A film in air is illuminated from directly behind by uniform white light - Physical Optics?

A soap film in the air is illuminated from directly behind (i.e. at normal incidence) by uniform white light (the light has equal intensity at all wavelengths). There is bright transmitted light at wavelengths of 620nm and 412nm. What is the thinnest soap film that will produce this effect due to interference? (ume nsoap = 1.33, the same as water)