A convex lens is placed on a flat glass plate and illuminated from above with monochromatic red light. When viewed from above, concentric bands of red and dark are observed. What does one observe at the exact center of the lens where the lens and the glass plate are in direct contact?
A) a bright red spot
B) a dark spot
C) a bright spot that is some color other than red
D) a rainbow of color

Question

Quizplus · Accepted Answer

At the exact center where the lens and the glass plate are in direct contact, there is no gap for the light to interfere constructively or destructively, resulting in a dark spot due to the absence of a path difference.

A Convex Lens Is Placed on a Flat Glass Plate