[填空题]
A fine metal foil separates one end of two piemhbt; jrq4gs,5c -ni2 ces of opticallyni3rtg vo7+1 z (b3mym flat glass, as in Fig. 24–33. When light of wavelength 670 nm is incident normally, 28 dark lines are ob+1v3(tn zgbyom7mr 3iserved (with one at each end). How thick is the foil? $\mu m$
