[填空题]
What is the minimum (non-zero) thickn7 wfi.rk*of* tess for the air layer betweewg ypxpy;4i0(f7* 9hflty d*sv+- zh en two flat glass surfaces if the glass is to appear i7v; l ytfgxpd0(w p h9-*eyh+zf4s *ydark when 640-nm light is incident normally? nm
What if the glass is to appear bright? nm