It happens so because of the interference between the two light waves ; one from the top of the thin film;another after getting refracted from the film and reflected back.The path difference between the two waves is λ/2.
How does thin film interference occur?
Constructive and destructive interference of light waves is also the reason why thin films, such as soap bubbles, show colorful patterns. This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface.
What is thin film technology?
Thin Film Deposition is the technology of applying a very thin film of material – between a few nanometers to about 100 micrometers, or the thickness of a few atoms – onto a “substrate” surface to be coated, or onto a previously deposited coating to form layers.