[填空题]
A very thin sheet of plasticv i0ce0w )k/yhz7; wam (n=1.60) coverz2i*tvxxf9clzt,n5 x4/ w szo9tym6 6s one slit of a double-slit apparatus illuminated by 640-nm light. The cent/xc9tzw 42 xzmvin6t * f6ylz,o9t5s xer point on the screen, instead of being a maximum, is dark. What is the (minimum) thickness of the plastic? nm