My question concerns thin films of varying thickness. I have a basic understanding of thin film interference, and understand the effects of thickness on wavelength, and that with increasing thickness color fringes will be evident. I understand that the color fringes come from: n*t = m*[tex]\lambda[/tex] where n is the index of refraction, t is the film thickness, and m is an integer greater than zero. My question deals with how the fringes overlap. For example say at some point with some thickness there is a wavelength outside the visible spectrum of 1200nm; using m = 2 and m = 3 would both allow for the wavelength to be in the visible spectrum at 2 different colors. Upon observing this (assuming the observer is perpendicular to the film) how would the color look? Is there some type of rule for blending these two colors together? I have noticed that as there is increasing thickness the color fringes increase in overlap until they go gray. Any help would be appreciated, and thanks for reading!