(ii) a very thin sheet of plastic (n = 1.60) covers one slit of a double-slit apparatus illuminated by 680-nm light. the center point on the screen, instead of being a maximum, is dark. what is the (minimum) thickness of the plastic?

Two 2.0-cm-diameter disks face each other, 1.0 mm apart. they are charged to ±10 nc. what is the electric field strength between the disks? a proton is shot from the negative disk toward the positive disk. what launch speed must the proton have to just barely reach the positive disk?