Question

why do the edges of a thin film appear dark when illuminated by monochromatic light?

Answer 1

This happens because of the interaction between two light waves; the monochromatic light and the top of the thin film which reflects back. Due to this  superimpose of the two waves the result is a destructive interference ( this happens when two waves of equal frequency and opposite phase, which leads to cancellation where the positive displacement of one always concur with the negative displacement of the other) this then makes the edges of the thin film to appear dark