[填空题]
A very thin sheet ofe2pf,p l78grq plpo4 5w;v qwyskg f,3l,astic (n=1.60) covers one slit of a double-slit apparatus illuminated by 640-nm light. The center point on the scres;q, ,y vwlofw5 p3k4gen, instead of being a maximum, is dark. What is the (minimum) thickness of the plastic? nm
