passes through the lens and is reflected by the plane glass sheet with 180° phase shift. The distance of the air gap determines the interference relationship between the two light rays.
Where the two glass surfaces are in direct contact, the thickness of the air film is zero. Thus the path differential between the reflected and refracted rays is also zero. The refracted ray which reflects off the plane glass undergoes a 180° degree phase shift, so the two rays cancel each other via destructive interference. Thus a dark spot is formed at the center.