After a rain you can see bull's-eye shaped blobs of rainbow colors on street pavement. These are drops of oil or gasoline that have gotten very thin. When light reflects off these layers, the color whose wavelength is just twice the thickness of the oil film will be reflected and seen. If you investigate these blobs, notice that they always end up with white on the outside ring. Iridescent colors in general are usually the result of this phenomenon.
Peacock feathers get their brilliance from thin-film interference.
Opals, beetle-backs, and some butterfly wings owe their colors to this phenomenon. Another application comes with the coating of lenses. Like an oil film on water the coatings on lenses have intermediate optical density. In optical equipment such as cameras and binoculars, reflections from the lens surfaces seriously harm the images formed. The lenses are coated to prevent reflections.