[填空题]
A thin film of alcoh/ i7 ,gud8cjk4jy,ui ng.6adk l:td.o0 ahz 8aol $(n=1.36)$ lies on a flat glass plate $(n=1.51)$. When monochromatic light, whose wavelength can be changed, is incident normally, the reflected light is a minimum for $\lambda\;=\;512\;nm$ and a maximum for $\lambda\;=\;640\;nm$ What is the minimum thickness of the film? nm
