[填空题]
A fine metal foil separates one end of two pieces of optica;ayt+*,1x ivtqj0nb n , pi3xulrjopbw82ba- ;x9 ycn1.;rev lly flat glass, as in Fig. 24–33. When light of wavelength 670 nm is incident normally, 28 dar yacrjv.lopxwb ;8r 2b-en19; k lines are observed (with one at each end). How thick is the foil? $\mu m$
