Question

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

Answer 1

It happens as a result of the interference between the two light waves: one from the top of the thin film and another after getting refracted from the film and reflected back. The path difference between the two waves is equal to one half of the wavelength of monochromatic light. Therefore when the waves superimpose on each other the destructive interference appears and we can see that the thin film is dark.