Convex lens

skrewd1

A convex lens is placed on a flat glass plate and illuminated from above with monochromatic red light. When viewed from above, concentric bans 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.

Doc Al

There is a thin layer of air between the surfaces, thus light reflects at the glass/air boundaries. The first reflection is at the lens/air boundary, the second at the air/plate boundary. Consider the interference between these two reflections. (What is their phase difference?)

skrewd1

bump
Still can't figure out

Doc Al

What have you done so far? What can you say about the two reflections?