“Thin films are a spectrum of colors produced by a soap bubble or by the oily film on a water puddle in a parking lot. The colors result from separation of white light by a prism or by absorption of colors in a pigment.” This is the definition from our textbook. Why do thin films show color? These colors are not from a ray model of light, they are from thin-film interference, a result of the constructive and destructive interference of light waves. An example of the light produced is given in the textbook. When you hold a soap film vertically, its weight makes it thicker at the bottom that at the top. However, this thickness changes slowly from top to bottom. So, when light hits the film, part is reflected and part is transmitted. The transmitted wave travels through the film to the back surface where part of it is reflected. The thickness of the film also affects how it is reflected and transmitted. If it is thinner, then it is reflected and transmitted faster. Different colors of light have different wavelengths. Also, as the thickness increases, the light with the shortest wavelength, which is violet, will be reflected most strongly, then blue, green, yellow, orange, and finally red, which has the longest wavelength. The result of this is a rainbow! Extra Credit: If the blue paint is in the can, with the lid on, then it is black, because anything without light hitting it is black....
