[填空题]
A very thin sheet of pla2(h-c)ed/e*,nb p agrey- imt 7 p0bxbr cm7m/cqp09estic (n=1.60) covers one slit of a double-slit apparatus illuminated by 640-nm light. The center point on the screen, instead of being a maximum, is dark. What is the (minimum) thickn7ebrxbc70m 09 qcp mp/ess of the plastic? nm
