[填空题]
Monochromatic light of variable wavelength is incident normally onytzxm(xqma ;23 t u:y: a thin sheetixwbxwqe;foq - km ie 0,:-5,hr7 sab( of plastic film in air. The reflected light is a minimum only x;ko0baw s(:,i wr-ei5hq, q fb7mxe-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
