[填空题]
Monochromatic light of variable wave.+.*ear th4 r tgx0zrslength is incident normally on a thin sc8j h7z ihn0+wheet of plasthh8wzn7+ c0ijic film in air. The reflected light is a minimum only for $\lambda\;=\;512nm$ and $\lambda\;=\;640nm$ in the visible spectrum. What is the thickness of the film $(n=1.58)$? [Hint: Assume successive values of m.] nm
